Java Collections Interview Questions and Answers

Java Collection Framework was introduced in JDK 1.2 which contains all the collection classes and interfaces. Java Collection is a framework that provides a mechanism to store and manipulate the collection of objects. It allows developers to access prepackaged data structures and algorithms for manipulating data. 

In this article, we’ve covered the top 50 Java Collections Interview Questions and Answers that cover everything from basic to advanced Java collection concepts such as navigation collection, WeakHashMap, streams Lambdas etc. Whether you are a fresher or an experienced Java developer, these Java Collections Interview Questions give you all the confidence you need to ace your next Java interview.

Java Collections Interview Questions

We have divided the 50 questions into two parts: Experienced and Freshers. Let’s begin with the questions for Freshers.

Java Collection Interview Questions For Freshers

1. What is Collection in Java?

The term collection refers to a group of objects represented as one unit. Classes in the Java collection class hierarchy are divided into two “root” interfaces: Collection (java.util.Collection) and Map (java.util.Map). Terms that you will encounter while learning about the collection in Java:

• Collection Framework: Java’s Collection Framework defines classes and interfaces for representing groups of objects as a single entity. C++ developers can compare the Collection framework with STL (Standard Template Library) and Container Framework with the Collection Framework if they come from a C++ background.
• Collection Interface: A class’s interface specifies what it should do, not how. In other words, it is the blueprint for the class. This interface provides the most common methods for all collection objects that are part of the Collection Framework. Alternatively, it represents the individual object as a whole.
• Collections Class: A member of Collection Framework, it is part of java.util package. The collection object is provided with many utility methods in this class.

2. What is a Framework in Java?

Frameworks are sets of classes and interfaces that provide a ready-made architecture. It is not necessary to define a framework in order to implement new features or classes. As a result, an optimal object-oriented design includes a framework containing a collection of classes that all perform similar tasks. The framework can be used in a variety of ways, such as by calling its methods, extending it, and supplying “callbacks”, listeners, and other implementations. Some of the popular frameworks in java are:

• Spring
• Hibernate
• Struts
• Google Web Toolkit (GWT)
• JavaServer Faces (JSF)

3. What is the difference between Array and Collection in Java?

Arrays are a collection of similar-typed variables with a common name in Java. There are some differences between arrays in Java and C/C++. On the other hand, Collections are groups of individual objects that form a single entity known as the collection of objects.

For more information, refer to the article – Difference Between Arrays and Collections in Java

4.  What are the various interfaces used in Java Collections Framework?

The collection is known as the root of the collection hierarchy. Collections represent groups of objects known as elements. The java platform does not provide any direct implementation of this interface but the Collection interface is being implemented by List and Set classes.

• Collection interface
• List interface
• Set interface
• Queue interface
• Dequeue interface
• Map interface

5. Explain the hierarchy of the Collection framework in Java.

All classes and interfaces required by the collection framework are contained in the utility package (java. util). Collection frameworks have an interface called an iterable interface, which allows the iterator to iterate over all collections. In addition to this interface, the main collection interface acts as a root for the collection framework. All the collections extend this collection interface thereby extending the properties of the iterator and the methods of this interface. The following figure illustrates the hierarchy of the collection framework. 

Java Collection Hierarchy

6. What are the advantages of the collection Framework?

Advantages of the Collection Framework: Since the lack of a collection framework gave rise to the above set of disadvantages, the following are the advantages of the collection framework. 

• Consistent API: The API has a basic set of interfaces like Collection, Set, List, or Map, all the classes (ArrayList, LinkedList, Vector, etc) that implement these interfaces have some common set of methods.
   
• Reduces programming effort: A programmer doesn’t have to worry about the design of the Collection but rather he can focus on its best use in his program. Therefore, the basic concept of Object-oriented programming (i.e.) abstraction has been successfully implemented.
   
• Increases program speed and quality: Increases performance by providing high-performance implementations of useful data structures and algorithms because in this case, the programmer need not think of the best implementation of a specific data structure. He can simply use the best implementation to drastically boost the performance of his algorithm/program.

7. What is ArrayList in Java? 

ArrayList is a part of the Java collection framework and it is a class of java.util package. It provides us with dynamic arrays in Java. The main advantages of ArrayList are, if we declare an array then it’s needed to mention the size but in ArrayList, it is not needed to mention the size of ArrayList if you want to mention the size then you can do it.

Image of Array List

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

8. What is the difference between Collection and Collections?

For more information, refer to the article – Collection vs Collections in Java with Example

9. Difference between ArrayList and LinkedList in the java collection framework?

ArrayList and LinkedList

For more information, refer to the article – ArrayList vs LinkedList in Java

10. What is an iterator?

Java’s Collection framework uses iterators to retrieve elements one by one. This iterator is universal since it can be used with any type of Collection object. Using Iterator, we can perform both reading and removing operations. This is an improved version of Enumeration with the addition of removing elements.

When enumerating elements in all Collection framework implemented interfaces, such as Set, List, Queue, Deque, and all implemented classes of Map, an Iterator must be used. The only cursor available for the entire collection framework is the iterator. Using the iterator() method in the Collection interface, you can create an iterator object.

Syntax:

Iterator itr = c.iterator();

Note: Here “c” is any Collection object. itr is of type Iterator interface and refers to “c”.

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

11. What is the difference between an Iterator and an Enumeration?

A major difference between iterator and enumeration is that iterators have a remove() method while enumerations do not. Thus, using Iterator we can manipulate objects by adding and removing them from collections. Since enumeration can only traverse objects and fetch them, it behaves like a read-only interface.

For more information, refer to the article –  Difference between Iterator and Enumeration

12. What is the difference between List and Set in Java

A major difference between a List and a Set is that a List can contain duplicate elements while a set contains only unique elements. The list is Ordered and maintains the order of the object to which they are added. The set is unordered.

For more information, refer to the article – Difference Between List and Set in Java

13. What are the best practices for Java Collections Framework?

Following are some of the best practices while using Java Collections:

• Programs should be written as interfaces, not implementations, so we can modify the implementation later.
• Whenever possible, use Generics to ensure type safety and avoid ClassCastExceptions.
• Choosing the appropriate type of collection based on the need. For example, if the size is fixed, we might want to use an Array over an ArrayList. When iterating over the Map, we should use LinkedHashMap. Set is the best way to avoid duplicates.
• Use immutable classes provided by JDK as keys in Map to avoid implementation of hashCode() and equals().
• In order to increase the readability of the code, we should use isEmpty() instead of finding the size of the collection and comparing it to zero.
• Rather than writing your own implementation, use the Collections utility class to get read-only, Synchronized, or empty collections instead. It enhances code reuse while resulting in greater stability.

14. What is a priority queue in Java?

PriorityQueues are used to process objects according to their priority. Queues follow the First-In-First-Out algorithm, but sometimes the elements of the queue need to be processed according to their priority, which is where PriorityQueue comes into play. Priority queues are based on priority heaps.

The elements of the priority queue are ordered according to the natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used.  

Priority Queues in Java

Declaration:

public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable
where E is the type of elements held in this queue

The class implements Serializable, Iterable<E>, Collection<E>, and Queue<E> interfaces.

15. What is the difference between List, set, and map in java?

For more information, refer to the article – Difference between List, Set, and Map in Java

16. What is the difference between Queue and Stack?

17. What is BlockingQueue in Java?

The BlockingQueue interface in Java is added in Java 1.5 along with various other concurrent Utility classes like ConcurrentHashMap, Counting Semaphore, CopyOnWriteArrrayList, etc. The BlockingQueue interface supports flow control (in addition to queue) by introducing blocking if either BlockingQueue is full or empty.

A thread trying to enqueue an element in a full queue is blocked until some other thread makes space in the queue, either by dequeuing one or more elements or clearing the queue completely. Similarly, it blocks a thread trying to delete from an empty queue until some other threads insert an item. BlockingQueue does not accept a null value. If we try to enqueue the null item, then it throws NullPointerException.

Usage of BlockingQueue 

Blocking Queue in Java

The Hierarchy of BlockingQueue

Hierarchy of Blocking Queue in Java

Declaration:

public interface BlockingQueue<E> extends Queue<E>

Here, E is the type of elements stored in the Collection.

For more information, refer to the article – BlockingQueue Interface in Java

18. What is the hashCode()?

Image to demonstrate Java Hash Code

hashCode() method returns the hashcode value as an Integer. It is defined in the Java Object class which computes the hash values of given input objects. Hashcode value is mostly used in hashing-based collections like HashMap, HashSet, HashTable….etc. This method must be overridden in every class which overrides the equals() method.

Syntax :

public int hashCode()
// This method returns the hash code value 
// for the object on which this method is invoked.

For more information, refer to the article – equals() and hashCode() methods in Java

19. Distinguish between ArrayList and Vector in the Java Collection Framework.

In collection interviews, this question is frequently asked; however, Vector is synchronized whereas ArrayList is not. ArrayList is faster than Vector. ArrayList’s Array size is increased by 50% when needed, while Vector’s capacity is doubled whenever it is needed.

Array List vs Vector in java

For more information, refer to the article – Vector vs ArrayList in Java

20. Differentiate between Iterator and ListIterator.

For more information, refer to the article – Difference Between an Iterator and ListIterator

21. What is the difference between an Iterator and an Enumeration?

Iterator: It is a universal iterator as we can apply it to any Collection object. By using an Iterator, we can perform both read and remove operations. 

Syntax: 

// Here "c" is any Collection object. itr is of
// type Iterator interface and refers to "c"
Iterator itr = c.iterator();

Enumeration: Enumeration (or enum) is a user-defined data type. It is mainly used to assign names to integral constants, the names make a program easy to read and maintain. 

Syntax: 

// A simple enum example where enum is declared 
// outside any class (Note enum keyword instead of 
// class keyword) 
enum Color 
{ 
    RED, GREEN, BLUE; 
}

For more information, refer to the article – Difference between Iterator and Enumeration 

22. What are the features of Java Hashmap?

HashMap is similar to HashTable, but it is unsynchronized. It allows us to store the null keys as well, but there should be only one null key object and there can be any number of null values.  This class makes no guarantees as to the order of the map. To use this class and its methods, you need to import java.util. HashMap package or its superclass.
 

HashMap in Java

 Syntax: 

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable

Parameters: It takes two parameters namely as follows:

• The type of keys maintained by this map (K)
• The type of mapped values (V)

For more information, refer to the article – HashMap in Java with Examples

23. What are Collection Interfaces?

The Collection interface is a member of the Java Collections Framework. It is a part of java.util package. It is one of the root interfaces of the Collection Hierarchy. The Collection interface is not directly implemented by any class. However, it is implemented indirectly via its subtypes or subinterfaces like List, Queue, and Set. 

For Example, the HashSet class implements the Set interface which is a subinterface of the Collection interface. If a collection implementation doesn’t implement a particular operation, it should define the corresponding method to throw UnsupportedOperationException.

The Hierarchy of Collection:

Collection Interface in Java

24. Explain the list interface.

Class Interface in Java

In Java, the List interface allows the user to store an ordered collection of objects. The list is the child interface of Collection. In Collection, a list is an ordered collection of objects which can have duplicate values. Since List preserves the insertion order, it allows positional access and insertion, which also allows duplicate values.

Syntax:

public interface List<E> extends Collection<E> ;

This list interface is implemented by various classes like ArrayList, Vector, Stack, etc. Since all the subclasses implement the list, we can instantiate a list object with any of these classes. 

Example: 

List <T> al = new ArrayList<> (); 
List <T> ll = new LinkedList<> (); 
List <T> v = new Vector<> (); 

Where T is the type of the object 

Array List in Java

The classes which implement the List interface are as follows:

• ArrayList
• LinkedList
• Vector
• Stack

25. Write a program to convert a given array into a collection with the asList() method.

To convert array-based data into Collection based we can use java.util.Arrays class. This class provides a static method asList(T… a) that converts the array into a Collection.

Array input: [Kamlesh, Abhay, Abhishek, Shivansh]
Converted elements: [Kamlesh, Abhay, Abhishek, Shivansh]

26. Differentiate between HashSet and HashMap

For more information, refer to the article – Difference between HashMap and HashSet

27. Differentiate between HashSet and HashTable.

28. What is the default size of the load factor in the hashing-based collection?

As the Load Factor increases, the capacity increases so that the operational complexity of the HashMap remains O(1) if the ratio of the current element to the initial capacity crosses the threshold. The meaning of operational complexity of O(1) means the retrieval and insertion operations take constant time. The default load factor size is 0.75. The default capacity is calculated by multiplying the initial capacity by the load factor.

For more information, refer to the article – Load Factor in HashMap in Java with Examples

Java Collection Interview Questions For Experienced

29. What is the difference between Comparable and Comparator in Java?

Java provides two interfaces to sort objects using data members of the class: 

• Comparable
• Comparator

For more information, refer to the article – Comparable vs Comparator in Java

30. What is the difference between fail-fast and failsafe?

Iterators in Java are used to iterate over the Collection objects. Fail-Fast iterators immediately throw ConcurrentModificationException if there is a structural modification of the collection. Structural modification means adding, or removing any element from a collection while a thread is iterating over that collection. Iterator on ArrayList and HashMap classes are some examples of fail-fast Iterator.

For more information, refer to the article – Fail Fast and Fail Safe Iterators in Java

31. Write a program to iterate the list using the lambda expression.

Iteration can be done using a lambda expression.

Syntax:

list_name.forEach(variable->{//block of code})

Geeks
for
Geeks

For more information, refer to the article – Iterate through List in Java

32. What is IdentityHashMap?

The IdentityHashMap implements the Map interface using Hashtable, comparing keys (and values) using reference equality instead of object equality. This class implements the Map interface, but it intentionally breaks Map’s general contract, which demands that objects are compared using the equals() method. This class is used when the user allows objects to be compared using references. It belongs to java.util package.

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

33. Write a program in Java to display the contents of a HashTable using enumeration.

The hashtable class implements a hash table, which maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method. Below is the program to display the contents of a HashTable using enumeration:

Geeks
for
Geeks

34. Write a program in java to get the collection view of the values present in a HashMap.

Java’s HashMap class has the java.util.HashMap.values() method for creating collections out of HashMap values. It basically returns a Collection view of HashMap values.

Initial Mappings are: {0=Welcome, 1=to, 2=Geeks, 3=4, 4=Geeks}
The collection is: [Welcome, to, Geeks, 4, Geeks]

For more information, refer to the article – HashMap values() Method in Java

35. Write a program to join two ArrayList into one single ArrayList.

Given two ArrayLists in Java, our task is to join these ArrayLists.

ArrayList 1: [Geeks, For, ForGeeks]
ArrayList 2: [GeeksForGeeks, A computer portal]
Joined ArrayLists: [Geeks, For, ForGeeks, GeeksForGeeks, A computer portal]

For more information, refer to the article – Join two ArrayLists in Java

36. How can you synchronize an ArrayList in Java?

Using the Collections.synchronizedList() method, we can synchronize our collections in Java. SynchronizedList() returns a synchronized (thread-safe) list backed by a selection.

Eat
Coffee
Code
Sleep
Repeat

37. What is a Properties Class in Java? 

The properties class is a subclass of Hashtable. The properties class stores a list of values whose key is a string and whose value is also a string. Properties can define other properties class lists, but the default is properties.

Features of Properties class:

• Property is a subclass of Hashtable.
• The Properties file is used to store and retrieve string data type for a list of values where the key is a string and the value is also a string.
• If the original properties list does not contain a certain key property, the default properties list will be searched instead.
• Objects can be shared by multiple threads without external synchronization.
• The properties class can be used to retrieve the properties of the system.

For more information, refer to the article – Properties Class in Java

38. What will happen if you use HashMap in a multithreaded Java application?

In a multi-threaded environment, if multiple threads alter the map structurally, such as adding, removing, or modifying mappings, the internal data structure of HashMap may become corrupted and there may be some missing links, incorrect entries, and the map itself may become completely useless. Thus, you should not use HashMap in a concurrent application; instead, use ConcurrentHashMap or Hashtable which is thread-safe. The ConcurrentHashMap includes all the Hashtable’s methods as well as full concurrency of retrievals and updates.

How did ThreadSafeConcurrentHashMap become thread-safe? 

• java.util.Concurrent.ConcurrentHashMap class provides thread safety by dividing the map into segments, which allows the lock to be taken only once per segment, i.e, once for each thread.
• The read operation in ConcurrentHashMap does not require a lock. 

For more information, refer to the article – How Does ConcurrentHashMap Achieve Thread-Safety in Java?

39. What will happen if two different keys of HashMap return the same hashcode()?

When two different keys of HashMap return the same hash code, they will end up in the same bucket; therefore, collisions will occur. n case of collision, i.e. index of two or more nodes is the same, nodes are joined by a link list i.e. the second node is referenced by the first node and the third by the second, and so on.

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

40. What is WeakHashMap?

WeakHashMap implements the Map interface. Unlike HashMap, WeakHashMap allows garbage collection even if the object specified as the key doesn’t contain any references despite being associated with WeakHashMap. In other words, Garbage Collector is better than WeakHashMap.

For more information, refer to the article – Hashmap vs WeakHashMap in Java

41. What is UnsupportedOperationException?

In the context of APIs or list implementations, the UnsupportedOperationException is a common exception. The exception is thrown when the requested operation cannot be performed. This class is a member of the Java Collections Framework.

Syntax:

public class UnsupportedOperationException
extends RuntimeException

For more information, refer to the article – UnsupportedOperationException

42. How to make a Collection Read-Only in Java?

Creating a Read-Only Collection involves restricting the object to only fetching the data and not adding or removing data. Java has different methods for different Collection types like unmodifiableCollection(), unmodifiableMap(), ununmodifiableSet(), etc. java.util.The collections class defines all methods. The unmodifiableCollection() method creates a Read-Only collection. It requires a reference to the Collection class. If we have an object of Set Interface, we can use ununmodifiableSet() to make Read-Only.

For more information, refer to the article – How to Make a Collection Read-Only in Java?

43. Difference between PriorityQueue and TreeSet in Java? 

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:

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

1. What are collections in Java interview questions?

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

2. 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

3. 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.

4. 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.