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package test;
class A {
public String f(A obj)
{
return("A");
}
}
class B extends A {
public String f(B obj)
{
return("C");
}
public String f(A obj)
{
return("D");
}
}
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B b = new B();
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A a;
a = b;
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public class TestObj
{
public static void main(String args[])
{ A a1 = new A();
A a2 = new B(); //×ÓÀà¶ÔÏóת»¯Îª¸¸À࣬³ÆΪÉÏתÐÍ£¬²»ÐèҪǿÖÆת»»¡£
B b = new B();
System.out.println(a2.f(b)); //“D”
System.out.println();
if (a2 instanceof B) {
B b1 = (B) a2; //¸¸Àà¶ÔÏóת»¯ÎªÏÂתÐÍ£¬´ËʱÐèҪʹÓÃÇ¿ÖÆת»»£¬´ËʱÐèÒªÏÈÅжÏҪת»»µÄÕâ¸ö¶ÔÏó£¨Ò²¾ÍÊÇa2Ö¸ÏòµÄÕâ¸ö¶ÔÏó£©ÊDz»ÊÇBÀàµÄʵÀý
System.out.println(b1.f(b));//"C"
 
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from: http://blog.csdn.net/cool_rain_man/archive/2009/02/16/3895210.aspx
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