Java Interview Questions and Answers 2025

Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let application developers write once, run anywhere (WORA), meaning that code that runs on one platform does not need to be recompiled to run on another.

Some of the main features of Java include: <ul class='uk-list uk-list-disc'><li>Object-Oriented: Java is based on the object-oriented programming paradigm, which allows for code reusability and modularity.</li><li>Platform-Independent: Java code can run on any platform that has a Java Virtual Machine (JVM).</li><li>Strongly Typed: Java enforces strict type checking at compile time and runtime.</li><li>Automatic Memory Management: Java has a built-in garbage collector that automatically manages memory allocation and deallocation.</li></ul>

The Java Virtual Machine (JVM) is an abstract computing machine that enables a computer to run Java programs. It provides a runtime environment in which Java bytecode can be executed, allowing Java applications to be platform-independent.

The Java Development Kit (JDK) is a software development kit that provides the tools necessary to develop Java applications. It includes the Java compiler, Java Runtime Environment (JRE), and various development tools such as debuggers and profilers.

JDK (Java Development Kit) is a software development kit that includes tools for developing Java applications, including the JRE (Java Runtime Environment) and development tools like the Java compiler. JRE is the runtime environment that allows Java applications to run on a computer, while JVM is the part of the JRE that executes Java bytecode.

A Java package is a namespace that organizes a set of related classes and interfaces. Conceptually similar to directories on your computer, packages help avoid name conflicts and can control access with protected and default access levels.

A Java class is a blueprint for creating objects in Java. It defines the properties (attributes) and behaviors (methods) that the objects created from the class will have. A class can also include constructors, which are special methods used to initialize new objects.

An object in Java is an instance of a class. It represents a specific entity that has state (attributes) and behavior (methods). Objects are created from classes and can interact with each other through method calls.

Inheritance is a fundamental concept in object-oriented programming that allows a new class (subclass) to inherit properties and behaviors from an existing class (superclass). This promotes code reusability and establishes a hierarchical relationship between classes.

Polymorphism is the ability of a single interface to represent different underlying forms (data types). In Java, polymorphism allows methods to do different things based on the object that it is acting upon, enabling method overloading and method overriding.

Encapsulation is the bundling of data (attributes) and methods (behaviors) that operate on the data into a single unit, typically a class. It restricts direct access to some of an object's components and can prevent the accidental modification of data. Encapsulation is achieved using access modifiers like private, protected, and public.

Abstraction is the concept of hiding the complex implementation details and showing only the essential features of the object. In Java, abstraction can be achieved using abstract classes and interfaces, allowing developers to define methods without providing a complete implementation.

An interface in Java is a reference type that is similar to a class but can only contain method signatures and final variables. Interfaces cannot contain instance fields or constructors. They are used to achieve abstraction and multiple inheritance in Java, allowing classes to implement multiple interfaces.

An abstract class in Java is a class that cannot be instantiated on its own and can contain abstract methods (methods without a body) as well as concrete methods (methods with a body). Abstract classes are used to provide a base for subclasses to extend and implement the abstract methods.

Abstract classes can have both abstract and concrete methods, while interfaces can only have abstract methods (prior to Java 8). Abstract classes can have instance variables, constructors, and access modifiers, whereas interfaces cannot (except for static and default methods introduced in Java 8).

Method overloading is a feature in Java that allows a class to have multiple methods with the same name but different parameter lists. It is a form of compile-time polymorphism and helps improve code readability and reusability.

Method overriding is a feature in Java that allows a subclass to provide a specific implementation for a method that is already defined in its superclass. The method in the subclass must have the same name, return type, and parameters as the method in the superclass.

Access modifiers in Java are keywords that determine the visibility and accessibility of classes, methods, and variables. The four access modifiers are: <ul><li>public: Accessible from anywhere.</li><li>protected: Accessible within the same package and by subclasses.</li><li>default (no modifier): Accessible within the same package.</li><li>private: Accessible only within the same class.</li></ul>

The 'final' keyword in Java is used to declare constants, prevent method overriding, and prevent inheritance. A final variable cannot be reassigned, a final method cannot be overridden, and a final class cannot be extended.

Exception handling in Java is a mechanism to handle runtime errors, ensuring the normal flow of the application. It is achieved using try, catch, finally, throw, and throws keywords. Exceptions can be checked, unchecked, or errors.

'throw' is used to explicitly throw an exception in a method or block of code, while 'throws' is used in a method declaration to specify the exceptions that the method can throw.

A static method in Java belongs to the class rather than any specific instance of the class. It can be called without creating an object of the class and can only access static variables and methods directly.

A static block in Java is a block of code that is executed when the class is loaded into memory. It is used for static initialization of variables and can be used to perform any setup required for the class.

A constructor in Java is a special method that is called when an object is instantiated. It has the same name as the class and does not have a return type. Constructors can be overloaded to provide different ways of initializing an object.

A constructor is used to initialize an object when it is created, while a method is used to define the behavior of an object. Constructors do not have a return type, while methods do. Constructors are called automatically when an object is created, while methods must be called explicitly.

The 'this' keyword in Java is a reference variable that refers to the current object. It is used to differentiate between instance variables and parameters with the same name, and it can also be used to call other constructors in the same class.

The 'super' keyword in Java is a reference variable that refers to the superclass of the current object. It is used to access superclass methods and constructors, and to differentiate between superclass and subclass members when they have the same name.

The Java Collections Framework is a unified architecture that provides data structures and algorithms for storing and manipulating groups of objects. It includes interfaces like List, Set, Map, and classes like ArrayList, HashSet, and HashMap.

Multithreading in Java is a feature that allows concurrent execution of two or more threads for maximum utilization of CPU. It is used to perform multiple tasks simultaneously within a program.

A thread is a lightweight sub-process that runs within a process, sharing the same memory space, while a process is a heavyweight, independent execution unit with its own memory space.

The synchronized keyword in Java is used to control access to a block of code or method by multiple threads, ensuring that only one thread can execute the synchronized block at a time.

A deadlock in Java occurs when two or more threads are waiting for each other to release resources, causing them to be stuck indefinitely. Proper synchronization and resource allocation can help avoid deadlocks.

The Java Memory Model (JMM) defines how threads interact through memory and what behaviors are allowed in concurrent execution. It ensures visibility, ordering, and atomicity of shared variables.

Design patterns in Java are reusable solutions to common software design problems. Examples include Singleton, Factory, Observer, and Strategy patterns, which help improve code structure and maintainability.

Checked exceptions are exceptions that must be declared in the method signature or handled using a try-catch block, while unchecked exceptions are runtime exceptions that do not require explicit handling.

Garbage collection in Java is an automatic process that reclaims memory occupied by objects that are no longer reachable or in use, helping to prevent memory leaks.

The Java Stream API is a feature introduced in Java 8 that provides a functional programming approach to process collections of data. It supports operations like filtering, mapping, and reducing.

A lambda expression in Java is a concise way to represent an anonymous function. It provides a clear and concise syntax for implementing functional interfaces, introduced in Java 8.

HashMap is not synchronized and allows one null key and multiple null values, while Hashtable is synchronized and does not allow any null keys or values.

Comparable is used to define the natural ordering of objects and is implemented by the class itself, while Comparator is used to define custom ordering and is implemented as a separate class.

The Fork/Join framework in Java is a framework introduced in Java 7 that is used to take advantage of multiple processors by breaking tasks into smaller subtasks and combining their results.

Callable is an interface that can return a result and throw a checked exception, while Runnable does not return a result and cannot throw checked exceptions.

String is immutable, StringBuilder is mutable and not synchronized, while StringBuffer is mutable and synchronized, making it thread-safe.

The Java Reflection API allows runtime inspection and modification of classes, methods, and fields, enabling dynamic behavior in Java applications.

A shallow copy copies only the references to objects, while a deep copy creates a new instance of the objects being copied, duplicating the entire structure.

The volatile keyword in Java ensures that changes to a variable are immediately visible to all threads, preventing caching of the variable in thread-local memory.

wait() causes the current thread to wait until another thread invokes notify() or notifyAll() on the same object, while notify() wakes up a single waiting thread and notifyAll() wakes up all waiting threads.

Soft references are cleared only when memory is low, weak references are cleared as soon as they are no longer reachable, and phantom references are used to perform cleanup actions before an object is removed from memory.

The transient keyword in Java is used to indicate that a field should not be serialized when an object is converted to a byte stream.

A bounded wildcard restricts the types that can be used as arguments (e.g., <? extends Number> or <? super Integer>), while an unbounded wildcard (<?>) allows any type.