CodeSubmit Interview Series

9 Best Java Interview Questions in 2023

Want to hire your next Java developer? You need the right Java interview questions. Here are the 9 best Java interview questions and answers to ask your candidates. 

Java interviews

Java is a class-based and object-oriented programming language that is one of the most popular programming languages in the world. It’s created to include as few implementation dependencies as possible. Java is a general-purpose language, meaning that its code can run on all platforms that support Java. 

But how do you hire the right Java developer? Here are some of the best Java interview questions. 

Beginner Java interview questions

What questions can you ask junior Java developers? Here are the top questions to ask beginner Java developers. While their technical knowledge might still be limited, these questions show what their understanding of Java is. 

1. What is Java?


Java is an object-oriented, class-based, general-purpose, platform-independent, secure, robust, and high-performance programming language. It was developed by Sun Microsystems in the 1990s and is today one of the most popular programming languages. 

This language is intended to run on all platforms that support Java without recompilation. Java applications are usually compiled to bytecode that can run on any Java virtual machine, irrespective of the underlying computer architecture. 

Java is similar to C and C++ but with fewer low-level facilities. Its runtime provides dynamic capabilities, something that typically isn’t available in traditional compiled languages. 

2. What are the benefits of Java?


Java has several benefits, including:

  • Object-oriented: Java uses the object-oriented paradigm with which developers can maintain code as the combination of different types of objects, which incorporate data and behavior. 

  • Portable: Java’s approach is “read once, write anywhere,” which means that Java can be executed on every machine. Java is compiled to bytecode (.class) so that it can be run on any virtual machine.

  • Simple: The syntax of Java is based on C++ and so Java is a relatively simple language to learn. 

  • Secure: Java doesn’t include explicit pointers and uses bytecode and exception-handling, all of which make it secure.

  • Robust: Java uses strong memory management, exception-handling, automatic garbage collection, and so on and is, therefore, a very robust language. 

  • Platform-independent: Other programming languages, like C and C++, need a platform to be executed. Java has its own platform and doesn’t depend on any operating system. 

  • Interpreted: Java is a just-in-time interpreter and uses it together with the compiler for program execution. 

  • High-performance: Java’s bytecode is close to native code and so Java is faster than other interpreted languages. However, it is slower than compiled languages, such as C++.

  • Multithreaded: Java applications can be written to deal with many tasks at once with multiple threads, which don’t occupy memory for each thread and are important for web applications, multimedia, and more. 

  • Distributed: Java enables distributed applications, which make it possible to access files from any machine on the internet. 

  • Dynamic: Java is a dynamic language, which supports dynamic loading of classes so that they can be loaded on demand. 

  • Architecture neutral: Java isn’t dependent on the architecture. 

3. What are classes in Java?


Java codes are defined in a class, which has methods and variables. Variables are attributes and they define the state of a class.

On the other hand, the business logic is done in methods, which contain a set of statements/instructions to meet particular requirements. 

4. What is an Inner Class and a Sub-Class?


An Inner Class is nested in another class. It can access all variables and methods that are defined in the class that is nesting it because it has access rights for that class.

A Sub-Class inherits from a Super Class, which gives it access to all public methods and fields of that class. 

Advanced Java interview questions

Now, let’s take a look at more advanced Java interview questions. These questions are more technical and work well for more senior Java developers.

5. What are Loops in Java?


Loops execute statements or a block of statements repeatedly. Java has three Loops. These are:

1. For Loops 

For Loops are used when the developer knows how many times a statement needs to be executed because For Loops are used to execute statements for a certain number of times. 

2. While Loops

If you need to execute a statement repeatedly until a condition is fulfilled, you use While Loops. The condition is checked before execution.

3. Do While Loops

Finally, Do While Loops statements are executed at least once. They are similar to While Loops, but instead of checking the condition is checked after execution. 

6. What is Java Classloader? 


Java Classloader loads classes on demand (lazy loading) into the Java Virtual Machine. These classes can be loaded from a remote file system, the local file system, or the web.

Three class loaders are used when the Java Virtual Machine is started. The first is Bootstrap Classloader, which loads the core Java API file rt.jar from the folder. The second is Extension Classloader, which loads jar files from the folder and the third is System/Application Classloader, which loads jar files from the path that is specified in the CLASSPATH environment variable. 

7. How do Interfaces impact performance compared to abstract classes?


Interfaces are more burdensome for developers because any time an Interface is implemented in a class, the developer must implement every method in the Interface. 

Interfaces are also slower compared to abstract classes because Interfaces require extra indirections. 

8. Would it be more secure to store sensitive data, such as social security numbers, credit card information, and so on, in a String or a character array?


Character arrays. If you use a mutable object like a character array to store the value (the sensitive data), you can set it to blank so that it won’t be retained in memory.

If you use Strings, which are immutable and stored in the String pool, the String stays in the pool in memory until it’s removed. So even after the value has been processed, it remains available in memory for a certain period of time and you don’t really have any control over that. Anyone with access to a memory dump could also access the sensitive data. 

9. What are packages in Java?


Packages are like folders in a file directory; they’re used to group related classes to avoid name conflicts and write better maintainable code. 

Packages are divided into two categories:

a) Built-in packages, which are packages from the Java API

b) User-defined packages, which are created by the user

Over to you!

There you have it! Now you know what the 9 best Java interview questions are.

Sure, you want someone with technical knowledge. But you also want someone who understands why they use Java, how it can benefit your organization, and who can communicate well -- that’s what your interviews are for.

A great way to assess your candidates' technical knowledge is by using skill assessment tests. 

If you want to incorporate skill assessments with your interviews, try CodeSubmit.

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