引用和對象實例的區別

父類：

package test;

public class FatherClass {

	protected int i=0;
}

　　子類：

package test;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Child extends FatherClass {

	public int i1=1;
	public void function(){
		System.out.println("call Child1 function");
	}
	public static void main(String []args) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException{
		//開始執行main
		System.out.println("the entrance.");
		System.out.println("----------------");
		//父類引用指向子類對象實例
		FatherClass f;
		f=new Child();		
		//f.function();//不符合語法規則，編譯不通過
		System.out.println(f);
		System.out.println(f.getClass());
		System.out.println("----------------");
		//子類引用指向子類對象實例
		Child c1;
		c1=(Child) f;
		
		c1.function();
		System.out.println(c1);
		System.out.println(c1.getClass());
		
		//運行時通過Method調用方法
		Method m=Child.class.getMethod("function");
		m.invoke(f);//輸出正常
		
		//子類引用指向了父類實例，父類中並沒有void function()
		//編譯正確但是運行錯誤
		Child c2=(Child) new FatherClass();
		m.invoke(c2);//Exception
		/**
		 * Exception in thread "main" java.lang.ClassCastException: 
		 * test.FatherClass cannot be cast to test.Child1
	at test.Child1.main(Child1.java:36)
		 */
	}
}

輸出：

the entrance.
----------------
test.Child@2a139a55
class test.Child
----------------
call Child1 function
test.Child@2a139a55
class test.Child
call Child1 function
Exception in thread "main" java.lang.ClassCastException: test.FatherClass cannot be cast to test.Child
at test.Child.main(Child.java:37)