Solved

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

Posted on 2003-11-18
17
493 Views
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.
0
Comment
Question by:panpioneer
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 6
  • 4
  • 3
  • +1
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 ;
}

0
 
LVL 35

Expert Comment

by:TimYates
ID: 9771723
you can also do:

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

to see what it is...
0
 
LVL 35

Expert Comment

by:TimYates
ID: 9771725
Sorry...

System.out.println( o.getClass().getName() ) ;
0
Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 

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() ) ;       
0
 
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....
0
 
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.
0
 

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
}
0

Featured Post

Salesforce Has Never Been Easier

Improve and reinforce salesforce training & adoption using WalkMe's digital adoption platform. Start saving on costly employee training by creating fast intuitive Walk-Thrus for Salesforce. Claim your Free Account Now

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

After being asked a question last year, I went into one of my moods where I did some research and code just for the fun and learning of it all.  Subsequently, from this journey, I put together this article on "Range Searching Using Visual Basic.NET …
Basic understanding on "OO- Object Orientation" is needed for designing a logical solution to solve a problem. Basic OOAD is a prerequisite for a coder to ensure that they follow the basic design of OO. This would help developers to understand the b…
Viewers learn about the “for” loop and how it works in Java. By comparing it to the while loop learned before, viewers can make the transition easily. You will learn about the formatting of the for loop as we write a program that prints even numbers…
This tutorial covers a step-by-step guide to install VisualVM launcher in eclipse.
Suggested Courses

751 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question