Java Exception handling for NullPointerException

Question

I am having the following exception:

Exception in thread "main" java.lang.NullPointerException
    at Studentlist.Studentlist(Studentlist.java:79)
    at Studentlist.main(Studentlist.java:38)

on line 38: I am passing to input to getstudentsprofile.

on line79: I am doing :

int z= array1.length;

How to debug this issue. I am reading lot of records and everything it is crashing at one place with this exception. How to debug this? how can I see when the exception happened what are the values?

Infact I have :

catch (IOException ee)
        {
            ee.printStackTrace();
        }

but no luck.

jmj · Accepted Answer · 2012-08-30T06:13:37.057

4

seems like array1 is null and NullPointerException is not a IOException so you need to catch

catch (NullPointerException npe) {
            npe.printStackTrace();
}

or even better don't give your code to raise this condition for that statement, check for null

int z = SOME_DEFAULT_VALUE;
if(arr!=null){
 z = arr.length;
}

edited Aug 30 '12 at 06:13

answered Aug 30 '12 at 06:00

jmj

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3

He shouldn't catch that exception. He should just fix it – Robin Aug 30 '12 at 06:08
Should not be catching a run-time exception. You end up masking the issue with your code. – Anupam Saini Aug 30 '12 at 06:11

score 4 · Answer 2 · edited May 23 '17 at 12:02

4

You should not be catching NullPointer exception as it's a RunTimeException and this occurs usually due to issue in your code that does not get caught at compile time. Please check these discussions for more details.

Is Catching a Null Pointer Exception a Code Smell?

If catching null pointer exception is not a good practice, is catching exception a good one?

edited May 23 '17 at 12:02

Community

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answered Aug 30 '12 at 06:06

Anupam Saini

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Nandkumar Tekale · Answer 3 · 2012-08-30T06:12:18.907

3

Check whether array1 is null

if(array1 ! = null) { // you should always avoid NPEs

  int z= array1.length;
}

Or

catch (IOException | NullPointerException e) // you can multi-catch exceptions, java7 new feature
   e.printStackTrace();
}

NullPointerException and IOException are different. Follow java tutorial for Exceptions .

edited Aug 30 '12 at 06:12

answered Aug 30 '12 at 06:00

Nandkumar Tekale

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score 2 · Answer 4 · edited May 07 '17 at 10:46

2

array1 is not initialised that's why you get the NullPointerException.

Obviously, an IOException is not a NullPointerException thats why it doesn't go in the catch block.

To debug a java program use an IDE that has built in debug functionality. You will find very easy to use Netbeans. Others like Eclipse and JDeveloper have nice features too.

edited May 07 '17 at 10:46

Rakibul Haq

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answered Aug 30 '12 at 06:05

MaVRoSCy

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dcernahoschi · Answer 5 · 2012-08-30T06:18:49.090

array1 at line 79 is null, but you should not catch NullPointerException as it most likely signals a bug in your program. Instead you should check if the reference is != null (if null is a valid value in your program) or, if not, signal the caller code that it provided an invalid value by, for example, throwing an InvalidArgumentException.

if(array1 ! = null) { //if array1 being null is a valid value in a program
    int z= array1.length;
}
//continue with your processing

or

if(array1 ! = null) { //if array1 being null is not a valid value in a program
    throw new IllegalArgumentException("You should pass a not null array for processing");
}

score 2 · Answer 6 · answered Aug 30 '12 at 06:11

2

NullPointerException is occurred because of you not initialize array1 that means the array1 is not point to any memory location.You have to initialize array1 like int[] arrray1 = new int[size];

answered Aug 30 '12 at 06:11

Rajshri

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score 2 · Answer 7 · answered Aug 30 '12 at 06:29

2

Just Handle it with try/catch.... No need to use if condition...

try{

     int z= array1.length;

 }catch(final IOException|NullPointerException ex){    

          // This Nested catch works with Java 7 and above

 }

answered Aug 30 '12 at 06:29

Kumar Vivek Mitra

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score 2 · Answer 8 · answered Aug 30 '12 at 06:31

Hey do not catch the NullPointerException and it is not a good practice. Rather , try to fix the problem. Runtime exceptions are given to fix the programming errors. In your case, array1 is NULL , because it is not been initialized. Do that and it should work.

Java Exception handling for NullPointerException

8 Answers8