Java-24，接口

• 多个无关的类可以实现同一个接口。
• 一个类可以实现多个无关的接口。
• 与继承关系类似，接口与实现类之间存在多态性。
• 定义Java类的语法格式：

<modifier> class <name> [extends <superclass>]
[implements <interface> [,<interface>]*]{
    <declarations>*
}

实现现实世界中多继承的现象，因为Java只能单继承。

• 接口（interface）是抽象方法和常量值的定义的集合。
• 从本质上讲，接口是一种特殊的抽象类，这种抽象类中只包含常量和方法的定义，而没有变量和方法的实现。
• 接口定义举例：

public interface Runner{
    public static final int id = 1;
    
    public void start();
    public void run();
    public void stop();
}

接口特性

• 接口可以多重实现；
• 接口中声明的属性默认为public static final的；也只能是public static final的；
• 接口中只能定义抽象方法，而且这些方法默认为public的，也只能是public的；
• 接口可以继承其它的接口，并添加新的属性和抽象方法。

示例1：

package com.nyist;

interface Singer{
    public void sing();
    public void sleep();
}

class Student implements Singer{
    private String name;
    Student(String name){
        this.name = name;
    }
    public void study() {
        System.out.println("studying");
    }
    public String getName() {
        return name;
    }
    public void sing() {
        System.out.println("student is singing");
    }
    public void sleep() {
        System.out.println("student is sleeping");
    }
}
public class TestInterface {
    public static void main(String[] args) {
        Student s1 = new Student("小明");
        System.out.println(s1.getName());
    }
}

示例2：

package com.nyist;

interface Singer{
    public void sing();
    public void sleep();
}

interface Painter{
    public void paint();
    public void eat();
}

class Student implements Singer{
    private String name;
    Student(String name){
        this.name = name;
    }
    public void study() {
        System.out.println("studying");
    }
    public String getName() {
        return name;
    }
    public void sing() {
        System.out.println("student is singing");
    }
    public void sleep() {
        System.out.println("student is sleeping");
    }
}

class Teacher implements Singer,Painter{
    private String name;
    public String getString() {
        return name;
    }
    Teacher(String name){
        this.name = name;
    }
    public void teach() {
        System.out.println("teaching");
    }
    public void sing() {
        System.out.println("singing");
    }
    public void sleep() {
        System.out.println("sleeping");
    }
    public void paint() {
        System.out.println("painting");
    }
    public void eat() {
        System.out.println("eating");
    }
}
public class TestInterface {
    public static void main(String[] args) {
        Student s1 = new Student("小明");
        System.out.println(s1.getName());
        Teacher t1 = new Teacher("王老师");
        System.out.println(t1.getString());
    }
}

标准示例：

package com.nyist;

interface Singer{
    public void sing();
    public void sleep();
}

interface Painter{
    public void paint();
    public void eat();
}

class Student implements Singer{
    private String name;
    Student(String name){
        this.name = name;
    }
    public void study() {
        System.out.println("studying");
    }
    public String getName() {
        return name;
    }
    public void sing() {
        System.out.println("student is singing");
    }
    public void sleep() {
        System.out.println("student is sleeping");
    }
}

class Teacher implements Singer,Painter{
    private String name;
    public String getString() {
        return name;
    }
    Teacher(String name){
        this.name = name;
    }
    public void teach() {
        System.out.println("teaching");
    }
    public void sing() {
        System.out.println("singing");
    }
    public void sleep() {
        System.out.println("sleeping");
    }
    public void paint() {
        System.out.println("painting");
    }
    public void eat() {
        System.out.println("eating");
    }
}
public class TestInterface {
    public static void main(String[] args) {
        Singer s1 = new Student("le");
        s1.sing();
        s1.sleep();
        Singer s2 = new Teacher("steven");
        s2.sing();
        s2.sleep();
        Painter p1 = (Painter)s2;
        p1.paint();
        p1.eat();
    }
}

结果：

student is singing
student is sleeping
singing
sleeping
painting
eating

面向对象总结：