Top 25 Java Collections Framework Questions for Interview

Java collections framework is undoubtedly the most popular and frequently asked topic during Java technical interviews. Besides interviews, Java web developers use it quite often to process the large set of data in their normal application development. Another important factor that makes it more significant among developers is the support of a variety of collection API in Java. In this blog post, you will get to know all the essential information about Java collections backed by 60 best interview questions to help you crack any Java interview.

Java collections framework overview:

The Java Collections Framework defines a collection of interfaces and classes that serve to store and handle the data in an efficient manner. It lists out many useful classes that support operations like searching, sorting, insertion, manipulation, and deletion. The framework includes many interfaces (List, Set, Queue, Deque, etc.) and classes (ArrayList, LinkedList, Vector, PriorityQueue, TreeSet, HashSet, LinkedHashSet, etc.).

Next, it is also important to understand the fundamental objectives behind the conception of the collections framework.

  1. Attain high speed and performance. This framework equips with highly efficient collections of data structures like generic arrays, link list, trees and hash tables.
  2. Achieving versatility by supporting the distinct type of collection types.
  3. Maintain consistency and interoperability in the implementation and usage of different collection types.
  4. Easy customization of primitive collections types as per developer’s requirements.

For your note, Java collections framework is bundled into the Java’s java.util package. And it also provides some map interfaces and classes. Maps use to hold data in the form of key/value pairs. Here, you must know that maps are distinct from a collection object though they are an integral part of the collections framework.

Let’s now have a look at the hierarchy of interfaces and classes available in the Java collections framework. Here, there is one catch which we would like you to note that the Collection interface extends the Iterable interface (in java.lang.Iterable). That’s why the Iterable interface is one of the root interfaces of the Java collection classes. And all subtypes of Collection, in turn, implement the Iterable interface.

But Iterable interface doesn’t belong to the java.util package so we didn’t include it at the top of the Collection interface in the below diagram. Though, we sketch the whole structure of the Java collections framework in the below picture for your clarity.

Next, we request you to have a closer look at the below screenshot to learn the architecture of Java collections framework, also please carefully see the color and symbol legends used to represent the interface, classes, and their relationship.

Java Collections Framework Hierarchy.

Java Collections Framework Hierarchy.

Apart from the interview questions, we host a no. of Java quizzes on our blog. If you like to give a try, please open any of the below links.

Now, it’s time to start exploring the Java collections framework interview questions and answers.

Java Collections Interview Questions and Answers:

Question(1): Explain the difference between Collection and Collections Framework? State the main benefits of Java Collections Framework?


Collection: A collection is an object container that can hold multiple elements and functions as a single unit.

Collections Framework: Collections framework provides a centralized system for managing and representing collections.

Advantages of Java Collections Framework:

  1. Provides better program management and improves performance.
  2. Enhances the reusability of source code and the compiled modules.
  3. Significantly reduces development efforts.

Question(2): What is the root interface in collection hierarchy?


The correct response to this question is the Collection interface in the collection hierarchy. Though few may disagree and will say that the Collection interface extends the Iterable interface so Iterable must be the root interface. But you can counter the argument by giving the fact that the Iterable interface is a part of java.lang package, not the java.util package. And even the Oracle’s Java collections documentation states that Collection interface is a member of the Java Collections framework which belongs to the java.util package.

Question(3): State the difference between Collection and Collections?


The Collection is an interface while Collections is a Java class, both are present in java.util package and the components of java collections framework.

Question(4): Mention the collection classes that are synchronized or thread-safe?


Following collection classes can be used in a multi-threaded environment because they are synchronized and thread-safe.

  1. Stack,
  2. Properties,
  3. Vector, and
  4. Hashtable.

Question(5): What are the core Collection interfaces?


Following is the list of primary collection interfaces.

  1. Collection,
  2. Set,
  3. Queue,
  4. List, and
  5. Map.

Some of the extended interfaces are as follows.

  1. SortedSet,
  2. SortedMap,
  3. Deque, and
  4. ListIterator etc.

Question(6): Elaborate the difference between List and Set?


  1. Set holds unique elements whereas List can have duplicate elements.
  2. The Set is an unordered collection whereas List is the ordered one.
  3. The List preserves the order of the object addition.

Question(7): Tell us the difference between Map and Set?


Map object use unique keys to hold values whereas Set contains unique values.

Question(8): Which classes implement List and Set interface?


Following classes implement the List interface.

  1. ArrayList,
  2. Vector, and
  3. LinkedList.

Following classes implement the Set interface.

  1. HashSet, and
  2. TreeSet

Question(9): What do you think an Iterator is?


The Iterator is an interface. You can locate it in the java.util package. It lists methods to iterate over any Collection.

Question(10): State the difference between Iterator and Enumeration?


  • Iterator:
    • Enumeration functions like a read-only interface as it only allows to traverse and retrieve the object.
    • Enumeration is doubly faster than the Iterator and uses very less memory.
    • Enumeration is very necessary and fulfills the primary requirements.
  • Enumeration:
    • The Iterator is much safer than Enumeration as it always blocks other threads to alter the collection object.
    • Iterator gives the ability to remove elements using remove() method whereas Enumeration doesn’t.
    • Iterator method names are more logical to make their functionality clear.

Question(11): What is the design pattern that Iterator uses?


  • It uses iterator design pattern. Iterator design pattern enables the ability to sail through the collection of objects by using a standard interface abstracting the underlying implementation.
  • Enumeration is also an example of Iterator design pattern.

Question(12): What are the methods to override for you to use any object as a key in HashMap?


To use an object as the key in HashMap, you can implement equals() and hashCode() methods.

Question(13): Explain the difference between Queue and Stack?


  • The Queue is a type of data structure which follows the FIFO (first in first out) mechanism. An example of Queue in the real world is casting your votes in the electoral center.
  • The Stack is a data structure which follows the LIFO (last in first out) mechanism. A real-time example of Stack is when a person wears bangles, the last bangle worn is the first to get removed and the opening bangle you would take out in the end.

Question(14): What is the best way to reverse a List in Collections?


The Collections class provides a reverse(List list) method which accepts the list as a parameter.

e.g. Collections.reverse(listOfobject);

Question(15): Which approach would you choose to convert the array of strings into the list?


There is one method in the Arrays class of java.util package as List() which takes the array as its parameter.

Question(16): Iterator doesn’t have a method to get next element. Can you explain why?


It would be easy to introduce a method on top of current Iterator interface, but its use will be rare. Hence, it doesn’t make sense to add it to the interface that everyone has to implement.

Question(17): State the difference between the Iterator and ListIterator?


  1. You can use Iterator to traverse the collections like Set and List whereas ListIterator you can utilize with Lists only.
  2. Using Iterator, you can cross in the forward direction only whereas ListIterator supports to traverse in both the direction.
  3. ListIterator inherits from Iterator interface and brings add-on functionalities like the ones listed below.
    1. Adding an element,
    2. Replacing an element, and
    3. Retrieve index position for the previous and next elements.

Question(18): Can you explain iterator’s fail-fast property?


  1. Iterator fail-fast property verifies for any change that occurs in the structure of the underlying collection whenever you try to get the next element.
  2. If it detects any change, it throws the ConcurrentModificationException.
  3. All the Iterator implementations in the Collection classes are fail-fast by design except the concurrent collection classes like ConcurrentHashMap and CopyOnWriteArrayList.

Question(19): State the difference between fail-fast and fail-safe?


  • Fail-fast is nothing but the mechanism to support the instant reporting of any failure and whenever a problem occurs fail fast system fails.
  • In Java, a fail-fast iterator may throw concurrent modification exception whereas a fail-safe iterator won’t. The exception can occur due to one of the two reasons as given below.
    • When one of the threads is iterating through the collection while the other one is trying to modify it.
    • Or after invoking the remove() method if you attempt to change the collection object.

Question(20): What are the different ways to sort a list of Objects?


Following methods can be applied to perform sorting.

  1. To sort the array of objects, you can use Arrays.sort() method.
  2. When you need to sort a collection of the objects, then use Collections.sort() method.

Question(21): How does HashMap work in Java?


HashMap follows the concept of hashing. It employs put(key, value) and get(key) methods to manage (storing and retrieving) the objects from the HashMap. When we call the put() method with a key and value object, the HashMap mechanism triggers the hashCode() method on the key object and returns the hash value. Then, the hashing function locates a bucket location using the hash value where it can save the key-value pair in the form of a nested class called Entry (Map.Entry).

When you need to fetch a value from the HashMap, you call the get() method with the desired key as the parameter. The key object produces the same hash value which leads to same bucket location where the object was stored in the put() method.

Question(22): What is the difference between HashMap and Hashtable?


  • Synchronization or Thread Safe
  • Null keys and null values
  • Iterating the values
  • Default Capacity

Question(23): Why Map interface doesn’t extend Collection interface?


  • The Set is a unordered collection and does not allow duplicate elements.
  • The List is ordered collection allows duplicate elements whereas the Map is a key-value pair.
  • It is viewed as a set of keys and collection of values.
  • A Map is a collection of key-value pairs so by design they separated from collection interface.

Question(24): What is the use of Properties class?

The Properties is a subclass of Hashtable. You can use it to manage the list of values using the keys, both the key and value are of String type.

Question(25): How to remove the redundant elements from an ArrayList object?

You can apply the following to cut off the redundant element from the ArrayList.

  1. Offload all of the ArrayList elements to a LinkedHashSet. It helps to eliminate the duplicates and keeps the insertion order intact.
  2. Emptying the ArrayList
  3. Copy all elements of LinkedHashSet (non-duplicates) to an ArrayList.

You can refer the below example for help.


We wish that the above post would help you to pass your expected Java programming interview. If you like the list of Java collections framework questions, please extend your support to share this post.

All the very best,


Leave a Reply