Liskov于1987年提出了一个关于继承的原则“Inheritance should ensure that any property proved about supertype objects also holds for subtype objects.”——“继承必须确保超类所拥有的性质在子类中仍然成立。”也就是说，当一个子类的实例应该能够替换任何其超类的实例时，它们之间才具有is-A关系。

public abstract class AbstractBird {
 
    protected String color;
    protected String name;
 
    public AbstractBird(String color,String name) {
        this.color = color;
        this.name = name;
    }
 
    public void show() {
        System.out.println("看那是" + this.name + ":颜色是" + this.color);
        drinking();
        goWalk();
        sleep();
    }
 
    public abstract void goWalk();
 
    public abstract void sleep();
 
    public abstract void drinking();
}
 
public class Zoo {
 
    private AbstractBird abstractBird;
 
    public Zoo(AbstractBird abstractBird) {
        this.abstractBird = abstractBird;
    }
 
    public void show() {
        this.abstractBird.drinking();
    }
}
    public static void main(String[] args) {
// Zoo zoo = new Zoo(new Canary("红色","金丝雀"));
        Zoo zoo = new Zoo(new Magpie("蓝色","喜鹊"));
// Zoo zoo = new Zoo(new Sparrow("黑色","麻雀"));
        zoo.show();

对象本身有一套对自身状态进行校验的检查条件，以确保该对象的本质不发生改变，这称之为不变式(Invariant)。

public class Stone {
 
    public Number getNumber() {
        return new Integer(99);
    }
}
 
public class Adamas extends Stone {
 
    @Override
    public Integer getNumber(){
        return new Integer(22);
    }
}
public class StoneClient {
 
    public static void main(String[] args) {
        Stone stone = new Adamas();
        System.out.println(stone.getNumber());
    }
}
答案
22
 
如果把父类的参数缩小，子类的参数扩大，就问题出问题
 
public class Stone {
 
    public Number getNumber(HashMap map) {
        return new Integer(99);
    }
}
public class Adamas extends Stone {
 
    public Number getNumber(Map map) {
        return new Integer(22);
    }
}
public class StoneClient {
 
    public static void main(String[] args) {
        //Stone stone = new Stone();
        Adamas stone = new Adamas();
        System.out.println(stone.getNumber(new HashMap()));
    }
}
答案 2个打印出都是99 ，我们想要的，99，22

总结就是子类的方法参数一定要小于等于父类的参数。