Top Java Collections Interview Questions to Know

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Are you an experienced Java programmer looking to ace your next job interview? Being knowledgeable of Java Collections is essential in mastering your craft. Here, we provide a comprehensive list of the most essential Java collections interview questions to know before taking that crucial job interview. Understanding these topics will help you confidently answer difficult questions and make a lasting impression on potential employers.


Dive deeper to learn about the most common Java Collections interview questions and how to answer them. 


Basic Java Collections Interview Questions

Java Collection interview can be very difficult to get into, so it’s important to know what you’re getting into. We bring to you the top java collection Interview Questions asked with sample answers that would help demonstrate your knowledge of the industry and the job role. 


Some of the most basic java collection interview questions are as follows:


What is Collection in Java?

In Java, a Collection is a framework that provides classes and interfaces to manage and store groups of objects. It allows you to organize, manipulate, and access these objects easily. Collections offer various data structures like lists, sets, and maps, enabling efficient data management in Java applications. It is very crucial to go through all Java Interview questions for better learning.


What is the difference between Array and Collection in Java?

The main difference between an Array and a Collection is in their nature and features in Java. Arrays are fixed in size and can store elements of the same data type, while Collections are dynamic and can store elements of different data types. Collections offer more convenient methods for adding, removing, and manipulating elements than arrays.


Explain the differences between ArrayList and LinkedList

ArrayList and LinkedList are both Java implementations of the List interface, but they differ in their underlying data structures and performance characteristics. ArrayList uses a dynamic array to store elements, providing fast random access but slower insertion and deletion. LinkedList uses a doubly-linked list, offering quick insertion and deletion but slower random access.


Let’s move to the next java collection interview question.


How does the HashSet ensure the uniqueness of elements?

HashSet ensures the uniqueness of elements by using a hashing mechanism. When an element is added to the HashSet, it calculates its hash code and finds the appropriate bucket to store it. If another element with the same hash code already exists in the bucket, the new element is compared for equality. The new element is not added to maintain uniqueness if they are equal.


What is the purpose of the Comparator interface?

The Comparator interface in Java defines custom comparison logic for objects. It allows sorting collections or arrays of objects based on specific criteria that are not the natural order defined by the objects’ classes. This enables developers to sort data flexibly and customizable, depending on their application’s requirements.


Differentiate between Collection and collections in the context of Java.

In Java, “Collection” (with an uppercase “C”) is an interface that represents a group of objects, providing methods for basic collection operations. On the other hand, “collections” (with a lowercase “c”) refer to the utility class java.util. Collections, provides various static methods to manipulate and operate on collections, like sorting, searching, and creating immutable collections. Java Fundamentals are core to your Java programming skills.


Let’s move to the next java collection interview question.


What is an Iterator? What is the default size of the load factor in the hashing-based collection?

An Iterator in Java is an interface that allows you to traverse or iterate through the elements of a collection, such as ArrayList, HashSet, or LinkedList. It provides methods like next(), hasNext(), and remove() to access and manipulate the elements in a collection sequentially.


The default size of the load factor in a hashing-based collection is 0.75. The load factor determines the threshold when the collection should be resized (rehashed) to maintain a balance between the number of elements and buckets in the underlying hash table. A load factor of 0.75 ensures a good balance between performance and memory consumption for most scenarios.


What are different ways to iterate over a list?

There are several ways to iterate over a list in Java. The most common approaches are using a for loop, for-each loop (enhanced for loop), Iterator, and ListIterator. The for loop provides index-based iteration, while the for-each loop and Iterator offer simpler, more concise ways to traverse the list’s elements. ListIterator allows bi-directional iteration with additional features like adding and removing elements during iteration. Choose the method that best suits your specific requirements and coding style.


Let’s move to the next java collection interview question.


Java Collections Interview Questions for Experienced

A few java collection interview questions for the experienced level are as follows:


What is BlockingQueue in Java?

BlockingQueue in Java is an interface that extends the Queue interface and provides additional methods to support blocking operations. It represents a thread-safe queue where different threads can add or remove elements concurrently. If the queue is full, the put() method blocks until space becomes available, and if it’s empty, the take() method blocks until an element is available for retrieval. This feature is helpful in implementing producer-consumer scenarios and other multi-threaded applications.


What is the difference between Comparable and Comparator?

In Java, Comparable and Comparator are interfaces used for custom sorting of objects. Comparable is implemented by the class itself to define its natural ordering. Comparator is a separate class that allows sorting objects based on different criteria without modifying their original class.


How to remove duplicates from ArrayList?

To remove duplicates from an ArrayList in Java, you can follow these steps:


  1. Create a new empty ArrayList or use the existing one to store unique elements.
  2. Traverse the original ArrayList using a loop or Iterator.
  3. For each element in the original ArrayList, check if it already exists in the new ArrayList using the contains() method.
  4. If the element is not present in the new ArrayList, add it to the new ArrayList using the add() method.
  5. After iterating through the original ArrayList, the new ArrayList will contain only unique elements.

Let’s move to the next java collection interview question.


What is a stack? Give one of its advantages

A stack is a linear data structure in computer science that follows the Last In, First Out (LIFO) principle. Elements are added and removed from the top, resembling a stack of plates. One of its advantages is its simplicity and efficiency in managing function calls and keeping track of program execution flow during recursion.


What is a priority queue in Java?

In Java, a priority queue implements the Queue interface that stores elements based on their priority. Elements with higher priority are dequeued before elements with lower priority.


What is the difference between Queue and Stack?

The main difference between a Queue & a Stack lies in their order of element removal. In a Queue, elements are removed in the order they were added (FIFO – First In, First Out), whereas, in a Stack, the last element added is removed first (LIFO – Last In, First Out).


What is the hashCode()?

hashCode() is a method in Java used to generate a unique integer value (hash code) for an object. It is used in hash-based data structures like HashMap and HashSet to store and retrieve objects efficiently.


Let’s move to the next java collection interview question.


What is Enumset?

EnumSet is a specialized Set implementation in Java that is designed to work exclusively with enum types. It offers memory-efficient and high-performance operations for enum values.


Differentiate between Iterator and Enumeration

Feature Iterator Enumeration
Availability Part of the newer Java Collections Framework. Part of the older Java Collections Framework.
Bidirectional Support Supports both forward and backward traversal. Supports only forward traversal.
Removal Support Supports element removal using remove() method. Does not have a built-in removal method.
Fail-Fast Behavior Iterator is fail-fast and throws ConcurrentModificationException if the collection is modified during iteration. Enumeration is not fail-fast and may not throw exceptions if the collection is modified during iteration.
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In this guide we have discussed some of the most important and common java collection interview questions with answers. Once you are prepared with these java collection interview questions, you have a greater chance of getting the selected in the interview and crack the offer. 

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To prepare for java collections interview questions, review core Java concepts thoroughly and practice coding exercises to strengthen your problem-solving skills. Additionally, be familiar with popular frameworks, libraries, and industry best practices.
Here are some common Java interview questions for freshers with 1-year experience:
  1. Explain the difference between ArrayList and LinkedList.
  2. What is the purpose of the "final" keyword in Java?
  3. Describe the usage of "static" keyword with methods and variables.
  4. How does Java handle multiple inheritance? Explain using interfaces.
  5. What are the different types of exceptions in Java?
Core Java concepts, OOP principles, exception handling, multithreading, and collections are essential topics for a Java interview. Understanding these concepts thoroughly will help you excel in the interview.
Java basics include fundamental concepts such as data types, variables, operators, control structures (if-else, switch), loops (for, while), and basic object-oriented programming (classes, objects, methods).

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