Top 25 Java Interview Questions In 2023

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Java has emerged as a thriving technology worldwide, renowned for its simplicity and robustness in coding and learning. Unlike the C language, Java offers the advantage of code reusability, making it an attractive choice for developers.

With its open-source nature, Java has found extensive utilization across various domains. It serves as a versatile platform, enabling users to accomplish tasks efficiently. In fact, Oracle reports that Java powers an impressive three billion devices globally. Its applications span a wide range of industries, including access control systems, automotive technology, IoT gateways, optical sensors, and more.

For those venturing into a Java development career, mastering the art of cracking Java interviews has become increasingly vital. The interview process has become more intricate, demanding comprehensive knowledge and expertise from candidates.

Most Common Java Interview Questions And Answers

Most Common Java Interview Questions and Answers” provides a concise compilation of frequently asked questions during Java programming job interviews, along with their corresponding answers. This resource serves as a valuable preparation tool for aspiring Java developers looking to excel in interviews. It covers a wide range of topics, including core Java concepts, object-oriented programming, data structures, algorithms, design patterns, multi-threading, exception handling, and more. 

Java is a high-level programming language and is platform-independent. Java is a collection of objects. It was developed by Sun Microsystems. There are a lot of applications, websites, and games that are developed using Java.
Java, objects play a crucial role as they encapsulate both state and behavior through methods and classes. A Java program revolves around multiple objects collaborating and communicating with each other to accomplish their respective tasks.
While Java is primarily an object-oriented programming language, it also incorporates non-object elements. it includes eight primitive data types boolean, byte, char, int, float, double, long, short that are not objects. Despite this, Java still emphasizes object-oriented principles and provides wrappers (classes) for these primitives to allow them to be used in an object-oriented manner.
Eclipse, IntelliJ IDEA, NetBeans, Visual Studio Code & JDeveloper are the IDE’s of JAVA.
Java language was designed to be platform independent by utilizing a compiler that converts code into byte code. This byte code can then be executed on various systems as long as a runtime environment (JRE) is installed, eliminating the need for hardware or software dependencies.
Java can be considered a complete object-oriented programming language because everything in Java is encapsulated within classes and accessed through objects. But,it is not considered a purely object-oriented language due to its inclusion of primitive data types like int, float, char, boolean, double, etc. These primitives do not directly belong to corresponding object classes like Integer. So, we can conclude that Java is not entirely object-oriented due to its support for primitive data types alongside object-oriented features.
In Java, a constructor is a special block of code used to initialize an object. It has the same name as the class and is automatically called when an object is created. There are two types of constructors: Default Constructor: This constructor has no inputs and is automatically created if no other constructor is defined. Its purpose is to initialize instance variables with default values and is commonly used for object creation. Parameterized Constructor: This constructor accepts arguments and initializes instance variables with the provided values. It allows for customization during object creation by passing specific values to initialize the object’s properties.
In Java, the super keyword is used to refer to the immediate parent class of a subclass. It is primarily used to access and invoke members (methods, variables, and constructors) of the superclass from within the subclass. With super, it is possible to call the superclass’s constructor, access superclass methods or variables, and override methods while still retaining the superclass implementation. It is useful in scenarios where a subclass wants to extend or modify the behavior inherited from its superclass. By using the super keyword, developers can efficiently reuse and build upon existing code in an object-oriented hierarchy, promoting code reusability and maintaining the integrity of the inheritance chain.
The following are the features in Java: Object-Oriented Programming (OOP): Java is a fully object-oriented programming language, emphasizing the use of classes and objects to create modular and reusable code. Platform Independence: Java’s “write once, run anywhere” principle allows Java programs to be executed on any platform that has a Java Virtual Machine (JVM) installed, without the need for recompilation. Garbage Collection: Java’s automatic memory management system handles memory allocation and deallocation, relieving developers from manual memory management tasks and reducing the risk of memory leaks. Strong Standard Library: Java provides a comprehensive standard library, offering a wide range of pre-built classes and APIs for common tasks like I/O operations, networking, data structures, and more, which accelerates development and reduces code duplication. Multi-threading and Concurrency Support: Java has built-in features for concurrent programming, allowing developers to create multi-threaded applications that can execute multiple tasks simultaneously, improving performance and responsiveness. Exception Handling: Java’s exception handling mechanism enables developers to effectively handle and manage errors, ensuring program stability and resilience by gracefully dealing with unexpected situations. Security: Java incorporates built-in security features, including a robust security model, encryption libraries, and authentication mechanisms, making it a suitable choice for developing secure applications and protecting sensitive data. Large and Active Community: Java boasts a vast and active developer community, providing access to a wealth of resources, forums, libraries, frameworks, and tools. This active community ensures continuous support and innovation within the Java ecosystem.
  • Inner Class: An inner class is a class that is contained within another class, creating a nested structure within the outer class. By being nested within the outer class, an inner class grants access rights to the variables and methods defined in the outer class. This means that the inner class can utilize and manipulate all the members of the enclosing class.

  • Subclass: A subclass is a class that inherits from another class called the superclass. It provides access to all public and protected methods and fields of its superclass.

Most Popular Java Interview Questions And Answers

In Java interviews, candidates often encounter a wide range of questions that assess their knowledge, problem-solving skills, and understanding of Java concepts. Commonly asked questions include ones about Java fundamentals, OOP concepts, exception handling, multi-threading, collections, and design patterns. Candidates may be asked to explain concepts like inheritance, polymorphism, and encapsulation, or demonstrate their ability to handle real-world scenarios through coding exercises. Additionally, questions about Java frameworks like Spring and Hibernate, as well as questions related to web technologies and database integration, are also popular. Being well-prepared with comprehensive answers to these frequently asked questions significantly increases the chances of success in a Java interview.

  • JVM, short for Java Virtual Machine, is a virtual machine that serves as a runtime environment for executing code. It is an integral component of the JRE Java Runtime Environment and plays a crucial role in converting bytecode into machine-level instructions.Also, the JVM handles memory allocation tasks during program execution.

The Just-In-Time Compiler is a vital component of the JRE responsible for compiling bytecode into native machine code for specific methods. This compiled code is then directly executed by the JVM without the need for interpretation, resulting in improved performance during runtime.

Yes, we have the ability to execute code before the main method. This can be done by using a static block of code within the class. When the class is loaded, the statements within this static block are executed immediately, even before creating objects in the main method. This allows us to perform specific actions or initialization tasks when the class is loaded into memory.

Inheritance is a fundamental concept in object-oriented programming where one class inherits properties and behaviors from another class. It allows a new class, called a subclass or derived class, to inherit and reuse the attributes and methods of an existing class, called a superclass or base class. By inheriting from a superclass, the subclass gains access to its fields, methods, and other members, enabling code reuse and promoting a hierarchical relationship between classes

Data types in Java determine the values and sizes that can be stored in variables. There are two main categories of data type, they are:

  • Primitive Data Types
  • Non-primitive Data Types

JSON stands for “JavaScript Object Notation,” and it serves as a lightweight and readable alternative to XML. It is a language-independent format that can be easily parsed in various programming languages. JSON finds extensive usage in client-server and server-server communication, facilitating data exchange between different systems. Due to its simplicity and widespread support, it has become a popular choice for transmitting structured data.

Reflection in Java is a runtime feature that allows you to inspect and modify the behavior of classes, interfaces, fields, and methods. It provides the ability to analyze and interact with these elements during program execution, even without knowing their names at compile time. Reflection enables tasks like creating new objects, calling methods, and accessing field values. It is particularly useful for extensibility by allowing the use of external classes with their fully-qualified names.

Non-primitive data types in Java are types that are not built-in and are derived from classes or interfaces. They include reference types like String, arrays, classes, and interfaces.

The Unicode system is a universal character encoding standard that represents characters from various writing systems and languages. It assigns a unique numerical value (code point) to each character, enabling consistent representation and interchange of text across different platforms and programming languages.

Java has five significant types of memory allocations.

  • Stack Memory
  • Heap Memory
  • Class Memory
  • Native Method Stack Memory
  • Program Counter-Memory

Constructor chaining allows calling one constructor from another constructor. It can be achieved in two ways:

  • Within the same class: In this case, the this() keyword is used to invoke another constructor within the same class.
  •  From the base class: Here, the super() keyword is used to call the constructor of the base class from the derived class.

Constructor chaining is closely related to inheritance. When a subclass constructor is invoked, it first calls the constructor of its superclass. This process continues recursively, initializing the data members from the top of the inheritance chain to the derived class. Constructor chaining can be performed across multiple classes, ensuring proper initialization of each class in the hierarchy.

 Yes. It is possible that the ‘finally’ block will not be executed. The cases are-

  • Suppose we use System.exit() in the above statement.
  • If there are fatal errors like Stack overflow, Memory access error, etc

When a string is created using the new(), a new object is always created. However, when a string is created using the string literal syntax (e.g., “example”), it may refer to an existing object with the same value. This is because Java optimizes memory usage by maintaining a string pool, which allows multiple string literals with the same value to share the same object. This behavior improves performance and reduces memory consumption when working with frequently used string values.

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 The main method in Java is made static to allow direct invocation without the need for an object. This eliminates the extra memory allocation required for invoking a non-static main method, as static methods can be called without object instantiation.


In Java, local variables do not have default values and must be initialized before use. If a local variable is not explicitly assigned a value before it is accessed, a compilation error will occur.

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