你好，標準答案是這樣的：
abstract class Animal {
public abstract void eat();
}
class Rabbit extends Animal {
public void eat() {
System.out.println("我吃草");
}
}
class Monkey extends Animal {
public void eat(){
System.out.println("我吃香蕉");
}
}
class Tiger extends Animal {
public void eat(){
System.out.println("我吃肉");
}
}

class Boy{
public void feed(Animal animal){
animal.eat() ;
}
}

public class TestAnimal {
public static void main(String[] args) {
Animal animal1 = new Rabbit() ;
Animal animal2 = new Monkey() ;
Animal animal3 = new Tiger() ;

Boy boy = new Boy() ;
boy.feed(animal1) ;
boy.feed(animal2) ;
boy.feed(animal3) ;
}
}

abstract class Animal
{ Animal(){
}
public void eat();
}
class monkey extends Animal
{ public void eat()
{System.out.println("我吃香蕉")；}
}
classTiger extends Animal
{ public void eat()
{System.out.println(”我吃猴子“);}
}
class boy
{ public static void main(String args[])
{
Aniaml animal1=new monkey();
Aniaml animal2=new Tiger();
animal1.eat();
animal2.eat();
}
}