Interface in Java

In Java, an interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. However, it cannot contain method implementations or instance variables. Interfaces are used to specify a set of methods that a class must implement. Think of an interface as a blueprint for classes to follow.

In the below PDF we discuss about Interface in Java  in detail in simple language, Hope this will help in better understanding.

Why Use Interfaces?

  • Abstraction: Interfaces allow developers to define a contract without specifying the implementation details. This promotes abstraction and helps in separating what a class does from how it does it.
  • Multiple Inheritance: Unlike classes, Java supports multiple inheritance of interfaces. A class can implement multiple interfaces, enabling it to inherit the functionality of all those interfaces.
  • Polymorphism: Interfaces enable polymorphic behavior, allowing different classes to be treated interchangeably based on their common interface.
  • Code Reusability: By defining common behavior in interfaces, you can reuse the same set of methods across multiple classes, promoting code reuse and reducing duplication.

Implementing Interface in Java:

Syntax of an Interface:
The syntax for declaring an interface in Java is straightforward:

public interface MyInterface {
// Method signatures
void method1();
int method2(String str);

In this example, MyInterface is declared using the interface keyword, and it contains two method signatures (method1 and method2). Any class that implements MyInterface must provide concrete implementations for these methods.

To implement an interface in Java, a class uses the implements keyword followed by the interface name. The class must then provide implementations for all the methods declared in the interface.

public class MyClass implements MyInterface {
public void method1() {
// Method implementation

public int method2(String str) {
// Method implementation
return str.length();

In this example, MyClass implements MyInterface and provides concrete implementations for method1 and method2.


In conclusion, interfaces are a fundamental concept in Java programming, providing a way to define contracts for classes to adhere to. Understanding interfaces and their usage is crucial for writing modular, maintainable, and flexible Java code. Whether you’re a beginner or an experienced Java developer, mastering interfaces is essential for building robust and scalable Java applications.

Must Read: OOPs Concepts In Java, Classes & Objects in Java and Abstraction in Java

Related Question

An interface in Java is a reference type, similar to a class, that can contain only constant declarations, method signatures, default methods, static methods, and nested types. It defines a contract for classes to implement, specifying a set of methods that must be implemented by any class that claims to conform to the interface.

Yes, interfaces can declare variables, but these variables are implicitly public, static, and final, essentially making them constants. They must be initialized at the time of declaration.

Interfaces in Java differ from classes in that they cannot be instantiated directly. They provide a blueprint for classes to implement, specifying what methods must be included without providing the implementation details. Classes can implement multiple interfaces but can only extend one class.

Yes, a class in Java can implement multiple interfaces. This allows for a class to inherit behavior from multiple sources, providing flexibility in designing software components.

Default methods in interfaces were introduced in Java 8 to provide a default implementation for methods in interfaces. This allows for the addition of new methods to interfaces without breaking existing code that implements those interfaces.


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