java中向上转型(upcast)和向下转型(downcast)

class Useful { int value = 20; public void f() {System.out.println("f() in the Useful class");} public void g() {System.out.println("g() in the Useful class");} } class MoreUseful extends Useful { int value = 21; public void f() {System.out.println("f() in the MoreUseful class");} public void g() {System.out.println("g() in the MoreUseful class");} public void u() {System.out.println("u() in the MoreUseful class");} public void v() {} public void w() {} } class MoreUseful2 extends Useful { int value = 21; public void f() {System.out.println("f() in the MoreUseful2 class");} public void g() {System.out.println("g() in the MoreUseful2 class");} public void u() {System.out.println("u() in the MoreUseful2 class");} public void v() {} public void w() {} } class ExtendsMoreUseful2 extends MoreUseful2 { int value = 22; public void f() {System.out.println("f() in the ExtendsMoreUseful2 class");} public void g() {System.out.println("g() in the ExtendsMoreUseful2 class");} public void u() {System.out.println("u() in the ExtendsMoreUseful2 class");} public void v() {} public void w() {} } public class RTTI { public static void main(String[] args) { //Useful useful = new MoreUseful(); //useful.u(); //这是错误的，u()方法没有在useful类型中调用。 Useful[] x = { new Useful(), new MoreUseful(), new MoreUseful2(), new ExtendsMoreUseful2() }; //声明一个变量数组，保存每个Useful对象的引用， //那么每一个Useful对象就会自动向上转型为Useful。 for(int i=0;i<x.length if instanceof moreuseful2 moreuseful2.u x g in the useful class moreuseful u extendsmoreuseful2 for i="0;i&lt;x.length;i++)" abstract classabstract1 classderived1 extends public void play1 system.out.println is classabstract2 play2 classderived2 e14_uncast static playdemo1 objectparameter playdemo2 objectparameter.play2 args main todo auto-generated method stub classabstract="new" string stringbuffer c ip: a int count="1;" hi hello a1 bonjour whatjune03 a.hi a.hello a1.hello a1.bounjour><div class="share_buttons" id="sharePanel"></div> <!--192.168.1.237 2012-7-24 13:05:06--> <div class="article_next_prev"> <li class="prev_article"> <span>上一篇：</span><a href="/qiuxiaolong007/article/details/6532422">android 学习之---Android之GoogleMap(转)</a> </li> <li class="next_article"> <span>下一篇：</span><a href="/qiuxiaolong007/article/details/6581483">drawable—hdpi、drawable—mdpi、drawable-ldpi区别《转》</a> </li> </div> </x.length>