Java开发网 - 一道迷惑的cast题

Topic: 一道迷惑的cast题

1.一道迷惑的cast题

Posted by: zming
Posted on: 2004-04-14 12:06

Consider the class hierarchy shown below:

  Animal
|
Mammal
/ / \ \
/ / \ \
/ / \ \
/ / \ \
/ / \ \
/ / \ \
/ / \ \
/ / \ \
Dog Cat Raccoon SwampThing
(implements) (implements)
( Washer ) ( Washer )

Consider the following code:
1.  Cat sunflower;
2.  Washer wawa;
3.  SwampThing pogo;
4.
5.  sunflower = new Cat();
6.  wawa = sunflower;
7.  pogo = (SwampThing)wawa;

Which of the statements below is true?

第7行为什么是运行时错误，而不是编译错误。
pogo = (SwampThing)wawa;
其中wawa是Washer类型和Animal~ SwampThing的层次一点关系也没有，居然不是编译错误。

2.Re:一道迷惑的cast题 [Re: zming]

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Posted by: zming
Posted on: 2004-04-14 12:12

Consider the class hierarchy shown below:

  
  Animal
     |
  Mammal
  / / \  \
              /  /   \    \
            /   /     \     \
          /    /       \       \
        /     /         \        \
      /      /           \         \
    /       /             \           \
  /        /               \             \
Dog      Cat      Raccoon      SwampThing
     (implements) (implements) 
     ( Washer   )   ( Washer   ) 

Consider the following code:
1.  Cat sunflower;
2.  Washer wawa;
3.  SwampThing pogo;
4.  
5.  sunflower = new Cat();
6.  wawa = sunflower;
7.  pogo = (SwampThing)wawa;

Which of the statements below is true?

第7行为什么是运行时错误，而不是编译错误。
pogo = (SwampThing)wawa;
其中wawa是Washer类型和Animal~ SwampThing的层次一点关系也没有，居然不是编译错误。

3.Re:一道迷惑的cast题 [Re: zming]

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Posted by: zming
Posted on: 2004-04-15 12:28

图示好像不是很清楚，再贴一次。

Consider the class hierarchy shown below:
    
    Animal
      |
    Mammal
    / / \ \
        /  /   \  \
      /   /     \   \
    /    /       \    \
  /     /         \     \
      /      /           \      \
    /       /             \       \
  /        /               \        \
Dog      Cat      Raccoon    Swamp
     (implements)     (implements) Thing
     ( Washer   )     ( Washer   ) 

Consider the following code:

1.  Cat sunflower;
2.  Washer wawa;
3.  SwampThing pogo;
4.  
5.  sunflower = new Cat();
6.  wawa = sunflower;
7.  pogo = (SwampThing)wawa;

Which of the statements below is true?

有谁知道啊？

4.Re:一道迷惑的cast题 [Re: zming]	Copy to clipboard
Posted by: zyin006 Posted on: 2004-04-15 19:07 cus wawa's type is interface,pogo's type is class,so no compile error. but wawa point to Cat object,pogo point to SwampThing object. so runtime error

5.Re:一道迷惑的cast题 [Re: zming]

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Posted by: zming
Posted on: 2004-04-19 12:50

i get the total specification in JLS.

JLS Quote:
"If S is a class type: ... If T is an interface type: ... If S is not a final class (§8.1.1), then the cast is always correct at compile time (because even if S does not implement T, a subclass of S might).

If Swampthing were to be a final class it would resu,t in a compile time error.

thanks,zyin006