0) Always, for every IF, write an ELSE. In a while loop, make sure to increment the index. Whenever you are calculating squares/multiplying, be very careful about overflow. arrays have length, strings have length(), collections have size(). You lexicographically compare two strings using the compareTo method, in the Comparator class you have to override the compare method, if a class implements the Comparable interface, you have to override the compareTo method.

1) Why does the ternary operator unexpectedly cast integers?

3) Is there any difference between return null and return (ListNode<Integer>) null ? i.e. does casting null somehow change null?

4) What is the best way to pass int by reference?

5) Why do we get one extra negative number?

6) C++ : This is compiler-specific, but if you're using gcc then you can compile with -ftrapv to issue SIGABRT when signed integral overflow occurs.

7) Is Java pass-by-value or pass-by-reference?

8) What is the difference between a simple comment and a Javadoc comment

9) What is the difference between Stack and Heap? Which kind of variable is stored where?

10) What is the difference between a "class method" and an "instance method"?

11) Using setters inside Constructors. When is it ok, and when is it not? //TODO

12) Creating a test directory in Intellij and running tests in it:
- IntelliJ IDEA. Writing Tests with JUnit 5
- Use Alt + Insert to "Add a Maven Artifact Dependency"

13) What is the difference between CLASSPATH and Build Path:
- //TODO:
Look here - On SO, What does "Could not find or load main class" mean? Explanation about what CLASSPATH actually is on SO.

15) What is the difference between myArray[index++] and myArray[++index]:

16) Java does not have a problem with forward referencing. What this means is that the below happens to be completely valid code in java. Note that the variable count id defined AFTER it is used in the varcount method. Not only does the code compile, but it also gives the correct result!

17) Using varargs
- What are they
- How an where they are used

18) What is System.err used for:
Every running program has these three streams

19) What are static initialization blocks are why do we need them?

21) Another Page: another page

21) Set up a new Linux VM:
- Step 1 - Youtube Link.
Do not eject the CD image. - Make VM full screen. First turn off the VM. Go to VirtualBox Manager - Settings - Display - Screen - Change Video memory to 128 MB. Restart the VM. Screen shoudl be full screen now.
- Install curl sudo apt install curl
- Install java sudo apt install default-jdk
- Install vim sudo apt install vim

22) What is the difference between fn(String... args) vs fn(String[] args)
- The only difference between the two is the way you call the function. With String var args you can omit the array creation.
- The difference is only when invoking the method. The second form must be invoked with an array, the first form can be invoked with an array (just like the second one, yes, this is valid according to Java standard) or with a list of strings (multiple strings separated by comma) or with no arguments at all (the second one always must have one, at least null must be passed).
- SO Link

23) What is the point of "final class" in Java?
- In Java, items with the final modifier cannot be changed. This includes final classes, final variables, and final methods. A final class cannot be extended by any other class. A final variable cannot be reassigned another value. A final method cannot be overridden.
- A final class is simply a class that can't be extended. It does not mean that all references to objects of the class would act as if they were declared as final.

59) Why are methods declared final?
- You can declare some or all of a class's methods final. You use the final keyword in a method declaration to indicate that the method cannot be overridden by subclasses.
- You might wish to make a method final if it has an implementation that should not be changed and it is critical to the consistent state of the object.
- Methods called from constructors should generally be declared final. If a constructor calls a non-final method, a subclass may redefine that method with surprising or undesirable results.
- Note that you can also declare an entire class final. A class that is declared final cannot be subclassed. This is particularly useful, for example, when creating an immutable class like the String class.
- Source on Oracle Java Docs here.

59.2) A variable declared as final in a class cannot be instantiated/set with a value when it is being done from a setter that is being called from within a constructor.
- Source on SO.
-

61) Why can final object be modified?
- If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object. final simply makes the object reference unchangeable. The object it points to is not immutable by doing this. Source on SO.

24) What does it mean when one class is said to inherit another in actual code?
- Stream.Builder<T> interface inherits from the Consumer<T> interface, (meaning, interface Builder<T> extends Consumer<T>)
- Also, remember that extends should go before implements.

24) for-each is not applicable to String
- So you cannot write:

25) What is the difference between putIfAbsent and computeIfAbsent in Java 8 Map ?
- SO Link
- Code example using putIfAbsent and computeIfPresent

26) Java 8 has a merge operation
- SO Link
- Take special note of the method implementation here:
default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V>remapppingFunction)
remappingFunction.apply(oldValue, value) The oldValue is the existing value mapped to the key in the map. The value is the second argument passed into the merge method.
- Solution to this problem on LC

27) What should I normally initialize a char to?
- char temp = ''; is a compile error. You cannot do this.
- char temp = '\u0000';
- This is just the unicode for null. SO Link. Another SO Link.

28) Never use == when comparing doubles or floats. Never use it to compare Integer, Long, Double either.

29.1) LeetCode Medium Top Interview Questions: link here

29.1) Array transformation problems:
- move-zeroes
- remove-element
- remove-duplicates-from-sorted-array
- 3sum

30) Things to do with Linked Lists:
- remove-linked-list-elements
- delete-node-in-a-linked-list
-

31) Bit Manipulation:
- Add Binary. Description has links to other problems that use the XOR operators. - https://leetcode.com/problems/single-number/
- https://leetcode.com/problems/missing-number/
- https://leetcode.com/problems/find-the-difference/
-

31.1) Recursion Questions:
- Refer this post for an explanation of how recursion works on a high level.
- Same Tree
- Symmetric Tree
- Subtree of Another Tree
- Invert Binary Tree
- Binary Tree Maximum Path Sum with explanation.

31.2) Traversing a Binary Tree:
- When to use Preorder, Postorder, and Inorder Binary Search Tree Traversal strategies
- There are two general techniques to traversing a tree: LC Solution
a) BFS: We scan through the tree level by level, following the order of height, from top to bottom. The nodes on higher level would be visited before the ones with lower levels.
b) DFS: In this strategy, we adopt the depth as the priority, so that one would start from a root and reach all the way down to certain leaf, and then back to root to reach another branch. The DFS strategy can further be distinguished as preorder, inorder, and postorder depending on the relative order among the root node, left node and right node.
- Binary Tree Pre Order Traversal. (From LeetCode solutions) Taking pre-order traversal as an example, in each iteration, we pop one node from the stack and visit this node. Then if this node has a right child, push its right child into the stack. If this node has a left child, push its left child into the stack. It is noteworthy that we push the right child first so that we can visit the left child first since the nature of the stack is LIFO(last in first out). After that, we can continue to the next iteration until the stack is empty.
- Binary Tree In Order Traversal (Traversal using iterative approach)
- Binary Tree Post Order Traversal. Lot of confusion on this one in the comment section about reversing the order or something. Refer this solution to see a way to do it without having to reverse the list. The explanation of what is going on is present in the comments: The algorithm is that we push each node twice onto the stack. Each time we pop a node out, if we see that there is a same node on the stack, we know that we have not done traversing yet, and need to keep pushing the current node's children onto the stack. However, if the stack is empty, or the top element is not the same as the current element, we know that we're done searching with this node, thus we can add this node to the result.
- Validate Binary Search Tree: good example of how to use iterative traversal to solve problem. Inorder traversal of a Binary Search Tree prints the nodes of the tree in the ascending order if the tree is a valid BST.
-

31.3) Graph Traversal:
- Binary Tree Level Order Traversal (Doing BFS using layers. Think Kleinberg-Tardos algo)
- Find all paths between two nodes (idiot this is an NP Hard problem)
- Find the shortest path between two nodes. Remember that the shortest path between two nodes is just the BFS iteration of the graph
- Find all the shortest paths between two nodes

31.4) BackTracking
- Card on LeetCode
-

32) Printing out an array:

32) Interconversion of types:
- Read here on Baeldung.
- Also, surrogate pairs for chars on SO.

32) Reversing a String:
- String a_reverse = new StringBuilder(a).reverse().toString();
- When the problem is to reverse a number, you can also do the same thing. Refer this leetcode question and look at the comments for the below code snippet. - Addendum:
StringBuilder result = new StringBuilder(s.charAt(i) + s.charAt(j));
This will not initialize a StringBuilder with the String formed by the concatenation of chars at location i and j. Instead, it will create a StringBuilder with the specified capacity, i.e. it will treat the chars as integers and do the arithmetic sum, not the String concatenation.
- The append method of StringBuilder class will append the specified string to the end of the string.

32) What is the difference between instanceof and getClass
- Here is SO link explaining it.
- objectWeHave instanceof classTypeWeWant tests whether the object on the LHS is an instance of the type on the RHS or some subtype.
- getClass() == ... on the other hand, tests whether the types are identical.
- And of course there is more to it.

33) You cannot iterate over a HashSet using the fori loop. You HAVE to use the for-each loop.

34) If you sort an array of strings using Arrays.sort(myArray), are they sorted lexicographically, or are they sorted by their lengths?
- They are sorted lexicographically.

35) Write about the different ways you can use the split method in Strings
- The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit is positive then the pattern will be applied at most limit-1 times, the array's length will be no greater than limit, and the array's last entry will contain all input beyond the last matched delimiter.
- The method signature is as follows: public String[] split(String regex, int limit)
- The string "boo:and:foo", for example, yields the following results with these parameters:
Regex Limit Result with separator as ":"
2 (Limit args passed in){ "boo", "and:foo" }
5 (Limit args passed in){ "boo", "and", "foo" }
-2 (Limit args passed in) { "boo", "and", "foo" }
Regex Limit Result with separator as "o"
5 (Limit args passed in) { "b", "", ":and:f", "", "" }
-2 (Limit args passed in) { "b", "", ":and:f", "", "" }
0 (Limit args passed in) { "b", "", ":and:f" }

36) Running s.substring method within a loop is a very slow computation. Avoid if possible.
Check problem on leetcode. Replace teh StringBuilder with just substring and you should see what I am talking about.

38) How to convert primitive array to List and vice-versa

38.1) Interconversions between List<String> and String[]

38.2) Adding and removing from a list while iterating over the same list
- Explanation of ConcurrentModificationException on SO.
- You are able to add and remove elements from a list while you are iterating over the same list if you are using a for-i type loop.
- But if you are using a for-each style loop, adding and removing elements while you are iterating over them will throw a ConcurrentModificationException. - A ConcurrentModificationException is thrown when you try to modify a Collection while iterating over it and without using the method remove of the iterator. When you use the syntax for (Integer i : list2), an iterator is created for you. If you like to delete items from your code you explicitly needs to iterate over an iterator or you need to loop using the old style for loop. Or you can use an Iterator. But remember that by using an iterator, you can only remove elements from the list while you are iterating over the same list. You cannot add elements to the list. Source on SO.
- But the question is, do you really want to modify the list while you are also iterating over it? Consider the following scenario. Source on SO. - So, back to our original question, how do you remove an element from a list while iterating over the same list? Use the removeIf method introduced in Java 8.
- Alternatively, you can use the for-i loop and traverse the list backwards to remove the required element as shown here in this SO answer.

38.3) Adding and removing from a Set/Map while iterating over the same set/map
- You cannot add or remove elements from a Set/Map while iterating over it using the enhanced for loop. - In order to remove elements from a Set/Map, similar to the case of List, you can use the removeIf method.
- Note in the below example that the way that the removeIf method is being called on a Set is different from the way that it is being called on the Map. Also note the way that you iterate over a Map using the forEach method. You are doing map.forEach((key, value) -> {System.out.println(key + ", " + value)}) and not map.forEach(entry -> {System.out.println(entry.getKey() + ", " + entry.getValue())}) - Note: there are a total of three ways that you can use the removeIf method to remove elements from a Map. Source. removeIf is a method available to Collections. A Map itself is not a Collection and does not have access to removeIf itself. But, by using: values, keySet or entrySet, a view of the Map's contents is returned. This view implements Collection allowing removeIf to be called on it. The Collection returned by values is backed by the Map and that changing either the Collection or the Map will alter the other. keySet and entrySet are also both backed by the Map's contents.
- Note the following usage of forEach method. forEach is a BiConsumer, which means that it accepts two values and does not return anything. If you try to return anything, the code will not compile.

38.5) Using the forEach() method vs using the for-each loop. Which should we prefer and why?
- Read here on SO.
- Also the counter arg answer for the above one. Note that the forEach() method IS NOT meant to be the replacement of the for-each loop. So dont mix the two. Continue using the for-each loop for iterating over the Set or HashMap for now.
- Interesting discussion in the answers and the comments.

38.6) Why can't you cast Object[] to String[]?
- Read here on SO.

41) How to store the Entries of a HashMap in a List
- Changing order to make Map related stuff appear together. Normal order continues below.

41.1) Map.Entry returns an unmodifiable entry. You can edit the entry's value only if the entry is backed by a map.
- Refer the following example to see. Also note that the key of the entry cannot be changed once the entry has been created. This is irrespective of whether there is a backing map or not. So the question now is, how do you use a tuple in java if this is not going to work?
- Also in the example, note the way that we are creating an Entry. It is being done using Entry.new(key, value)

39) A TreeMap can be sorted only on keys, not on values
- The Comparator goes into the constructor of the TreeMap.
- SO Link
- And this: link

40) How exactly do you sort something in the natural order vs. something in the reverse order
- There are 2 ways in which you can change the ordering of sorting, from ascending to descending.
- One way is you change the value returned on comparison. That is, when a > b, return -1 instead of +1 (for sorting in descending). Returning +1 sorts it in ascending, returning -1 sorts it in descending.
- Second way is you change the order in which you compare the objects. That is, if b > a, return 1. That will also change the ordering from ascending to descending.
- Both ways are shown in the below examples.
- Also refer the difference between Comparable and Comparator.

40.1) How to implement the Comparable interface and the compareTo method?
- Guide to implementing the compareTo Method on Baeldung.
- SO links:
a) How to implement the Java comparable interface?
b) How do I write a compareTo method which compares objects?
c) Why should a Java class implement comparable?

- First, let us take a look at the Comparable interface itself: - Note that the interface is making use of Generics. The type parameter that is passed into the interface name here in Comparable<T> is the same type parameter that is being expected to be passed in as the args to the compareTo(T o) method. Read the comment to this answer on SO to understand that.
- So what this means is that you can make a class implement the Comparable interface in the following ways:
a) You implement Comparable<ComparablePerson> b) You implement Comparable<Person> c) You implement Comparable - All of the above 3 will compile and order the ComparablePerson class as intended. Point that we are trying to note over here is that the same type parameter that is used in interface definition is also used in the compareTo method. Depending on the type that you use for the Comparable, you can only use it with the appropriate type of object. Comparable<ComparablePerson> can be compared only with another ComparablePerson, Comparable<Person> will be able to be compared with other Person types, and if you use Comparable<Object>, you can compare all Object types with it.

What is Comparable interface

?
- Read this SO Answer to understand this.
- Comparable defines a 'natural ordering'. What this means is that you're defining it when one object should be considered "less than" or "greater than". Things like String and Integer already have a 'natural ordering' associated with them. For example, you can sort strings lexicographically by calling Collections.sort() and the strings will be sorted for you. Same for Integers as well. All sorting methods in JDK will use this natural ordering by default.
- But what happens in the case of custom objects that you create? For example, the ComparablePerson object that we created above. If we need to sort a list of ComparablePerson objects, we need to specify a natural ordering to the objects, just like the String class or the Integer class. This is done by implementing the Comparable interface inside the class. Which is what we are doing in the above examples.
- Note that you can do the equivalent thing by defining a Comparator. The difference is that the Comparator defines the ordering logic outside the object. Maybe in a separate process I need to order the same objects by name and not id - in that case the ordering isn't necessarily a property of the object, or differs from the objects natural ordering. In that case, could use an external Comparator. This is just like sorting a list of strings by the length of the string, instead of sorting them lexically.
- The point is that the logic has to exist somewhere. That can be:
i) in the object itself, if it's naturally comparable (implements Comparable -e.g. integers)
ii) supplied in an external comparator, as in the example above.

Some key facts to remember while you are overriding compareTo:

(Source on SO)
i) CompareTo must be in consistent with equals method e.g. if two objects are equal via equals() , there compareTo() must return zero otherwise if those objects are stored in SortedSet or SortedMap they will not behave properly.
ii) CompareTo() must throw NullPointerException if current object get compared to null object as opposed to equals() which return false on such scenario.

42) What would be faster - comparing two String using .equals method or computing the hash codes of two strings and comparing those hash values to see if the two strings are equal?
- Using .equals will be faster.
- SO link

43) How to create a Queue, Stack, PriorityQueue?
- Why use a deque instead of a Stack explained here on Baeldung.
- Don't do this now. Read this on the other page here.
- Create a Queue: Deque<Integer> queue = new ArrayDeque<Integer>(); SO Link
- Create a Stack: Deque<Integer> stack = new ArrayDeque<Integer>(); SO Link
- Create a PriorityQueue: PriorityQueue<String> pq = new PriorityQueue<>(initialCapacity, customComparator); SO Link

43.1) How does the Deque implementation differ between LinkedList and ArrayDeque?
- The difference lies between how each of the DS implementations handle null values. When the Deque interface uses the LinkedList implementation, you are allowed to add null values to the Deque. But if you use a ArrayDeque implementation, adding null values to the Deque is going to throw a NullPointerException
- There was also the confusion that if you are using the ArrayDeque as a stack, and doing a push operation on it, does it shift the remaining element in the stack to the right? So would that mean that using a LinkedList implementation in this case be better then? No. As mentioned in the javadoc here, most ArrayDeque operations run in amortized constant time. Exceptions include remove, removeFirstOccurrence, removeLastOccurrence, contains, iterator.remove(), and the bulk operations, all of which run in linear time. So you should continue to use ArrayDeque implementations when using it as a stack as well (source on SO).

43.2) TreeSet vs PriorityQueue? Finding k maximum elements in a collection.
- Top K Frequent Words problem on LC.
- Java PriorityQueue with fixed size SO Link
- Is there a PriorityQueue implementation with fixed capacity and custom comparator? SO Link
- When should I use a TreeMap over a PriorityQueue and vice versa? SO Link
- The PriorityQueue docs on Oracle Java here states the runtime of the PriorityQueue operations as follows:
a) this implementation provides O(log(n)) time for the enqueing and dequeing methods (offer, poll, remove() and add()); linear time for the remove(Object) and contains(Object) methods; and constant time for the retrieval methods (peek(), element(), and size()).
- [element() is just another version of peek(). A collection designed for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either null or false, depending on the operation). Source on Queue docs on Oracle Java]
- Note that the TreeSet interface that is an implementation of the Set interface has additional methods that are not present in the Set interface and that are useful.
- Time Complexity of the TreeSet methods are mentioned in this SO Post.
a) Operations which work on a single element are all O(ln n) comparisons except first and last which are O(1) comparisons or O(ln N) node search time.
b) comparator(), iterator(), clear(), first(), isEMpty(), size(), last(), pollFirst(), pollLast() are O(1)
c) add(), ceiling(), contains(), floor(), headSet(), higher(), lower(), remove(), subSet(), tailSet() are O(ln N)
d) clone(), equals(), hashCode(), toArray() and toString() are O(n)
- From Oracle Java Docs: TreeSet implementation provides guaranteed log(n) time cost for the basic operations (add, remove and contains). (Source on Oracle Java Docs).

44) What does package-javafx-util-does-not-exist error mean?
- Don't add a library bigger than the JDK itself, when all you need, is just a Pair type. You can use Map.entry(a, b) instead, or AbstractMap.SimpleImmutableEntry, if you need support for null or pre-Java 9 versions. Or any of the other 3rd party libraries providing a Pair type without the baggage of a UI library/webkit engine.

45) Checked vs Unchecked Exceptions
- In Java, there are two types of exceptions: checked exceptions and un-checked exceptions. A checked exception must be handled explicitly by the code, whereas, an un-checked exception does not need to be explicitly handled.
- For checked exceptions, you either have to put a try/catch block around the code that could potentially throw the exception, or add a "throws" clause to the method, to indicate that the method might throw this type of exception (which must be handled in the calling class or above).
- Any exception that derives from Exception is a checked exception, whereas a class that derives from RuntimeException is un-checked. RuntimeExceptions do not need to be explicitly handled by the calling code.
- Also read about exceptions on the Java Notes page here
- When to use Checked and when to use Unchecked exception:
- SO Link 1
- SO Link 2
- SO Link 3

46) Why am I seeing weird "String not found" errors in IntelliJ when I change the JDK from 8 to 11?
- Step 1, install JDK 11 by following this SO Link.
- When prompted by the installer, also choose the third option to change the JAVA_HOME variable value to JDK 11. Once the installation is complete, you can check the installed cersion by running java -version in your cmd. You can check the updated value by running echo %JAVA_HOME%.
- Next, we need to tell IntelliJ that we have a new JDK for it.
- Step 2: File -> Project Structure -> Platform Settings -> SDK -> Click on the + symbol and add the JDK. Point to the path where the JDK is installed. Should be: C:\Program Files\AdoptOpenJDK\jdk-11.0.8.10-hotspot
- The previously empty CLASSPATH should now be filled with new files.
- How to print environment variables to the console in PowerShell here.
- How to prepend a path to the path variable: set path = C:\Program Files\Java\jdk1.8.0_281\bin;%path%;
- java -version prints the version of JVM. javac -version prints the version of java compiler.
- If there are multiple versions of java present on your machine, the version which is the highest in the list is going to be picked first.

47) How to create an in-memory/embedded database using Spring Boot + H2 that auto creates tables every time that the application is run.
- The application.properties file should look like this: - The database can be viewed at the foll URL: http://localhost:8080/h2-console
- Make sure that the JDBC URL in the window maps to the correct URL: jdbc:h2:mem:testdb

48) What does it mean to catch and Exception vs throws an Exception?
- Precisely what is going on over here.

49) What is the meaning of this when used in constructors?
- Check this SO Link for explanation.
- This SO Link shows the different ways in which the this keyword can be used.
- From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. (Java Doc Source)

50) Why can't this() and super() be used together in the same constructor?
- There is a difference between super() and this().
super: calls the base class constructor. If no super() is explicitly written,the compiler will add one implicitly.
this: calls current class constructor
- Both this() and super() are constructor calls. Constructor call must always be the first statement. So you either have super() or this() as first statement.
- In places where you have to use both this() and super(), make sure that last constructor to be called is the super() constructor. SO Link Source.
- You can also use super to call an overridden method of a super class. In that case, the super call does not necessarily need to be the first statement in the method body.

51) How does reduce in Stream actually work?

52) When is the finalize() method called in Java?
- Read here, here, here.
- TODO

52) How to package programs as jar files?
- Read here
- TODO

53) How are IP Addresses mapped to MAC Addresses?
- Kishori Sharan in Java 8 chapter on Network programming says: "This (Network Interface) layer resolves the destination hardware address from the IP address and places it in the frame header". How does it do that? - Read Link here.
- TODO

54) What is Heap-Pollution?
- Read here on SO.

55) What is a Class Loader in Java?
- SO here.

56.0) Intellij being Intellij I guess?
- Why am I seeing weird "library has broken clases path" error all of a sudden: - If Intellij is showing compile errors that it cannot find the library that you have imported, one way to fix it would be to first "Reload all Gradle Projects" from the "Gradle" tab in Intellij. Once done, right click on the name of the module here, and select "Rebuild Project". That fixed the compile errors. - But even if that fixed the compile errors, the "Project Structure" window still shows the errors. The way to fix them is as follows:
- Bug on Intellij to track feature.
- How to fix: click on the [Fix] and this should open a dropdown "Remove invalid root". That will remove the invalid path from "Libraries" tab in the above window. You can reimport the sources by following the steps here on SO on how to tell Gradle to download all sources.
- Classpath things: When you downloaded the Budi Kurniawan project from here and opened up in Intellij, you got errors saying that the classes like HttpServletRequest, HttpServletResponse, were not found. You had to download the jar files that contained these classes. Normally when you download a project, all these dependencies are managed through tools like maven, gradle. They download whatever dependencies are required. But in this case, there wasn't any build file specified, hence you had to resort to downloading the dependencies manually (as in when you hovered over the name of the file that was missing, Intellij asked you whether you want to download the dependency, and you said yes, because what else were you supposed to do, and that created a folder like this that contained the dependencies). Point to note is that the files did not go into the "External Libraries" section. - But this was not enough. Now you need to tell Intellij that these files are available and that they can be imported. This is where the classpath thing comes in. This explainshow to add directory to classpath in an application run profile in IntelliJ IDEA?. The top voted answer does not work. Instead this one does. But the other answers are also worth going through. This SO question talks about Adding Jar files to Intellij Idea classpath. This is the official Intellij doc for how to add dependencies to Modules. If you have multiple modules in your project, then you don't want to go to each module and add a dependency for all of them. Instead, do File - Project Structure - Libraries. You will see all the libraries listed there. Right-click on the library that you want to add to the multiple modules, and then click Add. From here, select the Modules into which you want to add the library to.

56) Maven related stuff in IntelliJ
- "Java file outside of source root intelliJ" error: SO Solution here.
- Show dependency hierarchy as UML here on SO
- How to view effective POM in IntelliJ here on SO.
- Maven build vs Maven install on SO here.
- Running mvn clean install will also show the location where the resulting jar has been packaged and placed. You can check it out by manually navigation to the specified directory.
- The provided scope means that you intend that library to be provided by the environment in which you deploy your code. Why would you do such a thing?
- Download all the sources for the files that you have in your project: here.

57) Gradle related stuff in IntelliJ
- "Could Not target platform" error on SO here.
- Build error with gradle Could not find method testCompile() which leads to What's the difference between implementation, api and compile in Gradle?

57.1) Git related stuff
- Let's say, my current git log is (latest) commit3 :: commit2 :: commit1 (earliest). I reset the head to commit2. Now I want to go back to commit3, but I forget the sha-1 of commit3, so I want to look at the log , but "git log" will only show commit2, commit1, my question is how to reset to commit3 or let the log show also commit3 now? We use git reflog in this case (Source on SO). Note that after you run this command you will see a "HEAD Detached" warning in IntelliJ. You can rectify that by following this SO post linked here. It explains what a detached head really means, and what you can do about it.
57.2) I mistakenly added files to Git using the command: git add myfile.txt I have not yet run git commit. How do I undo this so that these changes will not be included in the commit? Source on SO.
- To unstage a specific file: git reset fileName. To unstage all files from the current change set: git reset.

58) Difference between Abstract classes and interface in java 8
- Java Doc Source here.
- An abstract class is a class that is declared abstract - it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
- An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:
abstract void moveTo(double deltaX, double deltaY);
- If a class includes abstract methods, then the class itself must be declared abstract.
- When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract.
- Methods in an interface that are not declared as default or static are implicitly abstract, so the abstract modifier is not used with interface methods. (It can be used, but it is unnecessary.)
- You extends a abstract class. You do not implements an abstract class.

- Abstract Classes Compared to Interfaces:
- Read differences here on SO.
- All methods in an interface are implicitly public and abstract (but not final). All fields in an interface are implicitly public, static and final. Whereas in Abstract classes you can define private variables.

60) Oh boy.. What are interfaces in Java?
- (Source on Java Docs).
- My notes on Interfaces

60) 'Optional.get()' without 'isPresent()' check

62) What is the difference between Compiled vs Interpreted Languages?
- On SO.

63) What is meant by DAO Layer, Service Layer/Business Layer, Web layer? - Pivotal Certified Spring Web Application Developer page 17

64) Java: compiled or interpreted? Compiler vs Interpreter? JIT Compiled vs Interpreted? Traditional interpreter vs Traditional Compiler vs JIT Compiler vs AOT Compiler?
- The main thing to keep in mind: JVM has, both, an interpreter as well as a compiler. With JVM, both interpreter and compiler (the JVM compiler and not the source-code compiler like javac) produce native code (aka Machine language code for the underlying physical CPU like x86) from byte code. Source on SO. So when people talk about compiling java code, there are two different compilers in picture - one is the javac compiler converting the .java files to .class files and then there is the compiler in the JVM itself that is converting parts of the bytecode to native instructions that can be run on the CPU/machine code (This is the JIT-Compilation that people refer to).
- Source of image on SO.

Java: compiled or interpreted

- First of all, there is no such thing as a "compiled language" or an "interpreted language".
- Generally, both interpreters and compilers first need to parse the source code and turn it into a representation, called AST (abstract syntax tree). A compiler then turns the AST into executable code (through various transformations), while an interpreter might just directly 'interpret' the AST or sometimes compile and execute it (just-in-time compilation). The statement that there is nothing like a "compiled language" or an "interpreted language" is correct in that this has nothing to do with the language: in theory, you can write an interpreter and compiler for any language. Which one to use really depends on the use-case, scenario and environment. A compiler has the advantage that it only needs to do it's job once, regardless of how often you then execute the program. An interpreter needs to parse the source every time (or do some caching), thus you have an overhead for each execution which might take way longer than the actual execution time of the final program. On the other hand, an interpreter is more flexible (it can take into amount the current environment and thus do optimizations a compiler is not allowed to do). But the differences don't stop here, these are just two obvious points. Source on SO.

Compiler vs Interpreter

- So we first need to understand what is the difference between a compiler and an interpreter. Check Source on SO here.
- To convert source code into machine code, we use either a compiler or an interpreter. Both compilers and interpreters are used to convert a program written in a high-level language into machine code understood by computers. However, there are differences between how an interpreter and a compiler works. Look at the interpreter vs compiler table at this link and this SO answer.
- On a higher level, the difference really comes down to 'prepare the code for later execution' (compilation) or 'execute the code right now' (interpretation). Some interpreters (e.g. Unix shells) read the description one small piece at a time and act on each piece as they see it; some (e.g. Perl, Python) read the entire description, internally convert it to a simpler form and then act on that. Some interpreters (e.g. Java's JVM, or a Pentium 4 chip) only understand a very simple description language that is too tedious for humans to work with directly, so humans use compilers to convert their high-level descriptions to this language. Compilers never do the work. Interpreters always do the work.
- A compiled language is one where the program, once compiled, is expressed in the instructions of the target machine. For example, an addition "+" operation in your source code could be translated directly to the "ADD" instruction in machine code. An interpreted language is one where the instructions are not directly executed by the target machine, but instead read and executed by some other program (which normally is written in the language of the native machine). For example, the same "+" operation would be recognised by the interpreter at run-time, which would then call its own "add(a,b)" function with the appropriate arguments, which would then execute the machine code "ADD" instruction. (Note that, in the case of interpreted languages, all of this is happening during run-time.)
- Advantages of compiled languages:
a) Faster performance by directly using the native code of the target machine. It takes large amount of time to analyze the source code but the overall execution time is comparatively faster.
b) Opportunity to apply quite powerful optimisations during the compile stage
- Disadvantages of compiled languages:
a) Generates intermediate object code which further requires linking, hence requires more memory.
b) It generates the error message only after scanning the whole program. Hence debugging is comparatively hard.
- Advantages/disadvantages of interpreted languages:
a) Easier to implement (writing good compilers is very hard!!)
b) No need to run a compilation stage: can execute code directly "on the fly". It takes less amount of time to analyze the source code and start running it, but the overall execution time is slower.
c) Can be more convenient for dynamic languages
d) No intermediate object code is generated, hence are memory efficient.
e) Continues translating the program until the first error is met, in which case it stops. Hence debugging is easy.
Note that modern techniques such as bytecode compilation add some extra complexity - what happens here is that the compiler targets a "virtual machine" which is not the same as the underlying hardware. These virtual machine instructions can then be compiled again at a later stage to get native code (e.g. as done by the Java JVM JIT compiler). Source on SO here.
- You can now see the problem with a purely interpreted language. A traditional compiler is translating all the source code to machine code before the execution of the program. Hence at runtime, all the CPU has to do is run the machine code instructions (for eg. ADD). On the other hand, for a purely interpreted language, the source code needs to be translated to the machine code for every instruction - at runtime. If an instruction is present in a loop, the interpreter might end up translating the same source code to machine code millions of time. This is a huge overhead.

JIT Compilers

So that brings us to the solution - that is : Just-In-Time (JIT) Compilation.
- Source on SO here.
- An interpreter pretty much converts each byte-code instruction to corresponding native instruction by looking up a predefined JVM-instruction to machine instruction mapping (see below pic). Interestingly, a further speedup in execution can be achieved, if we take a section of byte-code and convert it into machine code - because considering a whole logical section often provides rooms for optimization as opposed to converting (interpreting) each line in isolation (to machine instruction). This very act of converting a section of byte-code into (presumably optimized) machine instruction is called compiling (in the current context). When the compilation is done at run-time, the compiler is called JIT compiler. Since Java designer went for (hardware & OS) portability, they had chosen interpreter architecture (as opposed to c style compiling, assembling, and linking). However, in order to achieve more speed up, a compiler is also optionally added to a JVM. Nonetheless, as a program goes on being interpreted (and executed in physical CPU) "hotspot"s are detected by JVM and statistics are generated. Consequently, using statistics from interpreter, those sections become candidate for compilation (optimized native code). It is in fact done on-the-fly (thus JIT compiler) and the compiled machine instructions are used subsequently (rather than being interpreted). In a natural way, JVM also caches such compiled pieces of code.

Bringing it all together with a Java JIT Compiler

- Source on SO.
- Java implementations typically use a two-step compilation process. Java source code is compiled down to bytecode by the Java compiler. The bytecode is executed by a Java Virtual Machine (JVM). Modern JVMs use a technique called Just-in-Time (JIT) compilation to compile the bytecode to native instructions understood by hardware CPU on the fly at runtime. Some implementations of JVM may choose to interpret the bytecode instead of JIT compiling it to machine code, and running it directly. While this is still considered an "interpreter," it's quite different from interpreters that read and execute the high level source code (i.e. in this case, Java source code is not interpreted directly, the bytecode, output of Java compiler, is). It is technically possible to compile Java down to native code ahead-of-time and run the resulting binary. It is also possible to interpret the Java code directly.
- To summarize, depending on the execution environment, bytecode can be:
a) compiled ahead of time and executed as native code (similar to most C++ compilers)
b) compiled just-in-time and executed
c) interpreted
d) directly executed by a supported processor (bytecode is the native instruction set of some CPUs)

JIT Compilers vs AOT Compilers

- Source on SO here.

References:
1) Is Java a Compiled or an Interpreted programming language ?
2) How does an interpreter/compiler work
3) Compiled vs. Interpreted Languages
4) JIT vs Interpreters
5) Understanding the differences: traditional interpreter, JIT compiler, JIT interpreter and AOT compiler
6) What is the industry definition of an interpreter (as opposed to a compiler)?
7) The difference between compiler and interpreter
8) Exact difference between Compiler and Interpreter?
9) There is no such thing as a "compiled language" or "interpreted language"
10) What is the difference between implementing a compiler and an interpreter?

63) Why do we have to add the suffix f after we declare a variable of type float?
- Source on SO.

64) Will browser download image twice if it's used in both an image's src as well as a div's background-image?
- Source on SO.
- Browsers are pretty smart when it comes to caching. It should only ask for it once. Additionally when it asks the server for an image, it usually sends with the request for the image, a couple of headers that tell the server.. Hey, I want this image, but I got one already that has these attributes that you told me about it last time you sent it to me. The server can then respond with a 200 meaning it's different content 304 meaning the one you have is the same, so I won't send it again, use the one you got.

65) Alternative to clearfix hack in HTML?
- display: flow-root

66.1) Can overridden methods return different types?
- Source on SO.
- Yes it may differ but there are some limitations. Before Java 5.0, when you override a method, both parameters and return type must match exactly. Java 5.0 it introduces a new facility called covariant return type. You can override a method with the same signature but return a subclass of the object returned. In other words, a method in a subclass can return an object whose type is a subclass of the type returned by the method with the same signature in the superclass.

66.2) Can we override a method by passing subclass of the parameter used in super class method?
- No, we cannot. Source on SO.
- In the below example, you can see that we are trying to override the eat method in the Dog class by passing in a subclass of the Food class, Flesh. But the method is NOT overridden. You can test this by adding a @Override annotation on top of the eat() method in the Dog class, and the compiler will throw an error saying that "Method does not override method from its superclass".
- So we set out to override methods, but we ended up overloading methods.
- And hence we get the behavior that we are now showing in the below snippet.

66.3) How can we trigger different method selection based on the parameter's real type?
- Source on SO. Excellent answers. Go through all of them.
- In the following code snippet, I expect the method selection to take into consideration the real (not the declared) parameter type. Hence the expectation is that following code would have printed foo(Object o), foo("foobar"), foo(12). But instead it prints foo(Object o) three times. What am I missing?
- Your expectation is wrong. In Java, dynamic method dispatch happens only for the object the method is called on, not for the parameter types of overloaded methods.
- Citing the Java Language Specification: When a method is invoked (S15.12), the number of actual arguments (and any explicit type arguments) and the compile-time types of the arguments are used, at compile time, to determine the signature of the method that will be invoked (S15.12.2). If the method that is to be invoked is an instance method, the actual method to be invoked will be determined at run time, using dynamic method lookup (S15.12.4).
- The last statement is a bit confusing. Can you clarify it further.
- Sure. The compile time type of the method parameters is used to determine the signature of the method to be called, in this case foo(Object). At runtime, the class of the object the method is called on determines which implementation of that method is called, taking into account that it may be an instance of a subclass of the declared type that overrides the method.
- Rewriting the above code showing this in action: - Ability to dispatch a call to a method based on types of arguments is called multiple dispatch. In Java this is done with Visitor pattern. However, since you're dealing with Integers and Strings, you cannot easily incorporate this pattern (you just cannot modify these classes). Source on SO.
- The key to understanding this is that the test for which method or constructor is most specific does not use the actual parameters: the parameters appearing in the invocation. They are used only to determine which overloadings are applicable. Once the compiler determines which overloadings are applicable and accessible, it selects the most specific overloading, using only the formal parameters: the parameters appearing in the declaration. Source on SO.
- We can also use Generics to solve this problem as shown in this SO answer.

66.4) Also read this:
- Calling a super method which calls an overridden method

67) You can create overloaded methods and constructors just by changing the order of the parameters in the method/constructor declaration
- Constructor overloading best practice on SO.
- Yes, this is valid code somehow.

67) Can an abstract class have a constructor? And if yes, what purpose does it satisfy?
- Yes, when we define a class to be an Abstract Class it cannot be instantiated but that does not mean an Abstract class cannot have a constructor. Each abstract class must have a concrete subclass which will implement the abstract methods of that abstract class.
- When we create an object of any subclass all the constructors in the corresponding inheritance tree are invoked in the top to bottom approach. The same case applies to abstract classes. Though we cannot create an object of an abstract class, when we create an object of a class which is concrete and subclass of the abstract class, the constructor of the abstract class is automatically invoked. Hence we can have a constructor in abstract classes.

68) What is an EnumSet and what is it used for?
-

68) What is bfcache
- Freakin' awesome: read here.

69) Can I spawn new Threads in JSP?
- Source on SO.
- It is absolutely not a good idea to manually spawn unmanaged threads in a JSP/Servlet web application. For sure not when done in the JSP side as it does not fit JSP's responsibilities in any way.
- Also there are now Concurrency Utils that enable you to achieve this so that you do not have to do this manually. Source on SO.

70) Where to place and how to read configuration resource files in servlet based application?
- Answer by BalusC explaining different ways that you can access a file from your code. Do you understand what a 'classpath' actually means now?
- Another SO Question: Java Package Does Not Exist Error. It has this oracle link for explaining Classpath.

71) How to make sure that the response/data is not cached in case of a HttpServletResponse
- Read this SO Answer.
- What does this even mean:

72) Suppose we use System.setProperty to store a property. Where is the property stored? But then you also say that once you have saved a property using setproperty, you can access that property anywhere in the entire java application. How is that fucking possible?
- Source on SO.
- System class has a static member variable named props which is of the type Properties. Properties extends HashTable. The HashTable is defined as Hashtable<Object,Object>. But note that each key and it's corresponding value in the property list is a String. So this should answer the first part of your question about where the property is saved, like literally. It is stored in a HashTable
- But where is the property stored. You haven't understood how java programs work. When you run a Java program, you are actually starting a JVM instance. That instance will have its own System properties. That is where you put your property. When you run another program, that will have its own System properties. So, you cannot expect a property which you set in one JVM instance to be accessible from another JVM instance!
- When you start a Java process a separate JVM instance is created that runs your process. Also since props is a Class variable(not an instance variable) just one copy of it will be present in the corresponding Class instance which will be set when that class is loaded. Now this is under the assumption that you do not have any of your custom class loaders in which case behavior might be different. But for simplistic scenario you System.setProperty() and System.getProperty() will set system properties that you can access via any class running as a part of that java process(JVM).

73) Why does byte b = 0b1000_0000; produce a compile error?
- Read on SO here.

74) What happens when you left shift a -ve number? Can it become +ve?
- Yes, with the correct bits, a -ve number will become +ve

75) What is the difference between >> and >>> ?
- >> stands for right-shift with sign-extension
- >>> stands for right-shift with zero-extension
- Difference between >> and >>> here on SO.

76) What is String Interning?
- Read this SO answer to understand how interning works
- Java String Pool on Baeldung.

77) What is the difference between [ab] and (a|b) in regex?
- Check here on SO and here.
- [ab] matches one character (a or b) and doesn't capture the group. (a|b) captures a or b, and matches it. In this case, no big difference, but in more complex cases [] can only contain characters and character classes, while (|) can contain arbitrarily complex regex's on either side of the pipe.

78) Is a switch statement faster than an if-else block in java?
- Yes, there could be performance gains of using a switch instead of an if-else block.
- Read this answer on SO. Also, a bit below this is written: javac implements switch several different ways, some more efficient than others. In general, the efficiency will be no worse than a straight-forward "if ladder", but there are enough variations (especially with the JITC) that it's hard to be much more precise than that.
- This SO answer explains the difference between lookupswitch and tableswitch. The linked answers on that page also contains some good answers.
- switch in the case of String was added only in Java 7. Using strings for switches requires calculating the hash value of the string. Hence, using switch for string might not always be the fastest way. Check this SO answer for details.

79) How to properly manage Date in your java classes?
- Check this link on SO to see how to validate dates in Java.
- This is a link to Oracle Java Tutorials on Date and Time.

80) TF is CORS?
- Add the following below your @RestController annotation: @CrossOrigin(origins = "*", allowedHeaders = "*") (but you should not be using wildcard characters while specifying origins because that might cause browsers to ignore the annotation altogether. Instead specify the exact domain.)
- Chrome Websockets CORS policy says: the server enforces an Origin. The Origin HTTP header is set by the browser to the origin of the HTML page containing the JavaScript that is opening the WebSocket connection. A server MAY check that header and deny. But since you say other browsers are working (which?), this is unlikely.
- Enabling CORS for a RESTful webservice. Docs on Spring site
- CORS in Spring on Baeldung
- CORS on Mozilla
- Origin is not allowed by Access-Control-Allow-Origin on SO.
- Why does my JavaScript code receive a "No 'Access-Control-Allow-Origin' header is present on the requested resource" error, while Postman does not?
- Excellent video by Steve Griffith on YT explaining how CORS works
- YT video explanation about how CORS works using AWS
- CORS Setup in a Spring Boot application
- Spring Boot tutorial on how to use CORS in your Spring application
- And of course there is an entire book on it
- Question with useful snapshots on SO
- But apparently, there is no same-origin policy for websockets?

- Good link on how to set cookies for cross-origin requests on SO.
- Note the following: You cannot set or read cookies on CORS requests through JavaScript. Although CORS allows cross-origin requests, the cookies are still subject to the browser's same-origin policy, which means only pages from the same origin can read/write the cookie. withCredentials only means that any cookies set by the remote host are sent to that remote host. You will have to set the cookie from the remote server by using the Set-Cookie header.
- How can I setup my server and frontend in order for cookies to be shared across domains: answer, including the basics like difference between site and domain, on SO

- Personal Local Env testing: What all is mandatory for a CORS cookie to work (atleast in localhost for now)?
- Having a CORS cookie as HttpOnly is not mandatory for CORS.
- Setting SameSite=Lax also works on localhost. But it should not work on actual servers. Also note that: If no SameSite attribute is set then the cookie is treated as Lax.
-

- From the FrontEnd, this is how we are initialising the process: - This is what the backend code looks like so that we can handle the request correctly:
- Controller: - Adding cookie to the response:

81) Why does sockJS add a '/info' to a given websocket url path?
- Because it is in the official docs, as shown here on SO.
- YT tutorials on WebSockets: From Zero to Hero with Spring WebSocket, Deep dive into Spring WebSockets

82) What is mixed content in a browser?
- Explained on MDN.
- When you visit a secure web page (that is, using HTTPS), your connection is encrypted with SSL. If the HTTPS page also includes content retrieved through a regular HTTP connection, the connection is only partially encrypted. This is called a web page with mixed content. By default, mixed content is blocked in all major browsers. (Source)
- I have not found a way that can display mixed content in browsers. Is it even possible?
- I still have to try this though: How to get Chrome to allow mixed content?
- Since your application is serving the content over HTTPS, its a must to access other resources also through HTTPS to solve the mixed content issue. The best way to approach is to use a proxy. For that you can either setup a proxy manually using EC2 or using a managed service like AWS CloudFront or AWS API Gateway depending on the nature of the resources you access. If its a REST API use API Gateway and if its more of content (JS, CSS, HTML and other files & etc.) use CloudFront (Source)
- A secure page (HTTPS) cannot get any data from an insecure page/request (HTTP) without a server-side solution.
- You cannot make an AJAX request to an https page if you are currently in http because of the Same Origin Policy. The host, port and scheme (protocol) must be the same in order for the AJAX request to work. You can either make sure that the originating page is on the same host and scheme or implement CORS (cross-origin resource sharing) on the target domain to permit this particular request. (But then you are going to run into the mixed content problem again).
- Alternatively to this, you can add this to your header: <meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests"> Source.
- Same thing about adding the meta tag explained on MDN. The HTTP Content-Security-Policy (CSP) upgrade-insecure-requests directive instructs user agents to treat all of a site's insecure URLs (those served over HTTP) as though they have been replaced with secure URLs (those served over HTTPS). This directive is intended for web sites with large numbers of insecure legacy URLs that need to be rewritten.
- Tried installing the nightly build of firefox because it has security.mixed_content.upgrade_display_content enabled which apparently allows FF to display mixed content as shown here on MDN site.
- Can I use HTTPS on localhost?
-

83) An inner class can either extend a single class or implement a single interface. Remember that you extend an abstract class and can either extend or implement an interface. You do not implement a abstract class.

84) Why would you want to use static variables in a class?
- Think of static variables as metadata, i.e., data about data. What this means is that if you want to store information about the class itself, in that case you would be better off using static variables.
- Consider the following example: you have a brand of a car, say Ferrari CX. There is a base price associated with the car. This base price is going to be constant across all instantiations of the same car. In that case, you would want to declare the base price as a static variable.
- (Source) The general rule is: Avoid static. However:
a) The main method must be static. When the program first starts, there are no instances (objects) yet to talk to, so you cannot start from an instance method. However, the classes themselves have been created, so static methods can be used.
b) When you want to have a variable that describes something about the class itself, not the individual objects of that class, make it static.
c) When you want to have a variable that always has the same value for every object of the class, forever and ever, make it static.
d) If you have a method that does not use any instance variables or instance methods, you should probably make it static.
- Source Key points to remember when using a static variable:
- static methods in Java are resolved at compile time. Since method overriding is part of Runtime Polymorphism, so static methods can't be overridden
- abstract methods can't be static
- static methods cannot use this or super keywords
- The following combinations of the instance, class methods and variables are valid:
a) Instance methods can directly access both instance methods and instance variables
b) Instance methods can also access static variables and static methods directly
c) static methods can access all static variables and other static methods
d) static methods cannot access instance variables and instance methods directly; they need some object reference to do so
- Question: when you use the new keyword to create an object, that object is created on the heap. Where are static variables stored then? Does it matter?

85) What are native methods in Java? How can I see the implementations of them?
- The native keyword is applied to a method to indicate that the method is implemented in native code using JNI (Java Native Interface). The actual implementation doesn't have to use JNI. () Certain JRE methods are handled intrinsically by the JVM. In fact, it's not even mandatory that the implementation is actually native code. It’s just "implemented in a language other than the Java programming language". Source on SO.
- You can look at the source code for OpenJDK (licensed under GPLv2 with Classpath Exception). Probably the best way to study the JDK implementation internals.

86) Adding a new Tomcat Server to the IntelliJ Idea Run Configuration
- First you will have to make sure that Tomcat is actually installed on your system. Download the version that you require from here. You can download the .exe installer. Change the shutdown server port to 8005 as explained here (C:\Program Files\Apache Software Foundation\Tomcat 9.0\conf). Accept the other default options.
- In your system, for some reason you were unable to view the contents of the installed folder because the folder was in the Read Only Mode. You had to change the permissions for the folder by following the link here.
- Once that is done, the next step is to just add the Tomcat Dependency to IntelliJ. Which you can do by following the tutorial here.
- Once that is done, then from now on in your projects, you just need to Click on Run - Edit Configurations - + - Tomcat Server Local - Input the required information

87) trim vs strip method for String in Java
- Read here on SO.

88) What are the rules for casting in Java? How does it work?
- Read here on Baeldung.
- The important thing to understand is that by casting a reference, you are not changing the actual object that the reference was pointing to in any way.
- You can either Upcast a reference (eg. casting a ArrayList to a List) which will limit the properties and methods that you can access on the object. Else, you can Downcast an object, which will increase the properties and methods that you can access on the object.
- Upcasting in the same inheritance tree is done implicitly (again, think about casting a ArrayList to a List. You never have to mention the cast. The compiler automatically does that for you).
- In upcasting, the following apply:
a) A reference variable can refer to an object if the object is of the same type as a variable or if it is a subtype.
b) Upcasting happens implicitly.
c) All Java objects are polymorphic and can be treated as objects of supertype due to upcasting.
- Then you have Downcasting. It's the casting from a superclass to a subclass.
- In the above example, we used the Animal reference to point to a Cat object. But as we noticed, it is not possible to call the meow() method on the Cat object if the reference is of type Animal. Hence, in this case, we need to Downcast the Animal to a Cat. - Running into ClassCastException.
- Why did we have to use the instance of operator to check whether the reference is of type Cat before we called the meow method on it?
- Let's remove that and see what happens. We get the following error: Exception in thread "main" java.lang.ClassCastException: class JoshuaBloch.Chapter3.Dog cannot be cast to class JoshuaBloch.Chapter3.Cat.
- Point to note is that the code compiled without any errors. It is only when we ran the program that we found out the issue. So the point is, ClassCastException is not going to be warned by the compiler. You as a developer are responsible for making sure that the code will not throw any such exception.
- Understand that the exception is thrown because you initially stored both, a Dog and a Cat object in the List (because you used the new operator to do it, even though the reference that you used to access the object was an Animal reference). But like we saw above, casting does not change the object in any way. All it does is change the way in which you access the methods and properties on the object. Hence even though the reference assigned to the Dog object is Animal, the object intrinsically is still a Dog object. And when you try to cast that Dog object to a Cat object, the JVM is unable to do it because you are trying to downcast to a type that does not match the type of the actual object.

What are the rules that go into casting?

- Read this answer here on SO.
- Another explanation on SO.
- Suppose we want to cast object d to A A a = (C) d;
- Internally, 3 rules will be checked by the Compiler and the JVM. The compiler is checking the first two rules at compile-time and the JVM is checking the last one at runtime.
a) Rule 1: Type of d and C must have some relation (child to parent or parent to child or same type). If there is no relationship then we will get a compile error (inconvertible types).
b) Rule 2: C must be either same type or derived type (subclass) of A otherwise we will get a compile error (incompatible types).
c) Rule 3: Runtime object type of d must be same or derived a type of C otherwise we will get a runtime exception (ClassCastException Exception).

89) What are <init> and <clinit>? What are the differences between them?
- Explained here on Baeldung. To better understand how initialization works in the JVM, it's highly recommended to read the JVM specification.
- Source on SO.
- <init> is the (or one of the) constructor(s) for the instance, and non-static field initialization.
- <clinit> are the static initialization blocks for the class, and static field initialization.

What commands can I use to see the detail of the <clinit> of some class of Java?

- The javap commands displays the <init> and the <clinit> methods. (Source on SO. Good explanation).

Who invokes the class initializer method <init> and when?

- <clinit> is a static method added by javac and called by JVM after class loading. We can see this method in class bytecode with bytecode outline tools. Note that <clinit> is added only if a class needs static initialization.

How is all of this related to the ExceptionInInitializerError that we are seeing?

- Read about the error here on Baeldung.
- Note the following points that are present in the page:
- The ExceptionInInitializerError indicates that an unexpected exception has occurred in a static initializer. Basically, when we see this exception, we should know that Java failed to evaluate a static initializer block or to instantiate a static variable. In fact, every time any exception happens inside a static initializer, Java automatically wraps that exception inside an instance of the ExceptionInInitializerError class. This way, it also maintains a reference to the actual exception as the root cause.
- And this is exactly what is happening in our case as well. The BaseComponent class has a static initialization block, and within that block is a method that calls another static method on the BaseComponentPropertyAdapter class (getTypeSingleton is the method that is called on the BaseComponentPropertyAdapter). And the getTypeSingleton throws a NPE. Hence, since the static initialization block has run into an exception in the form of a NPE, we get a ExceptionInInitializerError, as has been mentioned in the previous point.
- Note that the <clinit> method is a class initialization method in the JVM.

How does hot-swapping in the case of servers like Tomcat work

?
- Source on SO.
- A class static initializer is called each time a class is loaded.
- A class named com.example.Data loaded by ClassLoader C1 and a class named com.example.Data loaded by ClassLoader C2 are different classes, and trying to cast an instance of one class to the other class will result in a ClassCastException.
- With multiple classloaders, even though strictly speaking they would be different objects, you can't count on the initialization code being run exactly one time, so you at least have to keep that in mind when writing the init code.
- Instantiating directly with the new operator and again via reflection still uses only one ClassLoader.
- An application can create new classloaders and load classes with them. ClassA loaded with the new classloader is technically a different class than ClassA loaded with the default classloader. Tomcat (and I assume others) uses this technique to re-load a running web application. You can, for example, fix a bug in SomeClass and drop the new .class file into the running application without re-deploying the whole application or taking Tomcat down. Tomcat will notice the update and load new instances of the classes in the app. This is an example of the hot-fix or the hot-deploy functionality of the servers. This shows that it is possible and there are cases where a class may be loaded by more than one classloader and, therefore, the clinit could be called more than once.
- There is a difference between hot-deploy and hot-swap as mentioned in this blog post. This SO answer goes into details about how hot-deploy is done.
- This is a SO answer about how hot-swap works.

90) What does the following mean: void foo(@Nullable Object obj) { ... }
- Source

91) How to read UML diagrams?
- Refer Dashed line vs Solid line
- Association vs Dependency vs Composition - Arrowheads are of two types: closed arrowhead and open arrowheads.
- Closed arrowheads are used to represent relations for implements and extends. A dotted line with a closed arrowhead implies implements. A solid line with closed arrowhead implies extends.
- Open arrowheads are used to represent association/dependency relations. A dotted line with an open arrowhead means that there is a relationship of dependency. Dependency can exist in multiple cases like one class accepts the dependent class as an args for one of its methods, or returns the other class type from one of its methods. A solid line with a open arrowhead means a relationship of association. - Composition is usually indicated by line connecting two classes with addition of a solid diamond at end of the class who owns the creational responsibility.

92) Private constructors of inner classes can be called from the enclosing parent class.
- Refer this SO Link for explanation about the different types of inner classes.
- This SO Answer explains why you are able to access the private constructor. If the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class that encloses the declaration of the member or constructor.

93) TODO Dump:
- ThreadLocal
- Lambda variables in a loop
- Regex stuff:
a) https://stackoverflow.com/questions/10172753/adding-variables-in-a-regular-expression-to-make-it-dynamic-in-java
b) https://stackoverflow.com/questions/172303/is-there-a-regular-expression-to-detect-a-valid-regular-expression
c) https://stackoverflow.com/questions/60160/how-to-escape-text-for-regular-expression-in-java
d) https://stackoverflow.com/questions/8534178/how-to-represent-empty-char-in-java-character-class
e) https://stackoverflow.com/questions/3976616/how-to-find-nth-occurrence-of-character-in-a-string?
f) https://docs.oracle.com/javase/tutorial/essential/regex/index.html
g) https://stackoverflow.com/questions/23536133/how-do-you-match-multiple-regex-patterns-for-a-single-line-of-text-in-java/23536172
h) https://stackoverflow.com/questions/22252297/match-a-string-against-multiple-regex-patterns
i) https://stackoverflow.com/questions/27666725/running-multiple-regex-patterns-on-string
j) https://stackoverflow.com/questions/869809/combine-regexp
k) https://www.regular-expressions.info/lookaround.html
l) https://stackoverflow.com/questions/4389644/regex-to-match-string-containing-two-names-in-any-order
m) https://stackoverflow.com/questions/20924693/how-to-find-words-containing-a-certain-letter-with-regular-expressions
n) https://stackoverflow.com/questions/18643157/regex-must-contain-specific-letters-in-any-order
o) https://stackoverflow.com/questions/5238491/check-if-string-contains-only-letters
- Grid:
a) https://stackoverflow.com/questions/43311943/prevent-content-from-expanding-grid-items
b) https://css-tricks.com/almanac/properties/o/overflow-wrap/
c) https://stackoverflow.com/questions/33454533/cant-scroll-to-top-of-flex-item-that-is-overflowing-container
d) https://developer.mozilla.org/en-US/docs/web/CSS/overflow-wrap
e) https://developer.mozilla.org/en-US/docs/Web/CSS/word-break
f) https://developer.mozilla.org/en-US/docs/Web/CSS/minmax()
g) https://stackoverflow.com/questions/8083430/how-to-change-the-font-and-font-size-of-an-html-input-tag
- XPATH:
a) https://www.youtube.com/watch?v=3uktjWgKrtI
b) https://stackoverflow.com/questions/16016521/how-do-i-select-child-elements-of-any-depth-using-xpath
c) https://stackoverflow.com/questions/1604471/how-can-i-find-an-element-by-css-class-with-xpath
d) https://stackoverflow.com/questions/6353259/how-do-i-verify-that-an-element-does-not-exist-in-selenium-2
e) https://stackoverflow.com/questions/23804123/selenium-gettext
f) https://www.tutorialspoint.com/how-to-get-entered-text-from-a-textbox-in-selenium
g) https://stackoverflow.com/questions/5856146/how-to-get-the-value-of-a-span-tag
h) https://www.browserstack.com/guide/find-element-by-xpath-in-selenium
i) https://stackoverflow.com/questions/18436102/selenium-findby-vs-driver-findelement
j) https://stackoverflow.com/questions/35606708/what-is-the-difference-between-and-in-xpath
k) https://riptutorial.com/xpath/example/6209/find-all-elements-with-certain-text
l) https://stackoverflow.com/questions/5643323/get-xpath-for-all-the-nodes
m) https://stackoverflow.com/questions/15077462/xpath-to-select-all-elements-with-a-specified-name/15077631
n) https://stackoverflow.com/questions/1022345/current-node-vs-context-node-in-xslt-xpath
o) https://www.w3schools.com/xml/xpath_syntax.asp
p) https://stackoverflow.com/questions/15077462/xpath-to-select-all-elements-with-a-specified-name
q) https://stackoverflow.com/questions/43512114/how-to-use-findby-for-driver-findelementsby-xpath-size0
r) https://stackoverflow.com/questions/49982869/what-is-the-use-of-findbys-and-findall
s) https://stackoverflow.com/questions/25914156/difference-between-findall-and-findbys-annotations-in-webdriver-page-factory
t) https://stackoverflow.com/questions/35597991/how-to-select-element-from-the-list-click-on-it-go-back-to-the-list-and-select
u) https://stackoverflow.com/questions/52667092/how-to-get-element-in-element-from-the-list-of-elements-in-java-and-selenium
v) https://stackoverflow.com/questions/22556423/xpath-search-by-id-attribute-giving-npe-java
w) https://stackoverflow.com/questions/3317018/xpath-check-for-non-existing-node
x) https://stackoverflow.com/questions/36622160/xpath-select-element-if-other-not-exist
y) https://stackoverflow.com/questions/3317018/xpath-check-for-non-existing-node
z) http://www.zvon.org/xxl/XPathTutorial/Output/example7.html
aa) https://stackoverflow.com/questions/10176657/xpath-count-function
ab) https://stackoverflow.com/questions/38942850/xpath-select-div-that-contains-class-and-whose-specific-child-element-contains
ac) https://stackoverflow.com/questions/1390568/how-can-i-match-on-an-attribute-that-contains-a-certain-string
ad) https://stackoverflow.com/questions/1604471/how-can-i-find-an-element-by-css-class-with-xpath
ae) https://stackoverflow.com/questions/7713797/how-to-get-css-class-name-using-selenium
af) https://stackoverflow.com/questions/11259890/how-to-get-the-class-name-through-selenium
ag) https://stackoverflow.com/questions/40611503/selenium-how-to-find-elements-by-attribute-with-any-value-in-java

ae) You cannot search for an exact class using xpath. = validates the attribute value is exactly equal to given value. As the attribute class value is "ng-untouched ng-pristine ng-invalid igx-input-group__input" you cannot say @class="ng-invalid". You should use contains instead. If you are searching for an exact class, try prefixing/suffixing the class name with a space instead. Otherwise, you can also do this: //div[contains(concat(' ',normalize-space(@class),' '),' foo ')] as mentioned here. But note that there must be spaces on either side of the class name 'foo'.
af) Difference between //, .//, and ..// in XPath?
JavascriptExecutor related stuff:
ah) https://stackoverflow.com/questions/49871432/what-does-arguments0-and-arguments1-mean-when-using-executescript-method-fro
ai) https://stackoverflow.com/questions/31632923/using-javascriptexecutor-to-sendkeys-plus-click-on-web-element

94) When assigning boxed values to the primitive types, if the boxed type is null, a NPE will be thrown.
- Consider the following code snippet. Running this will produce a NPE in the method call when you try to pass in a null value. Assigning null to a native type (like boolean, int, etc.) will result in a NullPointerException
- Also remember that while boolean is initialized to false, Boolean is initialized to null. Hence, if you declare a type of Boolean in your class, you must initialize it before you put it to use. Else, NPE. Source on SO.

95) Can we instantiate an abstract class?
- Source on SO.
- Asked this question on SO for the behavior that you were seeing.
- No, you are not creating the instance of your abstract class here. Rather you are creating an instance of an anonymous subclass of your abstract class. And then you are invoking the method on your abstract class reference pointing to subclass object.
- Put simply: = my() {}; means that there's an anonymous implementation, not simple instantiation of an object, which should have been: = my(). You can never instantiate an abstract class.

96) What is the difference between using this.variableName vs. ClassName.this.variableName?
- SO Source 1, SO Source 2
- Shadowing section in the Nested Classes tutorial on Oracle tutorials. This is the example that they have over there.
- If a declaration of a type (such as a member variable or a parameter name) in a particular scope (such as an inner class or a method definition) has the same name as another declaration in the enclosing scope, then the declaration shadows the declaration of the enclosing scope. You cannot refer to a shadowed declaration by its name alone. - Where this is useful is in the case of anonymous classes and if you are changing anonymous class to an instance of lambda expression instead.
- To access the instance of the enclosing-class from inside the inner classes or the anonymous classes, you need to use this syntax: EnclosingClass.this.someMethod();
- The Classname.this syntax is used to refer to an outer class instance when you are using nested classes

97) You can pass code as data in java using lambda expressions. How do you do this?
- SO Link.
- Suppose there is a function that you want to execute as a callback. For eg. when the user clicks on the close button on a popup, and in response you want to call a method that will be executed. The straight-forward way of doing this would be to add a close handler to the popup (assume the popup has such a method that allows you to do so), such that when you clicked on the close button, the method was called. But for this it is necessary that the function you want to add as a callback is in scope.
- Consider the following code. In the below example, the onBindingEditorPopupClose is defined in the same class, and hence we can add the method as a callback. - But now suppose if you wanted to refactor the BindingTextBox class such that the createPopup no longer returned the instance of the popup that it had created. In that case you wouldn't have a reference to the popup in the PropertyRow class, hence not allowing you to add the addCloseHandler. You can try to add the close handler in the same place where you are instantiating the bindingEditorPopup, but then you would need a reference to the same PropertyRow class because that is the class on which the onBindingEditorPopupClose is defined, which in turn means changing the constructor of the BindingEditorPopup, and all the other places where the constructor is being used as well. This will in-turn end up changing the dependency graph of the objects are being created as well. Not such a clean way.
- There is an alternate way though. The problem can be solved by passing in the onBindingEditorPopupClose as data.

Why does setCloseHandlerCallback use the Runnable interface

: The method that we intend to pass in as the callback in this case is onBindingEditorPopupClose. onBindingEditorPopupClose does not take in any arguments and returns void. Hence, we need to refer the onBindingEditorPopupClose function using an interface that also has the same definition. We could create an interface of ourselves, but java already has such an interface in the form of Runnable, and hence that is the interface that we end up using.

What would we do if the onBindingEditorPopupClose took in params instead

: In the below code example, all we are changing is that the onBindingEditorPopupClose now takes in an args and returns void. Hence, we change the interface to instead use the Consumer interface, since that has the same definition as well.

97) Why does java have transient fields?
- SO Answer here.
- Before understanding the transient keyword, one has to understand the concept of serialization. Serialization is the process of making the object's state persistent. That means the state of the object is converted into a stream of bytes to be used for persisting (e.g. storing bytes in a file) or transferring (e.g. sending bytes across a network). In the same way, we can use the deserialization to bring back the object's state from bytes. This is one of the important concepts in Java programming because serialization is mostly used in networking programming. The objects that need to be transmitted through the network have to be converted into bytes. For that purpose, every class or interface must implement the Serializable interface. It is a marker interface without any methods.
- By default, all of object's variables get converted into a persistent state. In some cases, you may want to avoid persisting some variables because you don't have the need to persist those variables. So you can declare those variables as transient. If the variable is declared as transient, then it will not be persisted. That is the main purpose of the transient keyword.
- For example, you may have fields that are derived from other fields, and should only be done so programmatically, rather than having the state be persisted via serialization. Consider this Image example. The thumbnailImage is a thumbnail image that is generated by invoking the generateThumbnail method. The thumbnailImage field is marked as transient, so only the original image is serialized rather than persisting both the original image and the thumbnail image. This means that less storage would be needed to save the serialized object.

97) Understanding WebPack
- YT Tutorial here.
- You cannot run .ts files directly in the browser. For instance, this would not work: <script src="dist/main.ts"></script> and you would get a mime error, and in order to fix it, you will have to convert the ts file to js. This is where webpack comes in. It converts the ts file to js, and then you include this new js file in your script tag.

1) Consider the following computation:

2) lol

3) The tale of 0xcafebabe

4) What will be the output of the following:

5) What will this output:

6) Data types in JAVA:

#	Datatype	Size	Description
1	boolean	This data type represents one bit of information, but its "size" isn't something that's precisely defined. Source, Source
2	byte	8 bit	(-128 to +127)
3	short	16 bit	signed, (-32,768 to 32,767)
4	char	16 bit	unsigned, (0 to 65,535)
5	int	32 bit	(-2^31 to 2^31 - 1) 2^31 = 2,147,483,648
6	long	64 bit	(-2^63 to 2^63 - 1)
7	float	32 bit	(refer specs/never use where accurate calculation is required)
8	double	64 bit	(refer specs/never use where accurate calculation is required)

7) Why won't this code compile:

8) What will the following code output:

9) What will the following code output:

10) What will the following code output:

11) What is String Interning?

12) What does it mean to "escape" a String?

13) So what does it mean to escape in regex?

14) So, what will this code output now?

16) List the bugs in this code:

17) What is the output of the following code:

18) What is wrong with the following code:

19) What would this code print:

20) What is the problem with this code:

21) What is the problem with this code:

22) What will make these loops run indefinitely?

23) What does the following program print:

24) What does the following code print: