Solved

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

Posted on 2003-11-18
17
489 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
  • 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
Efficient way to get backups off site to Azure

This user guide provides instructions on how to deploy and configure both a StoneFly Scale Out NAS Enterprise Cloud Drive virtual machine and Veeam Cloud Connect in the Microsoft Azure Cloud.

 

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

ScreenConnect 6.0 Free Trial

Want empowering updates? You're in the right place! Discover new features in ScreenConnect 6.0, based on partner feedback, to keep you business operating smoothly and optimally (the way it should be). Explore all of the extras and enhancements for yourself!

Question has a verified solution.

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

Suggested Solutions

INTRODUCTION Working with files is a moderately common task in Java.  For most projects hard coding the file names, using parameters in configuration files, or using command-line arguments is sufficient.   However, when your application has vi…
Introduction This article is the first of three articles that explain why and how the Experts Exchange QA Team does test automation for our web site. This article explains our test automation goals. Then rationale is given for the tools we use to a…
The viewer will learn how to implement Singleton Design Pattern in Java.
This video teaches viewers about errors in exception handling.

776 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