Java Interface Methods

In Java, an interface is an abstract type used to specify the behaviour of a class. One of the most important rules when working with interfaces is understanding how methods are declared and implemented. In this article, we are going to learn the interface methods in detail.

Note: Interface methods are always public and abstract.

Example:

void methodOne();

public void methodOne();

abstract void methodOne();

public abstract void methodOne();

• public: To make this method available for every implementation class.
• abstract: The Implementation class is responsible for providing the implementation.

Why Every Method Declared in an Interface is Public?

When a class agrees to follow an interface, its method is available to any class that implements that interface, and that's why interface methods are public by default. This ensures that any class implementing the interface can access those methods.

Why Every Method Declared in an Interface is Abstract?

In Java, an interface only tells what a method should do, it does not know how to do it. The classes that implement the interface need to provide the implementation for that method, and that's why methods are abstract by default.

Note: We can not use the following modifiers with abstract methods in an interface. Using these will cause compile-time error.

• private
• protected
• final
• static
• synchronized
• native
• strictfp

When a class implements an interface, the method must be declared public in the class because we can not assign a weaker access modifier to a method that is already public in the interface.

Example:

// Demonstrating the working 
// of interface 
interface A {
    
    // implicitly public and abstract
    void Method1(); 
}

class B implements A {
    public void Method1() {
        System.out.println("GeeksforGeeks");
    }
}

class C {
    public static void main(String[] args) {
        B b = new B();
        b.Method1();
    }
}

GeeksforGeeks

If we define a weaker access modifier then we will get a compilation error and the below example demonstrates the same.

Example:

interface A {
    
     // implicitly public and abstract
    void Method1(); 
}

class B implements A {
    
    // Compilation error
    void Method1() {  
        System.out.println("GeeksforGeeks");
    }
}

class C {
    public static void main(String[] args) {
        B b = new B();
        b.Method1();
    }
}

Output:

Types of Methods in a Java Interface

1. Abstract Methods: An abstract method in Java is a method that has no body and the actual code is provided by the class that inherits it.

Example:

// Demonstrating abstract method
interface Animal {
    
    // abstract method
    void sound(); 
}

2. Default Methods (Java 8+): These methods have a body and provide default code inside an interface.

Example:

// Demonstrating Default Methods
interface Animal {
    default void eat() {
        System.out.println("This animal eats food.");
    }
}

3. Static Methods (Java 8+): These methods are written inside the interface and can be called directly using the interface name not with an object.

Example:

// Demonstrating static methods
interface Animal {
    static void info() {
        System.out.println("Animals are living beings.");
    }
}

// Call the method like this
Animal.info();

4. Private Methods (Java 9+): These methods reduce code duplication in static and default methods.

Example:

// Demonsntrating Private method
interface Animal {
    private void helper() {
        System.out.println("Helper method inside interface.");
    }
}

It can only be called from within default/static methods inside the interface.