class Fruit{
    void eat(){}
}
class Apple extends Fruit{
    void eat(){}
}
class Test{
    Fruit fruit = new Apple();  //------向上转型(父类引用指向子类对象)
}

class Fruit{
    void eat(){
        System.out.println("Fruit eat");
    }
}
class Apple extends Fruit{
    void eat(){
        System.out.println("Apple eat");
    }
}
public class TestClass{
    public static void main(String[] args){
        Fruit fruit = new Apple();
        fruit.eat();        //------动态绑定
    }
}output:
Apple eat

class Fruit{
    void eat(){
        System.out.println("Fruit eat");
    }
}
class Apple extends Fruit{
    /*
    重写父类方法，实现多态
     */
    void eat(){
        System.out.println("Apple eat");
    }
}

abstract class Car{
    abstract void drive();
}
class Volvo extends Car{
    /*
    实现父类(抽象类)方法，实现多态
     */
    void drive(){
        System.out.println("Volvo drive");
    }
}

public class TestClass{
    public static void main(String[] args){
        Fruit fruit = new Apple();
        fruit.eat();
        Car car = new Volvo();
        car.drive();
    }
}output:
Apple eat
Volvo drive

interface People{
    String sayName();
}
class Lilei implements People{
    /*
    实现接口方法，实现多态
     */
    public String sayName(){
        return "My name is Lilei";
    }
}
class CreatePeople{
    /*
    匿名内部类也是实现接口多态的一种
     */
    public People getPeople(){
        return new People() {
            @Override
            public String sayName() {
                return "My name is Wangfang";
            }
        };
    }
}
public class TestClass{
    public static void main(String[] args){
        People people1 = new Lilei();
        People people2 = new CreatePeople().getPeople();
        System.out.println(people1.sayName());
        System.out.println(people2.sayName());
    }
}output:
My name is Lilei
My name is Wangfang

public class A {
    public String show(D obj) {
        return ("A and D");
    }

    public String show(A obj) {
        return ("A and A");
    } 

}

public class B extends A{
    public String show(B obj){
        return ("B and B");
    }
    
    public String show(A obj){
        return ("B and A");
    } 
}

public class C extends B{

}

public class D extends B{

}

public class Test {
    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();
        
        System.out.println("1--" + a1.show(b));
        System.out.println("2--" + a1.show(c));
        System.out.println("3--" + a1.show(d));
        System.out.println("4--" + a2.show(b));
        System.out.println("5--" + a2.show(c));
        System.out.println("6--" + a2.show(d));
        System.out.println("7--" + b.show(b));
        System.out.println("8--" + b.show(c));
        System.out.println("9--" + b.show(d));      
    }
}output:
1--A and A
2--A and A
3--A and D
4--B and A
5--B and A
6--A and D
7--B and B
8--B and B
9--A and D