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how to fix "The exception was: java.lang.ClassCastException" error?

Posted on 2003-11-18
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Last Modified: 2012-05-04
Hi experts,

I have some codes work on one latest verions of jdk and broken on older version.

The error is "The exception was: java.lang.ClassCastException", which I just can't see why it won't work. Is there a way to find out exactly what class the object is? So, I can compare with what it is cast to? thanks.
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Question by:panpioneer
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17 Comments
 
LVL 35

Accepted Solution

by:
TimYates earned 25 total points
ID: 9771718
> Is there a way to find out exactly what class the object is?

if( o instanceof String )
{
    String s = (String)o ;
}

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LVL 35

Expert Comment

by:TimYates
ID: 9771723
you can also do:

System.out.println( o.getClass.getName() ) ;

to see what it is...
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LVL 35

Expert Comment

by:TimYates
ID: 9771725
Sorry...

System.out.println( o.getClass().getName() ) ;
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Author Comment

by:panpioneer
ID: 9771962

THanks. It is very strange that both object are of the same type after the testing, however, when I assign one object to another, like the following, then I got the "java.lang.ClassCastException" error. why?

*****************************************
Research ac = new Research();
ac = (Research)request.getAttribute("ab");      

System.out.println((request.getAttribute("ab")).getClass().getName() ) ;
System.out.println(ac.getClass().getName() ) ;       
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LVL 86

Expert Comment

by:CEHJ
ID: 9772152
I'm not sure why if this:

>>System.out.println((request.getAttribute("ab")).getClass().getName() ) ;

is printing Research (in whatever package)

But there is a redundancy in your code:

>>Research ac = new Research();

creates a new instance. You should be doing:

Research ac = null;
try {
      Object o = request.getAttribute("ab");
      ac = (Research)o;
}
catch(ClassCastException e) {
      System.err.println("Object was of type " + o.getClass().getName());
}

0
 

Author Comment

by:panpioneer
ID: 9772303
thanks. But, if I copy your code, I will get another error message: o is not defined....
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LVL 86

Expert Comment

by:CEHJ
ID: 9772315
Sorry!

Research ac = null;
Object o = null;
try {
     o = request.getAttribute("ab");
     ac = (Research)o;
}
catch(ClassCastException e) {
     System.err.println("Object was of type " + o.getClass().getName());
}
0
 

Author Comment

by:panpioneer
ID: 9772321
CHEJ,

one more thing, if i move

Object o = request.getAttribute("ab"); to out side of try block, I am getting java.lang.null.exception.
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Author Comment

by:panpioneer
ID: 9772324

Oop! it is java.lang.NullPointerException
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 9772361
Always test for null:

if (ac != null) {
   // you got it
}
0
 

Author Comment

by:panpioneer
ID: 9772505
after the test, I still get the same object type of "Research", but, it still gives me the same ClassCastException...
0
 

Author Comment

by:panpioneer
ID: 9772616

I know this sound unbelievable. But, I re-start the Tomcat server and clean up some of the files on our C drive to give C more space and it works fine now.
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 9772724
Well Tom does cache files and classes...
0
 
LVL 92

Expert Comment

by:objects
ID: 9774578
> ac = (Research)request.getAttribute("ab");    

Probably better to test that the stored attribute is actually a Research instance (as mentioned by tim earlier), rather than handle the exception when it occurs.

Object o = request.getAttribute("ab")
if (o instanceof Research)
{
   Research ac = (Research) o;
   // do what you need with Research instance
}
else
{
    // Not a Research instance
}
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