Java 练习(多态性练习一)

随机生成 Animal 执行 eat 方法

package com.klvchen.java5;

import java.util.Random;


class Animal{
	
	protected void eat() {
		System.out.println("animal eat food");
	}
}

class Cat extends Animal{
	
	protected void eat() {
		System.out.println("cat eat fish");
	}
}

class Dog extends Animal{
	
	public void eat() {
		System.out.println("dog eat bone");
	}
}

class Sheep extends Animal{
	
	public void eat() {
		System.out.println("sheep eat grass");
	}
}


public class InterviewTest {

	public static Animal getInstance(int key) {
		switch(key) {
		case 0:
			return new Cat();
		case 1:
			return new Dog();
		default:
			return new Sheep();
		}
	}
	
	public static void main(String[] args) {
		int key = new Random().nextInt(3);
		
		System.out.println(key);
		
		Animal animal = getInstance(key);
		
		animal.eat();
	}
}

运行结果：

举例一：

Person.java

package com.klvchen.java;

public class Person {
	String name;
	int age;
	
	int id = 1001;
	
	public void eat() {
		System.out.println("人，吃饭");
	}
	
	public void walk() {
		System.out.println("人，走路");
	}

}

Woman.java

package com.klvchen.java;

public class Woman extends Person {
	
	boolean isBeauty;
	
	public void goShopping() {
		System.out.println("女人喜欢购物");
	}

	public void eat() {
		System.out.println("女人少吃，为了减肥");
	}
	
	public void walk() {
		System.out.println("女人窈窕地走路");
	}
}

Man.java

package com.klvchen.java;

public class Man extends Person{
	
	boolean isSmoking;
	
	int id = 1002;
	
	public void earnMoney() {
		System.out.println("男人负责挣钱养家");
	}
	
	public void eat() {
		System.out.println("男人多吃肉，长肌肉");
	}
	
	public void walk() {
		System.out.println("男人霸气的走路");
	}
}

PersonTest.java

package com.klvchen.java;

import java.util.Date;

public class PersonTest {
	public static void main(String[] args) {
		
		Person p1 = new Man();
		p1.name = "Tom";
		
		//不能调用子类所特有的方法、属性:编译时，p1 是 Person类型。
//		p1.earnMoney();
//		p1.isSmoking = true;
		
		//有了对象的多态性以后，内存中实际上是加载了子类特有的属性和方法的，但是由于变量声明为父类类型，导致
		//编译时,只能调用父类中声明的属性和方法。子类特有的属性和方法不能调用。
		
		//如何才能调用子类持有的属性和方法？
		Man m1 = (Man)p1;
		m1.earnMoney();
		m1.isSmoking = true;
		
		//使用强转时，可能会出现 ClassCastException 的异常
//		Woman w1 = (Woman)p1;
//		w1.goShopping();
		
		/*
		 * instanceof 关键字的使用
		 * a instanceof A: 判断对象 a 是否是类A的实例。如果是，返回 true，返回不是， 返回 false。 
		 * 
		 * 使用情境:为了避免在向下转型时出现 ClassCastException 的异常，我们在向下转型之前，先
		 * 进行 instanceof 的判断，一旦返回true,就进行向下转型。如果返回false,不进行向下转型。
		 */
		
		if( p1 instanceof Woman) {
			Woman w1 = (Woman)p1;
			w1.goShopping();
			System.out.println("*********** Woman ***********");
		}
		
		if(p1 instanceof Man) {
			Man m2 = (Man)p1;
			m2.earnMoney();
			System.out.println("*********** Man ***********");
		}
		
		if(p1 instanceof Person) {
			System.out.println("*********** Person ***********");
		}
		
		if(p1 instanceof Object) {
			System.out.println("*********** Object ***********");
		}
		
		//问题一：编译通过，运行时不通过
//		Person p2 = new Woman();
//		Man m2 = (Man)p2;
		
//		Person p3 = new Person();
//		Man m3 = (Man)p3;
//		m3.earnMoney();
		
		//问题二: 编译通过，运行时也通过
		Object obj = new Woman();
		Person p = (Person)obj;
		
		//问题三: 编译不通过
//		Man m4 = new Woman();
		
//		Object o = new Date();
//		String str1 = (String)o;
	}

}

运行结果：

举例二：

AnimalTest.java

package com.klvchen.java;

public class AnimalTest {
        public static void main(String[] args) {
		
		AnimalTest test = new AnimalTest();
		test.func(new Dog());
		
		System.out.println("*************************************");
		
		test.func(new Cat());
	}
	
	public void func(Animal animal) {// Animal animal = new Dog();
		animal.eat();
		animal.shout();
		
		if(animal instanceof Dog) {
			Dog d = (Dog)animal;
			d.watchDoor();
		}
	}

}


class Animal{
	
	public void eat() {
		System.out.println("动物, 进食");
	}
	
	public void shout() {
		System.out.println("动物, 叫");
	}
}


class Dog extends Animal{
	public void eat() {
		System.out.println("狗吃骨头");
	}
	
	public void shout() {
		System.out.println("汪！汪！汪！");
	}
	
	public void watchDoor() {
		System.out.println("看门");
	}
}

class Cat extends Animal{
	public void eat() {
		System.out.println("猫吃鱼");
	}
	
	public void shout() {
		System.out.println("喵！喵！喵！");
	}

}

运行结果: