from: http://blog.csdn.net/kaiming2008/archive/2007/04/27/1587843.aspx
在Java中,父类与子类之间的类型转换有两种,一种是子类向上转型父类(这永远是安全的),另一种是父类向下转型为子类(这得由相应的instanceof来判断数据类型,同时这也是一种不安全的转换)
Base class 与derived class 之间,为了达到从一个特定类型转为一个通用类型(有点多态的味道),可以借助向上]转型这一工具(即子类向上转为父类类型的),因为Java 里的对象引用都是dynamic binding或者lated bindin或者runing time binding,缺点就是在向上转型的过程中会丢失了属于子类专用的方法 ;向下转型中用得最多的就是interface的运用.
下面用一段代码讲示:
class Base { public void baseMethod() { System.out.println("into Base#baseMethod"); } public void fuc() { System.out.println("into Base#fuc"); } }
class Child extends Base { public void childMethod() { System.out.println("into Child#childMethod"); } public void fuc() { System.out.println("into Child#fuc"); } }
class CCChild extends Child { public void ccchildMethod() { System.out.println("into CCChild#ccchildMethod"); } public void fuc() { System.out.println("into CCChild#fuc"); } }
class DChild extends CCChild { public void dchildMethod() { System.out.println("into DChild#dchildMethod"); } public void fuc() { System.out.println("into DChild#fuc"); } }
public class DynamicBindingTest { public static void main(String[]args) { Base bs =new Child(); bs.fuc();//dynamic binding derived Child so call Child#fuc // bs.childMethod();//error:this method has been lost when upcasting to base class Base Base cbs =new CCChild(); cbs.fuc(); // bs.ccchildMethod();//error:this method has been lost when upcasting to base class Base System.out.println("================="); Child c=new Child(); CCChild ccc=new CCChild(); DChild dc=new DChild(); testCall(c); testCall(ccc); testCall(dc); } static void testCall(Base b) { b.fuc(); } } 控制台打印出来的结果
into Child#fuc into CCChild#fuc ================ into Child#fuc into CCChild#fuc into DChild#fuc
