Java Interfaces

Hello there, fellow coder! Today, we’re diving into the world of Java Interfaces. Ready to level up your Java skills? Let’s get started!


Java Interfaces are like a contract for your classes. They’re a powerful tool that can help you design robust, flexible code. But what exactly are they? And how do they work? Let’s find out!

Understanding Java Interface

What is a Java Interface?

An interface in Java is a blueprint of a class. It’s like an abstract class, with static constants and abstract methods. The interface plays a crucial role when it comes to the concept of inheritance.

Declaring an Interface

Declaring an interface is easy as pie. Here’s a simple example:

public interface MyInterface {
    // methods

Interface Methods and Variables

An interface can have methods and variables just like the class but the methods declared in the interface are by default abstract. Let’s see it in action:

public interface MyInterface {
    int myVariable = 10; // Variables
    void display(); // Method

Implementing Java Interface

How to Implement a Java Interface

When a class implements an interface, it inherits the abstract methods of the interface. Here’s how you can do it:

public class MyClass implements MyInterface {
    public void display() {
        System.out.println("Hello, World!");

Java Interface Implementation Example

Let’s see a complete example:

public interface Animal {
    void sound();

public class Dog implements Animal {
    public void sound() {
        System.out.println("Woof, woof!");

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.sound(); // Outputs "Woof, woof!"


In this example, we have an interface called Animal that declares a method sound(). The interface serves as a contract, stating that any class implementing the Animal interface must provide an implementation for the sound() method.

The Dog class implements the Animal interface. By implementing the interface, the Dog class is required to provide an implementation for the sound() method. In this case, the sound() method in the Dog class prints “Woof, woof!” to the console.

In the Main class, we create an instance of the Dog class called myDog. We can then call the sound() method on myDog, which will invoke the sound() method implemented in the Dog class. As a result, “Woof, woof!” will be printed to the console.

This example demonstrates how a class can implement an interface and provide its own implementation for the methods defined in the interface. By implementing an interface, a class can fulfill the contract specified by the interface and define its unique behavior.

Explanation using a diagram

The diagram illustrates the concept of Java Interface using the example from the article. It shows an interface Animal with a method sound(). The Dog class implements this interface, providing its own implementation for the sound() method. In the Main class, an instance of the Dog class is created and the sound() method is called.

the concept of Java Interface
Diagram: the concept of Java Interface with the example

Java Interface vs Abstract Class

The Difference

While both interfaces and abstract classes help us achieve abstraction, they each have their own rules. An abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods.

When to Use What

Use an interface when you want to provide a common protocol for classes that are not related, and an abstract class when you want to provide default behavior for subclasses.

Java Interface Methods

Default and Static Methods

Java 8 introduced default and static methods in interfaces. Default methods allow us to add new methods to interfaces without breaking the classes that implement them. Static methods, on the other hand, are utility methods that belong to the interface itself, not the implementing class.

public interface MyInterface {
    default void defaultMethod() {
        System.out.println("This is a default method");

    static void staticMethod() {
        System.out.println("This is a static method");

Explanation: In this example, we have an interface called MyInterface that contains two special types of methods: default methods and static methods.

The defaultMethod() is a default method introduced in Java 8. It provides a default implementation for the method. Any class that implements the MyInterface interface can choose to override this method or use the default implementation provided by the interface. In this case, the default implementation prints “This is a default method” to the console.

The staticMethod() is a static method defined within the interface itself. It belongs to the interface and can be called directly on the interface, without the need for an instance of a class. In this example, the static method prints “This is a static method” to the console.

Default methods and static methods in interfaces are powerful tools that allow us to add new functionality to interfaces without breaking the existing implementations. Default methods provide a default behavior that can be optionally overridden, while static methods provide utility methods that are associated with the interface itself.

By using default and static methods in interfaces, we can create more flexible and extensible code, enabling interface implementations to have additional behaviors and utility methods readily available.

Multiple Inheritance in Java

Java doesn’t support multiple inheritance with classes, but it does with interfaces! A class can implement multiple interfaces, which is a neat way to get around the multiple inheritance restriction.

public interface FirstInterface {
    void methodOne();

public interface SecondInterface {
    void methodTwo();

public class MyClass implements FirstInterface, SecondInterface {
    public void methodOne() {
        System.out.println("Method of FirstInterface");

    public void methodTwo() {
        System.out.println("Method of SecondInterface");

Explanation: In this example, we have two interfaces: FirstInterface and SecondInterface. Each interface declares a single method: methodOne() in FirstInterface and methodTwo() in SecondInterface.

The MyClass class implements both FirstInterface and SecondInterface. By implementing these interfaces, the class is required to provide implementations for all the methods declared in the interfaces.

In the MyClass implementation, we define the methodOne() and methodTwo() methods. The methodOne() implementation prints “Method of FirstInterface” to the console, while the methodTwo() implementation prints “Method of SecondInterface”.

By implementing multiple interfaces, the MyClass class inherits the methods from both interfaces. It effectively combines the behaviors specified by FirstInterface and SecondInterface. This allows the class to exhibit the functionality defined in both interfaces simultaneously.

The concept of multiple inheritance through interfaces in Java is powerful, as it enables classes to inherit and implement multiple behaviors from different sources. It provides flexibility and allows for the composition of multiple traits or capabilities into a single class.

In this example, MyClass benefits from the methods defined in both FirstInterface and SecondInterface, and it can be used interchangeably wherever either interface is expected.

Java Inheritance Examples

Implementing a Java Interface

public interface Flyable {
    void fly();

public class Bird implements Flyable {
    public void fly() {
        System.out.println("The bird is flying.");

public class Main {
    public static void main(String[] args) {
        Bird myBird = new Bird();; // Outputs "The bird is flying."

Explanation: In this example, we have an interface Flyable with a method fly(). The Bird class implements this interface, meaning it provides the implementation for the fly() method. In the main method, we create an instance of Bird and call the fly() method.

Using Multiple Inheritance with Java Interfaces

public interface Eater {
    void eat();

public interface Sleeper {
    void sleep();

public class Human implements Eater, Sleeper {
    public void eat() {
        System.out.println("The human is eating.");

    public void sleep() {
        System.out.println("The human is sleeping.");

public class Main {
    public static void main(String[] args) {
        Human myHuman = new Human();; // Outputs "The human is eating."
        myHuman.sleep(); // Outputs "The human is sleeping."

Explanation: Here, we have two interfaces Eater and Sleeper, each with a method. The Human class implements both interfaces, providing the implementation for both eat() and sleep() methods. In the main method, we create a Human instance and call both methods.

Wrapping Up

Java Interfaces are a powerful tool in a programmer’s toolkit. They help us write code that’s flexible, reusable, and easy to understand. Remember, practice makes perfect. So, keep coding and have fun!

Frequently Asked Questions (FAQ)

  • What is a Java Interface?

    A Java Interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It provides a way to achieve abstraction and multiple inheritance in Java.

  • How do you declare an Interface in Java?

    An interface is declared by using the interface keyword. It can include abstract methods and constants. For example:

public interface MyInterface {
    void myMethod();
  • What is the difference between an Interface and an Abstract Class in Java?

    The main difference is that an interface can only declare methods and constants; they cannot implement methods (except default and static methods introduced in Java 8), while an abstract class can have both abstract and non-abstract methods (methods with implementations).

  • How do you implement an Interface in Java?

    A class implements an interface by using the implements keyword followed by the interface name. The class must provide implementations for all the methods declared in the interface.

  • Can a class implement multiple Interfaces in Java?

    Yes, a class can implement multiple interfaces. They are separated by a comma. For example:

public class MyClass implements InterfaceOne, InterfaceTwo {
    // Class implementation here
  • What are default methods in a Java Interface?

    Default methods are methods in an interface that have a default implementation. They were introduced in Java 8 to allow developers to add new methods to an interface without breaking the classes that implement it.

  • What are static methods in a Java Interface?

    Static methods in an interface are similar to static methods in a class. They belong to the interface itself and not to the instances of the classes that implement the interface.

  • Can an Interface extend another Interface in Java?

    Yes, similar to classes, one interface can extend another interface using the extends keyword.

  • Can an Interface implement another Interface in Java?

    No, an interface cannot implement another interface. An interface can only extend another interface.

  • Can an Interface have a constructor in Java?

    No, interfaces cannot have constructors. Interfaces are not part of the object’s inheritance tree, and constructors are part of the object creation process, so it doesn’t make sense for interfaces to have constructors.

  • Understanding Abstract Classes in Java
  • Java Inheritance: A Complete Guide
  • Java Polymorphism Explained
  • Mastering Java: Classes and Objects
  • Java Encapsulation: A Practical Approach

That’s all, folks! Keep exploring, keep learning. Happy coding!

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