JavaJava Developer Interview Questions

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Java is one of the most used programming languages on the planet, rivaled only by big names such as Python and JavaScript. Java developers are highly sought-after and, as such, the job market can feel a bit competitive. To give yourself a leg-up on the competition, it is always wise to prepare prior to any job interview. To help you achieve that goal, we have compiled a list of the top Java interview questions and answers for 2021.

Preparation is key to any job interview – no matter what industry or title you are applying for. When it comes to jobs for software engineers, developers, and programmers, that preparation is even more vital because going in, you can expect that your skillset and knowledge about coding will be challenged. Sometimes that challenge comes in the form of some simple questions, and other times you will be asked to solve a riddle using programming logic or a bit of Java code.

Researching the company’s history, their product types and services offered, and learning a little about some of their employees through resources like Glassdoor or LinkedIn are also great ways to make sure you are prepared and are able to showcase that you have done your due diligence by asking questions related to the research you have conducted. Employers like to see that you have an interest in their organization and, for your own sake, you should.

At the end of the day, though, refreshing your memory about core Java concepts and programming principles is going to be key to getting you through the door. Without that knowledge – and the ability to prove you have it – you will never land the gig. With that in mind, let’s begin!

Java Job Interview Question Prep

The following are a list of questions (and their relevant answers) you can expect to encounter in an interview for a Java developer position. Not every employer will ask these questions, but they are commonly asked, so it is best to be prepared with their answers just in case.

Question: What is JVM: What Does It Stand For and What Does It Mean?
Answer: JVM stands for Java Virtual Machine and is a necessary piece of software systems use to run Java applications. It consists of several components, including:

Classloader: Loads class files when a Java app is executed.
Class Area: Holds the class level of every class file (including metadata and static variables).
Execution Engine: This is a virtual processor and an interpreter responsible for issuing instruction from the bytecode. It also has a JIT (Just-in-Time) compiler that detects low rates of execution and increases performance.
Java Native Interface: Used to communicate and interact with another piece of software developed in a different programming language, such as C.

Java Interview Questions JVM Concept

By Michelle Ridomi – Own work, CC BY-SA 4.0
Wikimedia Commons

Question: What Memory Allocations are Available in Java?
Answer: The main five types of memory allocation in Java are: Class memory, Heap Memory, Native Method Stack Memory, Program Counter-Memory, and Stack Memory.

Question: What is Object-Oriented Programming?
Answer: Object-oriented programming (OOP) is a programming concept based on the use of Objects, which are essentially containers that hold data as fields and code as procedures. These data fields are also referred to as attributes and properties. Procedures, meanwhile, are frequently referred to as methods.

Classes are another feature of most top-level OOP programming languages. Classes are essentially blueprints that Objects are created from, and, as such, you can define the common properties of all Objects (of one type) that get created from a Class.

Object-oriented programming also incorporates the concepts of inheritance, where one class inherits the properties of another class; encapsulation, a Java mechanism whereby code and data are wrapped up in a single unit; abstraction, the method of obfuscating the detail of implementation from a user, but still providing the functionality; and polymorphism, which describes the multiple forms that a function, Object, or variable might take.

Question: Is Java an Object-Oriented Programming (OOP) Language?
Answer: Yes and no. Java is partially an OOP language. The reason it is not fully has to do with the fact that it uses eight primitive data types: boolean, byte, cha, double, int, float, long, and short. All of these primitive data types are not considered Objects in Java.

Question: What Are Wrapper Classes in Java?
Answer: The easiest explanation for wrapper classes is to say that they are used to make the primitive data types in Java (boolean, byte, cha, double, int, float, long, and short), into Objects.

Question: What is a Java Singleton Class?
Answer: Java Singleton’s are Classes that can only have a single instance or object at a time. You can make a Singleton Class by settings its constructor private.

Question: Can you describe the JIT compiler?
Answer: The JIT compiler – which stands for Just-in-Time compiler – is a program that converts bytecode into a set of instructions and then helps send them to a processor. The purpose of JIT is to enhance performance in Java software at run-time. It earns the moniker “Just in Time” because it gets called upon every time a new method is invoked. Once this happens, JIT compiles the method’s bytecode into machine language “just in time” for it to execute.

Question: What is the difference between an instance variable and a local variable?
Answer: Java local variables are primarily used in a block, constructor, or method. They only have local scope, meaning that they are not used outside of the scope of a block. In fact, local variables are only visible within the block, constructor, or method that they are defined within. They do not have a default value, so you must always declare an initial value when creating them. To define a local variable, you could use the following code:

public class HelloMom
	public static void main(String[] args)
	String helloMomMessage;
	helloMomMessage = “Hey Mom! Look at me! No hands!”;

Instance Variables, meanwhile, are variables that belong to an object specifically and are declared within a class and outside of a method. If you change this variable type, it will not result in a change in other instances of the class it was created in – only in the original instance. Here is an example of an instance variable in Java:

class Superhero {
public String superName;
private int superAge;

In the above example, we created two instance variables. The first – superName – has public access. The second – superAge is an int type with private access.

More Java Programmer Interview Questions

There are literally hundreds of questions a Java programmer may face when being interviewed for a developer job. We have only just scratched the surface here so far. Be sure to bookmark this page, however, as we will be updating our Java Developer Interview Questions primer from time to time.

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