Java Interview Questions You Need to Know: Prepare for Your Dream Job

Questions about the Java Collection package or framework are the most common and challenging to answer in every Core Java interview Questions. If you are well-versed in Collection and threads, you will perform better in Java interviews because Collection and multithreading make passing any Java interview difficult. I considered creating interview questions about the Java collection framework, such as ArrayList, HashMap, and Hashtable, as well as recently added concurrent collections like ConcurrentHashMap.

We’ll review the most frequently asked java collection interview questions so that experienced candidates and new hires can do well in their upcoming interviews.

Java Collection Interview Questions for both Experienced and Freshers.

1. What does Java’s Collection framework do?

The collection framework is made up of interconnected classes and interfaces. It stores and changes data in the form of objects. It has several classes and interfaces to accomplish this, including ArrayList, Vector, Stack, HashSet, List, Queue, Set, and so on.

2. What is the primary distinction between a collection and an array?

When it comes to storing object references and manipulating data, arrays and collections have a lot in common, but they also have a lot of crucial differences. The following definitions explain the main differences between an array and a collection:

Unlike collections, whose size can be changed on the fly based on what is required, arrays always have the same length and cannot be expanded or contracted during runtime.

Collections can contain objects of various types, whereas arrays can only contain objects of the same type or with the same type.

Arrays lack the “ready-made” techniques that users require for sorting, searching, and so on, whereas collections do.

3. Define the difference between a Collection and a collection in Java.

Collection: A collection interface can be found in the Java.util.package. It demonstrates that a collection of different things is the same thing. It is analogous to the container in the C++ programming language. The collection interface is the collection framework’s primary interface. It has a lot of classes and interfaces that show how a bunch of different things work together. The collection interface’s main sub-interfaces are the List, Set, and Queue interfaces. Even though the interface is part of the Java collection framework, the map interface is not collected. The Collection interface’s most important functions are add(), delete(), clear(), size(), and contains() ().

Collections:  Collections are valuable in the Java.util.package. It describes various methods for working with collections, such as sorting and searching. The methods are all fixed. These methods simplify things for developers, making it easier for them to use Collection Framework. To arrange the collection elements, use the sort() function. It also has methods for obtaining the minimum and maximum values, respectively.

4.Why is Collection not built on the Cloneable and Serializable interfaces?

The Collection interface in Java describes a set of objects known as “elements.” The order and maintainability of the items are entirely dependent on the implementations provided by each Collection member. As a result, extending the Cloneable and Serializable interfaces is pointless.

5. Describe the main advantages of the Generic Collection.

The following is  the primary reason for using Java’s generic Collection:

  • More thorough type checks during compilation reduce the need for typecasting and allow you to use generic algorithms, making the code more flexible, type-safe, and easy to read .

6.What does “Iterator” in the Java Collection Framework mean to you?

The Iterator interface for the Collection framework is part of Java. util package in the Java programming language. It is a Java Cursor used to navigate through a collection of objects. Here are a few more essential features of the Iterator interface:

  • Visit each component of a group of things separately.
  • It is the Universal Java Cursor because it works with all Collection framework classes and supports the READ and REMOVE actions.
  • Iterator method names are simple to use.

7.How are Collection objects in Java ordered?

Java Collections sort items using the Comparable and Comparator interfaces. When the Collections. sort() method is called, the items are sorted in the natural order determined by the compareTo() method. When the Collections. sort(Comparator) method is used, the Comparator interface’s compare() function is used to sort the items.

Java Collection Interview Questions

8.What is the function of the List interface?

A list interface is a group of components that are ordered in Java. It allows you to store duplicate data and maintains the order in which it was added. This interface provides several methods for changing an element based on its index. ArrayList, LinkedList, Stack, and Vector are the main Collection framework classes that interact with the List interface.

9.In Java, what is an ArrayList?

An ArrayList is a List Interface implementation that allows members to be added and removed on the fly. The Collection framework’s ArrayList will enable you to access elements by position and add new ones. The list is in chronological order, and duplicates are permitted. If there are more items than the original size, the size of an ArrayList can be changed on the fly.

10.What exactly is a Java priority queue?

In Java, a priority queue is an abstract data type. It works similarly to a queue or stack, but each member has a unique property called “priority.” No matter the order in this queue, a high-priority item is served before a low-priority thing. The PriorityQueue is based on the priority heap. The articles in the priority queue are sorted by their natural order or by a Comparator available when the queue is created, depending on which function Object() [native code] is used.

11. What is the Java Stack class, and what methods does it have?

The Java Collection architecture includes the important Java Stack class. It is based on the fundamental “last-in, first-out” ordering principle. In other words, the back end contains both new and old components. A “push” is adding something to a stack, and a “pop” is removing something from a stack. 

You Must Watch: TypeScript: Boosting Your Productivity and Code Quality

12.What is the purpose of the Java HashSet class, and how does it store the parts?

The HashSet class in the Java collections framework implements the Set interface. The AbstractSet class serves as the foundation for this class. It collects different parts and stores them in an implicit hashtable. Hashing is used to store data in a hashset in a hashtable, which is a HashMap instance. Hashing converts data into a unique value known as a “hash code.” This hashcode is then used to index the key’s information. The hashcode can be generated from the informative key by performing some math.

13.Is it possible for a TreeSet or HashSet to have a null element?

A HashSet can have one null item, but a TreeSet cannot because its items are stored in a NavigableMap. This is because NavigableMap, a subtype of SortedMap, cannot accept null keys. If you try to add a null element to a TreeSet, you will get a NullPointerException.

14.What does “Map interface” mean in Java?

The elements are saved as pairs of keys and values in Java’s map interface, making them easier to find. Each key corresponds to a single value. These key-value pairs comprise the Map’s entries. This interface contains the method signatures for adding, removing, and retrieving elements based on a key. Dictionaries are the best way to connect a key to a value using these methods.

15.Why does Map not affect the Collection Interface?

The Map interface in Java comprises key/value pairs, whereas the Collection interface comprises a group of objects that are stored and accessed in a specific manner. The primary reason Map does not extend the Collection interface is that the Collection interface’s add(E e) function does not work with key-value pairs in the same way that the Map interface’s put(K, V) method does. Even though it adds nothing to the Collection interface, it is an integral part of the Java Collections framework.

16. What is a ConcurrentHashMap in Java? Do you make use of one?

ConcurrentHashMap, a Java class, uses the ConcurrentMap and Serializable interfaces. The hashMap class has been changed because it doesn’t work well in a setting with multiple threads. It works better than the HashMap when it comes to performance.


So, the Java Collections interview questions have come to a close. The most in-demand skill sets for a Java professional are those that you learnt about in this set of Java Collections interview questions. You will undoubtedly succeed in your job interview if you use these sets of Java Collection Interview Questions.

Press ESC to close