How to define an interface in Java?

1 year ago
Emily Johnson
2 minutes

In Java, interfaces can be defined using the keyword ‘interface’. Interfaces can contain constants and abstract methods, but cannot contain instance variables or non-abstract methods.

Below is the syntax for defining an interface:

public interface 接口名 {
    // 常量声明
    // 抽象方法声明
}

In an interface, you can declare constants and abstract methods, but you cannot have a method body. Constants in an interface are by default public static final, while abstract methods are by default public abstract.

For example, define an interface named Animal that contains a constant and an abstract method:

public interface Animal {
    int LEGS = 4; // 常量声明

    void sound(); // 抽象方法声明
}

In addition, interfaces can also inherit other interfaces using the keyword “extends”, and can inherit multiple interfaces at the same time. The syntax is as follows:

public interface 子接口名 extends 父接口名1, 父接口名2, ... {
    // 常量声明
    // 抽象方法声明
}

For example, create an interface named Walkable that extends the Animal interface.

public interface Walkable extends Animal {
    void walk();
}

Classes can implement interfaces using the “implements” keyword. A class can implement multiple interfaces, but must implement all abstract methods in the interface. The syntax is as follows:

public class 类名 implements 接口名1, 接口名2, ... {
    // 类的实现
}

For example, define a class named Dog that implements the Animal interface and the Walkable interface.

public class Dog implements Animal, Walkable {
    public void sound() {
        System.out.println("汪汪");
    }

    public void walk() {
        System.out.println("四条腿走路");
    }
}

In this way, the Dog class has implemented all the abstract methods in the Animal interface and the Walkable interface.

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