Popular Java Interview Questions and Answers that you should check out
Going for a job or even an internship interview is not easy for anyone, but being properly prepared for it can give your confidence a boost. Hence, if you have an interview for a Java development position, then it is important to gain an understanding of what you might be asked. Your preparation process for acing the interview should ideally start off with the right set of Java Interview Questions.
Traditionally, most java-based interviews used to focus majorly on the differences between programming languages. But today, the questions have become more advanced, and proper java training shall be needed to answer them. In addition to having a good degree of java knowledge, you should also know the basics of SQL and Linux, coding, software, networking fundamentals, and more at these interviews. You might also be asked questions associated with data structures, algorithms, and NIO patterns.
As Java is majorly used as an application programming language, interview questions linked with this domain put emphasis on API and core concepts. While there is no guarantee of what questions shall be asked at your relevant Java interview, gaining proper knowledge about the concepts associated with it and going through some frequently asked questions can increase your odds of faring well in it. For best outcomes, you may even join Java certification courses where important topics associated with Java shall be taught in a comprehensive manner.

Common Java Interview Questions

Q. Why is Java a platform-independent language?

Java was developed in a manner that doesn't depend on any kind of software or hardware. Instead, code is compiled by a compiler, which converts it into a platform-independent bytecode that can be run on several systems.

Q. Why can Java not be a pure object-oriented language?

As Java supports char, short, byte, long, boolean, int, and such primitive data types, it cannot be considered a pure object-oriented language.

Q. Is Java a completely object-oriented programming language?

Java is arguably the complete object-oriented language. You can access every Java component since they come under classes, by creating these objects. 

However, from another perspective, Java isn’t the complete package as an object-oriented programming language. According to this line of thinking, Java isn’t complete since it can directly access primitive types of data. This primitive data doesn’t belong directly to any integer classes.

Q. How are Java and C++ different?

  • C++ is a compiled programming language, while Java is both interpreted and compiled. 
  • C++ programs can run in just the machines where they have been compiled, while Java programs are system-independent.
  • While using C++, a user can use pointers in the program, which isn’t allowed with Java. Java uses pointers internally. 
  • C++ supports multiple inheritances as a concept while Java doesn’t. Moreover, avoiding the name ambiguity complexity results in the diamond problem.

Q. C and C++ use pointers. Why doesn’t Java use any?

A pointer is unsafe and complicated for new programmers to use. Java is more focused on simplifying code, which can become a challenge with pointers. Using pointers can also result in errors and compromise security by allowing users to access memory directly. Additionally, not using pointers with Java furnishes a new level of abstraction in the program. Additionally, using pointers makes the garbage collection process erroneous and slow. Java uses references instead as pointers can be manipulated, but references can’t. 

Q. What are the default values of instances and variables in Java?

  • Java variables have no default values. Developers need to initialise or enter values before using instances, else it could add compilation errors. In some situations, variables don’t need to be initialised. 
  • However, if you create this object, the default constructor initialises the default value depending on the type of data.
  • If the object is a reference, it is assigned a Null value.
  • If the object is numeric, it is assigned 0 as a value.
  • If the object is a boolean, the value assigned is ‘False’.

Q. What do you know about data encapsulation?

Data encapsulation can be described as a programming concept that is object-oriented and hides data attributes as well as their behaviours in unique units. Through data encapsulation, developers can follow modularity as they develop software. They ensure every object remains independent of the others through unique attributes, functionalities and methods for every object. The primary use of data encapsulation is to secure an object’s private properties and thus enable data hiding.

Q. What are the major benefits of using Java?

Being a high-level object-oriented programming language, Java can be used for developing applications, games, and device systems. In addition, it is platform-independent, swift, dependable, and fast.

Q. What is meant by Class and Object?

An object can be described as a collection of methods. Classes essentially execute operations and represent their state. Class is used for the purpose of defining new types of data that are subsequently used to create objects.

Q. What do JRE, JDK, and JVM imply?

JVM (Java Virtual Machine) provides a runtime environment for codes needed to be executed, while JRE (Java Runtime Environment) can be referred to as a collection of whiles required during runtime by JVM. JDK (Java Development Kit is ultimately used for writing and executing a program. It features JRE alongside certain development tools.

Q. Does all property of Immutable Objects have to be finalized?

No, this is not necessary. It is likely to achieve the same functionality by making members private but at the same time non-final and not making any modifications to them except in the constructor. Do not leak any reference for that member if it is a mutable object, and do not provide a setter method for them as well. It is important to remember that making a reference variable final just makes sure that it shall not be re-assigned a distinguished value. However, one may still change individual properties of an object pointed by the relevant reference variable. 
(This is among the popular Java Interview Questions.)

Q. What does looping imply?

Loops tend to be used for executing a block of statements or even just a particular statement repeatedly. Loops are categorized into three types, Do While Loops, While Loops, and For Loops.

Q. What is the key difference between Overriding and Overloading?

A situation where the two methods have the same properties and name but occur in a child and parent class, respectively, is known as Overriding. On the flipside, Overloading implies two methods with different properties but the same name. 

Q. What does Inheritance imply?

With Inheritance, one is able to let a derived class acquire methods from a base class.

Q.What is the JIT compiler?

An abbreviation for Just-In-Time, JIT is used with the aim of augmenting performance during the run time. Compiling parts of byte code that has similar functionality simultaneously are conducted by it to minimize the compilation of time involved in running the code. A compiler is a translator of source code to machine-executable code. JIT compiler essentially is a part of JVM. This is among the most interesting Java Interview Questions.

Q. Is restricting Inheritance possible?

Yes, one can opt to restrict Inheritance by making use of the method final or using the final keyword. It can also be restricted through a private constructor or (//) Javadoc comment.

Q.What is meant by Content Negotiation?

Content Negotiation takes place between the host server and the user. For example, while making an HTTP request, the user shall get their result in varied formats and languages. They subsequently can specify the content that they would accept back from the host through content negotiation.

Q. What does WORA mean?

WORA also known as Write Once Read Anywhere. It essentially is the property of a language to be run on any platform. Owing to its bytecode nature, Java is allowed in this property. However, it is not platform-specific, as WORA is somewhere between a source code and machine code.

Q. How can deadlock be avoided in Java?

By breaking circular wait conditions, it is possible to avoid deadlock in Java. However, to do so, one must make arrangements in the code for imposing the ordering on the acquisition and the release of locks. If the lock is acquired in a consistent manner and released in the opposite order, a single thread holding a lock acquired by another will not take place and vice versa.

Q. Why is a constructor important?

Constructors are used for initializing the state of an object. If the user creates a new object with the usage of a new keyword, a default constructor shall get invoked. It additionally should have a similar name to the class name.

Q. Is it possible to override static methods?

No, it is not likely to override static methods. While the declaration of static methods with the same signature can be made in the subclass, runtime polymorphism can't take place in these cases. Instead, dynamic polymorphism or overriding takes place during the runtime. However, static methods are loaded and looked up at the compile time statically, and they cannot be overridden as a result.

Q. What would be the core objective of garbage collection?

Freeing up valuable memory space that has been occupied by unreachable and unnecessary objects during the Java program execution is the core objective of garbage collection and is done by deleting such objects. This is done with the aim of making sure that the memory resource is used competently. Conversely, it also does not offer any guarantee that there shall be adequate memory for the program execution.

Q.What shall be the use of composition?

The composition can be used for holding the reference of one class within another class. In this situation, the contained object is not able to exist without the class that contains it, and it subsequently is a type of aggregation.

Q. What is meant by annotation?

Annotation is simply a tag that is used with the aim of symbolizing metadata representing a certain field, interface, class and so on. It is generally used by the JVM and the compiler and does not directly influence the operations.

Q. When it comes to storing confidential data, why is character array usually opted for over string?

A string cannot be modified in Java. Till it is not removed as garbage, it shall stay in the string pool subsequently to the declaration. Thus, string stays in the heap section of the memory for an unspecified and unregulated span of time after the execution of string value processing.

Q. Which components do you find in a Java download file? Are they the same or do they have differences?

Every Java Download file has two components. 

  • Java Development Kit or JDK
  • Java Runtime Environment or JRE

There are several differences between them both, which is as follows:



JDK is a kit dedicated solely to software development.

JRE is a bundle of software solutions and a library created to execute Java programs.

It’s a platform-independent component of Java Download files.

It’s a platform-independent component, unlike JVM.

JDK packages are bundles of tools that help with debugging and development. 

JRE packages only support files and libraries in system runtime environments.

JDK package comes together with any installer. 

The JRE Package doesn’t have an installer, has only a runtime environment

Q. What is a classloader?

In Java, classloaders are a Java Virtual Machine subsystem, which focuses on loading class files. Once a program gets executed, the classloader takes the first step toward loading executable files. The classloaders in Java are Bootstrap, Application and Extension. 

Q. Which memory allocations does Java have?

There are five leading memory allocation types.

  • Class memory
  • Stack memory
  • Heap memory
  • Native method stack memory
  • Program counter-memory

Q. Are there any differences between Stack and Heap memory?

There are fundamental differences between Stack and Heap memory. Stack Memory is typically used for storing local variables and the method execution order. Meanwhile, Heap memory stores objects securely. Once both these types of memories store information, dynamic memory reallocation and allocation are used. 

Q. Can a program run if you write ‘static public void main’ as a command?

In Java, there are no rules regarding specifiers that say they need to follow a particular order. So the program can run effectively with this command. 

Q. What is the default value that Local Variables store?

There are no default values stored in either Local Variables or any primitive or object references. 

Q. What is an Association?

An Association can be understood as a relationship where one component or object has no ownership of the other. For instance, one person can have multiple bank accounts and a bank can have multiple customers. However, neither one owns the other.

Q. What does aggregation mean?

Aggregation can be understood as the bond between two classes. It can best be described as a ‘has-a’ and ‘whole/part’ relationship. An aggregation is a highly-specialised type of association relationship. The reference of another class is contained in an aggregation and it also owns that class. 

Q. What is a Copy Constructor in Java?

In Java, a Copy Constructor is the constructor responsible for initialising one object through a second object belonging to the same class.

Q. What is a marker interface?

In Java, any empty interface is known as a Marker interface. Well-known examples of a Marker Interface are Cloneable and Serializable.

Q. What is Object Cloning in Java?

Object Cloning refers to the ability to recreate entire objects to mimic another existing object in Java. Java provides a method called clone() that enables the cloning of an existing object that will offer the same functionality as the one that already exists. 

Q. What is an object-oriented paradigm?

Object-oriented paradigms function according to the Objects concept and are made up of code and data. The data is contained as fields while regulation is contained as procedures. The most prominent feature object-oriented paradigms have is that the procedures of an object can access and modify the fields of data themselves. 

Q. What are Wrapper Classes in Java?

When a programmer declares a primitive data type in Java, a Wrapper class is responsible for converting the data types into objects.

Q. What is a Singleton Class in Java?

When a programmer makes a class’ constructor private in Java, that class can only generate one object. This class is what developers call a Singleton Class. 

Q. Define a Java package.

A package bundles together several interfaces and classes as well as the necessary JAR files and libraries. A package in Java helps reuse code. 

Q. Can you implement a pointer while using Java?

A Java VM implicitly helps in managing memory. The primary objective behind developing Java was to simplify programming. For this reason, accessing any memory or data directly using pointers is not recommended since it complicates the process. Thus, you cannot implement pointers in Java.

Q. Distinguish between local and instance variables.

Any variable is declared within a class, where the scope remains limited to one specific object. Local variables are contained within a method or a specific code block. Additionally, the scope remains limited to a code segment with a declared variable. 

Q. What do you know about Java String Pool?

Java String Pool is defined as a collection of strings within the Heap memory in Java. If you attempt to create any new string object, JVM tries to first locate a new object’s presence within the pool. If the object is available, the same object reference gets shared with this variable, otherwise a new object gets created. 

Q. What is an exception in the context of Java?

In Java, any unexpected event that disrupts the normal flow of a program is called an exception. You can fix these events using the Exception Handling process. 

Q. What will happen if the main() is not declared static?

If the main method isn’t declared static, the program can be correctly compiled but could end up with severe ambiguity. As a result, it throws up an error message that says ‘NoSuchMethodError’.

All these questions are not guaranteed to come in your next Java interview. These are the core Java interview questions that show you the level of questions that you can expect as well as how you should frame your answers when asked. There are several instances where candidates know the answer but stutter or lose confidence because they cannot frame their answer right. 

To train with industry experts and prepare for your interview with professional mentors, enrol in a training course on Koenig today.

Armin Vans
Archer Charles has top education industry knowledge with 4 years of experience. Being a passionate blogger also does blogging on the technology niche.



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