What does this mean ?java.lang.NullPointerException

What does this mean ?
Uncaught exception in evaluation: java.lang.NullPointerException.

How would you fix this ?
pheatsAsked:
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CEHJCommented:
Tricky! You'll have to look through the 'evaluation' method for objects that should have been initialised but weren't
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TimYatesCommented:
what are you doing?
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twansCommented:
heyas.. not too sure if your new to java soooo..

could you post come code?

also, to expand on CEHJ's suggestion: "look through the 'evaluation' method for objects that should have been initialised but weren't "

in newbie's terms this means the following:

Object whatever; => Uninitialised object
Object whatever = new Object(); => the 'new' operator calls the constructor of that class and thus initialises the object..

Now in regard to your 'evaluation' method.. it may look like:

public void evaluation( object Object) {..} .. the parameter, Object, that you pass this method MUST be initilalised.. a common error is to have the following..

if {}
else {
evaluation(Object)..
}

but since you have not initialised the object initially, there is a CHANCE that you might pass an object assigned as null. A quick fix is possibly to modify your initial instance variable from:
Object object;
to Object object = new Object();

the success of this will be dependent on your code.

hope this helps somewhat :)

twans
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anthony_castillonCommented:
NullPointerException is thrown by the JVM when you are invoking a method on a null object.  Make sure your object references refer to actual objects before you invoke methods on them.
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CEHJCommented:
Please don't repeat comments that have already been made
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objectsCommented:
> Please don't repeat comments that have already been made

I was about to make the same comment as I didn't see where anyone had explained that.
There are many ways an NPE can occur than  just an unitialised var, and is not a clear explanation of an NPE.
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applekannaCommented:
>>Uncaught exception in evaluation

Does this not mean that no exception occured still but we need to use a try catch to catch the Exception ...
Correct me if I am wrong

eg for pheats
if you have

public int Test() throws NullPointerException
{
 if ( a< 5 )
     throw new NullPointerExeption
else
return a;
}

and we use

int b = Test()

this needs to be used as
try{
int b = Test()
}
catch (NullPointerException e)
{
 System.out.println(e);
}

Hope this helps.
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CEHJCommented:
No, NullPointerException does not have to be caught
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objectsCommented:
> Does this not mean that no exception occured still but we need to use a try catch to catch the Exception ...

No it means an exception was thrown but it wasn't caught.
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CEHJCommented:
One way of reproducing this would be to call evaluation here:

public static void evaluation() {
      try {
            x();
      }
      catch(Exception e) {
            System.err.println("Uncaught exception in evaluation: " + e.getClass().getName());
      }
}

public static void x() {
      String s = null;
      int i = s.length();
}




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FesterWimCommented:
Maybe the simplest example would be:

String testString = null;
testString.substring(2); // Will throw a NullPointerException

Since testString does not refer to any real object, a NullPointerException is thrown.
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CEHJCommented:
8-)
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pheatsAuthor Commented:
I tried this and it proved that what is was doing was wrong.

I was able to catch the evaluation error.
Cheers
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