Null Pointer Exception (Looping through an array of objects)

Question

I haven't used Java in a while so I'm rusty. I have a method that returns an array of objects. In my main, I created a new object array and set that new array of objects to the return array of the method. Something like this:

Obj[] main_arr = new Obj[100];
main_arr = method(x);

for (int i = 0; i < main_arr.length; i ++){
  if(main_arr[i].number == 1) {  // the error seems to be here
      // do some stuff
  }
  else {
      // do some stuff
  }

}

I am trying to loop through main_arr and access the things in this object array, however I get a Null Pointer Exception. I've been in the debugger tool and main_arr seems to have the correct content. Any ideas why? Thank you in advance!

We need to see more code to be able to help. Please create a [mre] that demonstrates the problem. — John Kugelman, Apr 04 '20 at 00:30
By the way, it is wasteful to initialize `main_arr` to a new array if the next statement is just going to assign another value to the same variable. — cybersam, Apr 04 '20 at 00:43
Which statement is causing the NPE? In general, when you ask a question about an exception, you should show the stack trace. — cybersam, Apr 04 '20 at 00:43
If that line is throwing the NPE, then `main_arr[i]` is null. The problem is almost certainly in the method where the array is populated, rather than in the code presented here. — Ian McLaird, Apr 04 '20 at 00:48
I've been in the debugger tool, main_arr in my code isn't NULL, but correctly populated, hence why I was confused about the NPE — Hai Le, Apr 04 '20 at 00:51
It is not `main_arr` that is `null`. *Some elements* of `main_arr` are `null`. This could happen, for example, if `main_arr` has 100 elements but only the first 10 were actually assigned a value. — cybersam, Apr 04 '20 at 00:55

score 1 · Accepted Answer · answered Apr 04 '20 at 00:58

1

Some elements of main_arr are null.

This could happen, for example, if main_arr has 100 elements but only the first 10 were actually assigned a value.

answered Apr 04 '20 at 00:58

cybersam

63,203
6
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that was a quick fix, thank you! – Hai Le Apr 04 '20 at 01:01

score 0 · Answer 2 · answered Apr 04 '20 at 00:39

0

It's main_arr.length() not main_arr.length

answered Apr 04 '20 at 00:39

DebugDucker

43
6

1

I don't think that's the issue, plus length() is for strings i thought – Hai Le Apr 04 '20 at 00:41
1

just noticed that after the int i = 0 you put , instead of ; also I was wrong about the () – DebugDucker Apr 04 '20 at 00:44
that was a typo lmfao, sorry! – Hai Le Apr 04 '20 at 00:47

score 0 · Answer 3 · answered Apr 04 '20 at 00:50

Java Array Object does not have a method to get its length. Youneed to use the ‘array length attribute’, like this:

int arrayLength = myArray.length;
for (int i = 0; i <= arrayLength - 1; i++) {

   if (myArray[i] == whatever) {
      //do some job
   }
}

Credits: https://www.edureka.co/blog/array-length-in-java/

Null Pointer Exception (Looping through an array of objects)

3 Answers3