JAVA核心技术I---JAVA基础知识（多态）

一：类转型

类转型：子类可以转父类，父类不可以转子类(除非父类对象本身就是子类）

1.父类

public class Human {
    
    public void say() {
        System.out.println("Hello World");
    }
    
    public void eat() {
        System.out.println("eat food");
    }
}

2.子类

public class Men extends Human{
    public void plough() {
        System.out.println("Men is ploughing");
    }
    
    public void eat() {
        System.out.println("Men is eating");
    }
}

3.实现转型

public class HumanTest {
    public static void main(String args[]) {
        Men m=new Men();
        m.eat();
        //子类转父类
        Human hm=m;
        hm.eat();
        hm.say();
        //hm.plough();//The method plough() is undefined for the type Human
        //父类转子类（前提该父类是子类转过去的）
        Men m2=(Men)hm;
        m2.eat();
        m2.plough();
    }
}

Men is eating
Men is eating
Hello World
Men is eating
Men is ploughing

4.问题：编译时报错和运行时报错

class Pencil  {
    public void write (String content){
        System.out.println( "Write",+content);
    }
}

class RubberPencil extends Pencil{
    public void write (String content){
        System.out.println("Rubber Write"+content)；
    }

  public void erase (String content){
      System.out.println( "Erase "+content);
  }
}
执行下列代码的结果是哪项？
Pencil  p=new  Pencil();
(( RubberPencil) p).write("Hello");

C.运行时抛出异常

D.编译失败

强制转换是允许的，不过相当于会产生内存越界，导致运行时报错

二：多态

多态：子类转型为父类后，调用普通方法，依旧是子类的

1.父类

public class Human {
    
    public void say() {
        System.out.println("Hello World");
    }
    
    public void eat() {
        System.out.println("eat food");
    }
}

2.子类：对父类方法重写

public class Women extends Human{
    public void weave() {
        System.out.println("women is weaving");
    }
    
    public void say() {
        System.out.println("Hello World,I am a women");
    }
    
    public void eat() {
        System.out.println("Women eat food");
    }
}

public class Men extends Human{
    public void plough() {
        System.out.println("Men is ploughing");
    }
    
    public void eat() {
        System.out.println("Men is eating");
    }
    
    public void say() {
        System.out.println("Hello World，I am a man");
    }
}

public class Child extends Human{
    public void eat() {
        System.out.println("Child is eating");
    }
    
    public void say() {
        System.out.println("Hello World，I am a Child");
    }
}

3.多态实现

public class HumanTest {
    public static void main(String args[]) {
        Human[] HM= new Human[3];
        HM[0]=new Men();
        HM[1]=new Women();
        HM[2]=new Child();
        for(int i=0;i<3;i++) {
            HM[i].eat();
            HM[i].say();
        }
    }
}

Men is eating
Hello World，I am a man
Women eat food
Hello World,I am a women
Child is eating
Hello World，I am a Child

三：利用多态实现解耦

（一）父类实现解耦

public class HumanTest {
    public static void main(String args[]) {
        HumanSay(new Men());
        HumanSay(new Women());
        HumanSay(new Child());    
        HumanSay(new Child() {
            public void say() {
                System.out.println("Someone is eating");
            }
        });    
    }
    
    public static void HumanSay(Human a) {　　//方法不是依赖某个具体子类，可以使用父类，抽象类，甚至接口来实现解耦
        a.say();
    }
    
    public static void HumanEat(Human a) {
        a.eat();
    }
}

Hello World，I am a man
Hello World,I am a women
Hello World，I am a Child
Someone is eating

（二）接口实现：

1.接口

public interface Human {
    public void say();
    public void eat();
}

2.子类

public class Men implements Human{
    public void plough() {
        System.out.println("Men is ploughing");
    }
    
    public void eat() {
        System.out.println("Men is eating");
    }
    
    public void say() {
        System.out.println("Hello World，I am a man");
    }
}

public class Women implements Human{

public class Child implements Human{

3.其他一样，结果相同

（三）抽象类实现

1.抽象类

public abstract class Human {
    public abstract void say();
    public void eat() {
        System.out.println("Human is eating");
    }
}

2.子类

public class Men extends Human{
public class Women extends Human{
public class Child extends Human{

3.调用方式一样，结果一样