Abstract class vs Interface: The Interface Strikes Back!

Previously, methods with implementations could only exist in abstract classes, but starting from Java 8, they also appeared in interfaces.

Let's take a look at the syntax for declaring a default method:

default dataType methodName(parameters) {
// implementation 
}

We need default methods for the same purposes as regular methods in Java. We use them when we need an implementation in the method and when we want to implement only the interface without additional classes. Let's consider a small example using default methods:

Main.java


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package com.example;

public class Main {
    public interface Greeting {
        default void sayHello() {
            System.out.println("Hello from the interface!");
        }

        void greet(String name);
    }

    public static class GreetingImpl implements Greeting {
        @Override
        public void greet(String name) {
            System.out.println("Hello, " + name + "!");
        }
    }

    public static void main(String[] args) {
        GreetingImpl greetingImpl = new GreetingImpl();
        greetingImpl.sayHello(); // Output: Hello from the interface!
        greetingImpl.greet("Alice"); // Output: Hello, Alice!
    }
}

In this example, the interface Greeting contains a default method sayHello() with a default implementation. The class GreetingImpl implements this interface and overrides the greet() method. In the main method, an instance of GreetingImpl is created and both methods are called.

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Abstract class vs Interface: The Interface Strikes Back!

Previously, methods with implementations could only exist in abstract classes, but starting from Java 8, they also appeared in interfaces.

Let's take a look at the syntax for declaring a default method:

default dataType methodName(parameters) {
// implementation 
}

Main.java


              12345678910111213141516171819202122232425
            
package com.example;

public class Main {
    public interface Greeting {
        default void sayHello() {
            System.out.println("Hello from the interface!");
        }

        void greet(String name);
    }

    public static class GreetingImpl implements Greeting {
        @Override
        public void greet(String name) {
            System.out.println("Hello, " + name + "!");
        }
    }

    public static void main(String[] args) {
        GreetingImpl greetingImpl = new GreetingImpl();
        greetingImpl.sayHello(); // Output: Hello from the interface!
        greetingImpl.greet("Alice"); // Output: Hello, Alice!
    }
}

Was alles duidelijk?

Bedankt voor je feedback!

Sectie 1. Hoofdstuk 27

Default Methods

Abstract class vs Interface: The Interface Strikes Back!

Default Methods

Abstract class vs Interface: The Interface Strikes Back!