接口(interface)与多态

1. 接口（interface）是抽象方法与常量值的集合；

2. 从本质上来说，接口是一种特殊的抽象类，这种抽象类中只包含常量与方法的定义，而没有变量和方法的实现；

3. 接口中声明的属性默认为：public static final，也只能是：public static final，（接口中成员变量的定义）；

4. 接口中只能定义抽象方法，并且这些方法默认为public，也只能是public的；

5. 接口能够继承其余的接口，并添加新的属性和抽象方法；

6. 多个无关的类能够实现同一个接口，一个类能够实现多个无关的接口；

7. 与继承关系相似，接口与实现类之间存在多态性：

    多态的三个条件：继承，重写，父类引用指向子类对象；

    因为Java中没有多继承，一个类只能有一个父类。一个父类能够有多个子类，而在子类里能够重写父类的方法；

    用接口能够实现多继承，一个类或是能够同时实现多个接口；

    通常而言，接口有利于代码的扩展，而继承则有利于代码功能的修改

8. Demo

Demo_1

public interface Runner{
	int id = 1; // 至关于:public static final int id = 1;
	public void start();
	public void run();
	public void stop();
}
// 正确

 Demo_2

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

class Sudents implements Singer{
	private String name;
	
	public Sudents(String name) {
		this.name = name;
	}
	public void study(){
		System.out.println("Studying");
	}
	public String getName(){
		return name;
	}
	public void sing() { // @Override
		System.out.println("Student is Singing");
	}
	public void sleep() { // @Override
		System.out.println("Student is sleping");
	}
}
// 正确

 Demo_3

interface Singer{
	public void sing();
	public void sleep();
}
interface Painter{
	public void paint();
	public void eat();
}
class Student implements Singer{
	private String name;
	public Student(String name) {
		this.name = name;
	}
	public void study(){
		System.out.println("studying");
	}
	public String getName(){
		return name;
	}
	public void sing() { // @Override
		System.out.println("student is singing");
	}
	public void sleep() { // @Override
		System.out.println("student is sleeping");	
	}
}
class Teacher implements Singer, Painter{
	private String name;
	public Teacher(String name) {
		this.name = name;
	}
	public String getName(){
		return name;
	}
	public void teach(){
		System.out.println("teaching");
	}
	public void paint() { // @Override
		System.out.println("teacher is painting");
	}
	public void eat() { // @Override
		System.out.println("teacher is eating");
	}
	public void sing() { // @Override		
		System.out.println("teacher is singing");
	}
	public void sleep() { // @Override
		System.out.println("teacher is sleeping");
	}
}
public class Test {
	public static void main(String[] args){
		Singer s1 = new Student("s1");
		s1.sing();
		s1.sleep();
		Singer t1 = new Teacher("t1"); //至关于继承中的父类引用指向子类对象
		t1.sing();
		t1.sleep();
		Painter p1 = (Painter) t1; //至关于继承中的父类引用指向子类对象
		p1.paint();
		p1.eat();
	}
}
// 正确

 Demo_3的运行结果：

     student is singing     student is sleeping     teacher is singing     teacher is sleeping     teacher is painting     teacher is eating