Free Solution to Java Collection Interview Questions

Java collections framework is among the most important features of the java programming language.

There’s hardly any Java interview that won’t include questions about the Java collections framework. This article discusses some Java collection interview questions to help you prepare for your interview.

To be able to answer Java collection interview questions, you need a good understanding of different collection interfaces and their frequently used implementations.

Photo by Caspar Camille Rubin on Unsplash

Examples of Top Java Collection Interview Questions

What Is Java Collections Framework and Its Benefits?

Collections are used for sorting, searching and collecting data. Java collections framework is an effort to design an integrated framework to help developers. Its major benefit is its performance improvement over previous methods.

Another benefit is that it enhances code quality through the use of well-tested collections framework classes. In addition, interoperability and reusability are big advantages.

Differentiate Between Hashmap and Hashtable

This question is perhaps one of the most popular collection interview questions. The key difference between these two is thread safety. HashTable is a thread-safe collection, while HashMap is not thread-safe. 

Explain the Difference Between Vector and ArrayList

The first difference between the two is that the methods of ArrayList aren’t synchronized. On the other hand, the methods of Vector are synchronized. When resized internally, ArrayList increases by half of its size, while Vector doubles the size of its array when resized internally.

What Is the Difference Between TreeSet and HashSet?

One of the key differences between the two is that TreeSet maintains inserted elements in the sorted order. On the other hand, elements are maintained in random order in HashSet.

Also, TreeSet cannot store the null object, while TreeSet can. Null objects are allowed in HashSet. TreeSet does not allow null objects and throws a NullPointerException. 

What Are the Classes Implementing Set and List Interface?

A class implementing the Set interface includes HashSet and TreeSet. LinkedList, ArrayList, and Vector are class-implementing list interfaces.

Explain the Difference Between Set and List in Java

One key difference is that List allows duplicates. Conversely, Set doesn’t allow duplicates. In addition, Set does not maintain insertion and order, while List does.

Other Java Interview Questions to Expect

  • What is an iterator?
  • Arrange the following in descending order based on performance: ConcurrentHashMap, HashMap, Hashtable, and Collections.SynchronizedMap 
  • Why is it that the Map interface does not extend the Collection interface in the Java collections framework?
  • How do you sort objects?
  • What is the difference between stack and queue, considering the fact that they are both data structures?
  • Can you explain the importance of the hashCode() and equals() methods?


In order to prepare for an interview in the Java collections framework, you need to understand the several difference in collections mentioned above. All these collections work similarly.

However, the difference in their implementations and benefits will make a big impact on how they are used in practice. This is what interviewers want to check when they test you. You need to be confident and explain what you know.

Abir is a data analyst and researcher. Among her interests are artificial intelligence, machine learning, and natural language processing. As a humanitarian and educator, she actively supports women in tech and promotes diversity.

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