For more information, refer to the article – Difference Between PriorityQueue and TreeSet

44. What is the diamond operator in Java?

Diamond operators are used for simplifying the use of generics when creating objects while avoiding unchecked warnings in a program. When the Diamond operator was introduced in Java 7, we can create the object without mentioning the generic type on the right side of the expression as shown below.

Syntax:

List<String> list = new ArrayList<>();

For more information, refer to the article – Diamond operator

45. How TreeMap works in Java?

TreeMap stores the key-value pairs, but TreeMap sorts the keys ascending rather than descending like HashMap. Depending on which constructor is used, TreeMap will be sorted either based on its keys, or by a Comparator. In TreeMap, the elements are sorted based on a Red-Black tree. A red-black tree is a self-balancing binary search tree where each node has an extra bit, and that bit is often interpreted as the color (red or black). These colors are used to ensure that the tree remains balanced during insertions and deletions. 

Structure of a Node in Java

For more information, refer to the article – Internal Working of TreeMap in Java

46. List down ways to iterate over Map in java?

The HashMap class provides Java’s Map interface by storing data in (Key, Value) pairs and accessing them by an index of another type. To use this class it is necessary to import java.util.HashMap package or its superclass.

There are numerous ways to iterate over HashMap of which 5 are listed below:

1. Iterate through a HashMap EntrySet using Iterators.
2. Iterate through HashMap KeySet using Iterator.
3. Iterate HashMap using for-each loop.
4. Iterating through a HashMap using Lambda Expressions.
5. Loop through a HashMap using Stream API.

For more information, refer to the article – How to Iterate HashMap in Java

47. What is CopyOnWriteArrayList in Java?

CopyOnWriteArrayList in Java

JDK 1.5 introduced an enhanced version of ArrayList called CopyOnWriteArrayList, where all modifications (add, set, remove, etc) are implemented by a new copy. It can be found in java.util.concurrent. It is a data structure created to be used in a concurrent environment. In a Thread-based environment, the CopyOnWriteArrayList is intended for frequent reading and infrequent updating. CopyOnWriteArrayList is a thread-safe version of  ArrayList.

For more information, refer to the article – CopyOnWriteArrayList in Java

48.  What is EnumMap in Java?

EnumMap is an implementation of the Map interface specific to enumeration types. EnumMap class is a member of the Java Collections Framework & is not synchronized. It extends AbstractMap and implements the Map interface in java. EnumMap belongs to java.util package.

Syntax: 

public class EnumMap<K extends Enum<K>,​V> extends AbstractMap<K,​V> implements Serializable, Cloneable

// K must extend Enum, which enforces the requirement that the keys must be of the specified enum type. 

Parameters:

• Key object type
• Value object type

EnumMap in Java

For more information, refer to the article – EnumMap class in Java

49. How does Hashmap internally Works?

HashMap works on the principle of Hashing. HashMap contains an array of Node and Node can represent a class having the following objects:

• int hash
• K key
• V value
• Node next

The internal workings of HashMap:

• Hashing
• Buckets
• Index Calculation in Hashmap

For more information, refer to the article – Internal Working of HashMap in Java

50. Why iterator in hashmap is considered fail-fast?

fail-fast iterators immediately throw concurrent modification exceptions if any thread from outside attempts to modify the collection on which they are iterating. The fail-fast feature ensures that the iterator fails immediately if it detects that any modification of the collection will lead to anomalous behavior in the future.

Fail fast feature ensures that if iterator feels that modification of collection would result in anomalous behaviour at any point of time in future, it fails immediately.

Example:

// Java code to demonstrate remove
// case in Fail-fast iterators

import java.io.*;
import java.util.ArrayList;
import java.util.Iterator;

public class GFG {
    public static void main(String[] args)
    {
        ArrayList<Integer> arr = new ArrayList<>();
        arr.add(1);
        arr.add(2);
        arr.add(3);
        arr.add(4);
        arr.add(5);

        Iterator<Integer> it = arr.iterator();
        while (it.hasNext()) {
            if (it.next() == 2) {
                // will not throw Exception
                it.remove();
            }
        }

        System.out.println(arr);

        it = arr.iterator();
        while (it.hasNext()) {
            if (it.next() == 3) {
                // will throw Exception on
                // next call of next() method
                arr.remove(3);
            }
        }
    }
}

Output:

[1, 3, 4, 5]
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    at java.util.ArrayList$Itr.next(ArrayList.java:851)
    at FailFastExample.main(FailFastExample.java:28)

Conclusion

Java Collections is important to understand for Java developers or programmers because Java is widely used in various industries. It’s important for developers to have a solid understanding of core concepts of Java Collections. Java is one of the most widely used languages in top companies such as Uber, Airbnb, Google, Netflix, Instagram, Spotify, Amazon, etc. To get into these companies or any other IT companies, you need to master these mostly asked Java Collections interview questions in order to crack their Java-based online assessment and technical interview.

If you’re looking to practice coding questions, the JAVA Collection Programs may be a helpful resource.

Java Collections Interview Questions – FAQs

What are collections in Java interview questions?

Collection in Java is a framework used for storing and manipulating collections of objects.

What are the 4 collection classes in Java?

There are many collections in Java but out of them most used collections are:

1. ArrayList
2. LinkedList
3. HashSet
4. Stack

Can HashMap have duplicate keys?

No, HashMap can’t have duplicate keys. As HashMap is one of the collections in Java, it stores the value in the form of key-value and every key has its own value attached to it. So, as no key can have two value means we can’t have duplicate keys in HashMap.

Why array is not a collection?

The array is not a collection, it is all because of the difference in the functionality between collections and arrays few of which are mentioned below:

• The size of the Array can’t be changed once declared
• Arrays can hold only homogeneous data types elements.
• The array can hold both primitive datatypes and objects whereas in collections it is only able to hold wrapper objects.