java中转型(upcasting)与动态绑定(dynamic binding)或者后期(late binding)细节

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