初学Java-15-多态

package com.dyz.test; interface HelloWorld{ 	void sayWhat(); }

package com.dyz.test; interface HiWorld{ 	void sayHi(); }

package com.dyz.test; public abstract class AbstractSuper{ 	protected abstract void sayGood(); }

package com.dyz.test; public class Child extends AbstractSuper implements HelloWorld,HiWorld{ 	 	public void sayWhat(){ 		System.out.println("good good study,day day up"); 	}  	public void sayHi(){ 	System.out.println("hi"); 	}  	public void sayGood(){ 		System.out.println("good"); 	} }

package com.dyz.test;  public class ChildTest{ 	public static void main(String[] args){ 			HelloWorld hello = new Child(); 				hello.sayWhat(); 			HiWorld hi = new Child(); 				hi.sayHi();	 			AbstractSuper as = new Child(); 				as.sayGood();		   		} 	 }

package com.dyz.test;  public class ChildTest{ 	public static void main(String[] args){ 			HelloWorld hello = new Child(); 				hello.sayWhat(); 			HiWorld hi = new Child(); 				hi.sayHi();	 			AbstractSuper as = new Child(); 				as.sayGood();		  			((Child) hello).sayHi();  		} 	 }

package com.dyz.test;  public class ChildTest{ 	public static void main(String[] args){ 			HelloWorld hello = new Child(); 				hello.sayWhat(); 			HiWorld hi = new Child(); 				hi.sayHi();	 			AbstractSuper as = new Child(); 				as.sayGood();		  			if(hello instanceof Child){ 				((Child) hello).sayHi();  				((Child) hello).sayGood(); 			}	 		} 	 }

为什么Java会具有多态这个特性？答案已经呼之欲出了，继承、重载和重写、以及我们允许父类（接口类）的引用指向子类（实现类）对象。

package com.dyz.test; public class SuperClass{ 	public void print(){ 		System.out.println("父类的print方法"); 	} }

package com.dyz.test; public class SonClass extends SuperClass{ 	public void print(){ 		System.out.println("子类的print方法"); 	} 	 	public static void main(String[] args){ 		SuperClass sc = new SonClass(); 		sc.print(); 	} }

请注意，我们在做强制类型转换时，一定要使用instanceof来判断这个转换是不是能成功，能成功才能做强制类型转换，否则程序运行时会抛出异常！

package com.dyz.test; public class Animal{ 	public void eat(){ 		System.out.println("进食"); 	} }

package com.dyz.test; public class Cat extends Animal{ 	public void eat(){ 		System.out.println("我吃鱼"); 	} }

package com.dyz.test; public class Cow extends Animal{ 	public void eat(){ 		System.out.println("我吃草"); 	} }

package com.dyz.test; public class AnimalUse{ 	public void eat(Animal x){ 		x.eat(); 	} 	public static void main(String[] args){ 		Animal a = new Cat(); 		Animal b = new Cow(); 		Animal c = new Animal(); 		a.eat(); 		b.eat(); 		c.eat(); 		 		AnimalUse user = new AnimalUse(); 		user.eat(a); 		user.eat(b); 		user.eat(c); 	} }

package com.dyz.test; public interface Animal{ 	public void eat(); }

package com.dyz.test; public class Cat implements Animal{ 	public void eat(){ 		System.out.println("我吃鱼"); 	} }

package com.dyz.test; public class Cow implements Animal{ 	public void eat(){ 		System.out.println("我吃草"); 	} }

package com.dyz.test; public class AnimalUse{ 	public void eat(Animal x){ 		x.eat(); 	} 	public static void main(String[] args){ 		Animal a = new Cat(); 		Animal b = new Cow(); 		a.eat(); 		b.eat(); 		AnimalUse user = new AnimalUse(); 		user.eat(a); 		user.eat(b); 	} }

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

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

class C extends B{ }

class D extends B{ }

public class E{ 	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)); 	} 	 }

封装：将对象的信息隐藏起来，只向外部提供必要的接口来提供服务。它让我们可以在类的内部修改代码，而不必改外部调用方的代码。

文章来源: 初学Java-15-多态