第二十八讲：基础三里氏代换原则

JAVA中一个非常重要的原则:多态.

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//
public class MainClass {
    public static void main(String[] args) {
        Person person = new Person();
        display(person);
        
        Person man = new Man();
        display(man);
    }
    //public static void display(Person person){
    public static void display(Man man){
        //person.display();
        man.display();
    }
}

//JAVA中一个非常重要的原则:多态.
public class Man extends Person {
    public void display(){
        System.out.println("this is man");
    }
}

public  class Person{
    public  void display(){
        System.out.println("this is a person");
    }
}

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从智能工具上来讲企鹅当然是属于鸟类,从程序设计上来讲企鹅它到底是鸟类吗？

传父类的地方可以用子类替换.

企鹅是鸟类吗？同样是大话设计模式里面提到的知识点. 

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java里面的父类引用指向子类对象和里氏替换原则是极其类似的.父类引用指向子类对象同样也是多态的条件.多态的三个条件:第一个要有继承,第二个要有重写,第三个要有父类引用，父类引用指向子类对象.这就是多态.

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package com.ibeifeng.ex2;

public interface Bird {
    public void fly();//飞
    //public void bushi();//捕食
    
}

package com.ibeifeng.ex2;

public class Laoying implements Bird {

    @Override
    public void fly() {
        // TODO Auto-generated method stub
        System.out.println("老鹰在飞");
    }



}

package com.ibeifeng.ex2;

public class MainClass {
     public static void main(String[] args) {
        fly(new Laoying());
        fly(new Maque());
        //fly(new Qie());//不能把一个企鹅传进来,因为企鹅根本就不能飞.
        
    }
     
     public static void fly(Bird bird){
         bird.fly();
     }
    
}

package com.ibeifeng.ex2;

public class Maque implements Bird {

    @Override
    public void fly() {
        // TODO Auto-generated method stub
         System.out.println("麻雀在飞");
    }

}

package com.ibeifeng.ex2;

//public class Qie implements Bird {
    public class Qie {

    /*@Override
    public void fly() {
        // TODO Auto-generated method stub
         
    }*/

}

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《java与模式》里面的一个问题:

小学生会说:正方形是一种特殊的长方形.正方形的长和宽是相等的.

里氏替换原则是能够放父类的地方就可以放子类.里氏替换原则的一个重要的地方是使用父类的地方可以使用子类.

正方形和长方形是相互独立的两个类,没有继承关系那就不存在里氏替换关系.

Java的特性:父类引用可以指向子类对象.成功地骗过了编译器.但是它真的符合里氏替换原则吗？

所以正方形根本不是长方形的子类,只不过是我们把它强求在里面而已.所以说强扭的瓜不甜.我们强求地把ZhengFX extends  ChangFX

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里氏替换原则的重点是:如果使用父类的地方一定适用于子类.你可以欺骗java编译器.java编译器不能检查是否符合里氏替换原则,因为java编译器不能理解很多业务逻辑,它所能识别的是子类是不是继承父类以及实现父类的方法.

因为使用了里氏替换原则才使得前面的开发封闭原则具有扩展性.设计软件的时候会设计一个接口接受父类,而真正使用到的是它的子类.使用到父类的地方也可以使用子类保证了子类可用.

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package com.ibeifeng.ex3;

//public class ChangFX {
public class ChangFX implements Sibianxing{
     private long width;
     private long height;
    public long getWidth() {
        return width;
    }
    public void setWidth(long width) {
        this.width = width;
    }
    public long getHeight() {
        return height;
    }
    public void setHeight(long height) {
        this.height = height;
    }
     
}

package com.ibeifeng.ex3;

public class MainClass {
    public static void main(String[] args) {
        ChangFX changfx = new ChangFX();
        //Sibianxing changfx = new ChangFX();
        
        changfx.setHeight(10);
        changfx.setWidth(20);
        test(changfx);
        //resize(changfx);
        
        
        
        //ChangFX zhengfx = new ZhengFX();
         ZhengFX zhengfx = new ZhengFX();
        zhengfx.setHeight(10);
        test(zhengfx);
        //resize(zhengfx);
    }
/*    public static void test(ChangFX changFx){
        System.out.println(changFx.getHeight());
        System.out.println(changFx.getWidth());
    }*/
        public static void test(Sibianxing sibianxing){
        System.out.println(sibianxing.getHeight());
        System.out.println(sibianxing.getWidth());
    }
/*    public static void resize(ChangFX changfx){
        while(changfx.getHeight() <= changfx.getWidth()){
            changfx.setHeight(changfx.getHeight() + 1);
            test(changfx);
        }
    }*/
/*       public static void resize(Sibianxing sibianxing){
        while(sibianxing.getHeight() <= sibianxing.getWidth()){
            sibianxing.setHeight(sibianxing.getHeight() + 1);
            test(sibianxing);
        }
    }*/
}

package com.ibeifeng.ex3;

public interface Sibianxing {
/*   public void getWidth();
   public void getHeight();*/
   public long getWidth();
   public long getHeight();
}

package com.ibeifeng.ex3;

//public class ZhengFX {
//public class ZhengFX extends ChangFX{
public class ZhengFX implements Sibianxing{
    private long side;

    public long getSide() {
        return side;
    }

    public void setSide(long side) {
        this.side = side;
    }

    @Override
    public long getWidth() {
        // TODO Auto-generated method stub
        //return super.getWidth();
        return this.getSide();
    }

    //@Override
    public void setWidth(long width) {
        // TODO Auto-generated method stub
        //super.setWidth(width);
        this.setSide(width);
    }

    @Override
    public long getHeight() {
        // TODO Auto-generated method stub
        //return super.getHeight();
        return this.getSide();
    }

    //@Override
    public void setHeight(long height) {
        // TODO Auto-generated method stub
        //super.setHeight(height);
        this.setSide(height);
    }
    
}