java.lang.NullPointerException on ArrayList

Question

I have the following code:

for (int i=0; i<wallMaterialName.size(); i++){
if (Math.abs(calculatedWallUValue - alternativeWallUValue) > 0.01){
   if(wallMaterialName.get(i).charAt(0) == 'A'){                   //*****ERROR IN HERE*****
      PreparedStatement prepStateMat = con.prepareStatement("select * from concretestonefloor where value<? order by value desc limit 1");
      prepStateMat.setFloat(1, (Float)wallMaterialLambda.get(i));
      ResultSet rsMaterial = prepStateMat.executeQuery();
      //...
      }
  //...
  }
}

When I run the code, a "java.lang.NullPointerException" appear. The error comes from this code:

if(wallMaterialName.get(i).charAt(0) == 'A')

I don't know why the arraylist is empty since I have checked that it contains some data. Anybody knows how to solve this problem? Thanks.

The data inside the arraylist is null

Alex
– Alex

2012-10-29 12:29:50 +00:00
Commented Oct 29, 2012 at 12:29 — Alex
– Alex, Commented Oct 29, 2012 at 12:29
This means that wallMaterialName.get(i) is null.

Jesper
– Jesper

2012-10-29 12:29:54 +00:00
Commented Oct 29, 2012 at 12:29 — Jesper
– Jesper, Commented Oct 29, 2012 at 12:29
It obviously does not contain enough data...

Thilo
– Thilo

2012-10-29 12:29:55 +00:00
Commented Oct 29, 2012 at 12:29 — Thilo
– Thilo, Commented Oct 29, 2012 at 12:29
Could you add more code to analyze?

Jericho
– Jericho

2012-10-29 12:30:31 +00:00
Commented Oct 29, 2012 at 12:30 — Jericho
– Jericho, Commented Oct 29, 2012 at 12:30

Sergey Kalinichenko · Accepted Answer · 2012-10-29 12:30:19Z

6

The array list is not empty, otherwise you'd get an index out of bounds exception (which is impossible to produce in your code, because your loop checks i < wallMaterialName.size()). However, your array list contains a null object at index i.

Change your code as follows to fix the error:

String materialName = wallMaterialName.get(i);
if(materialName != null && materialName.charAt(0) == 'A') ...

answered Oct 29, 2012 at 12:30

Sergey Kalinichenko

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3 Comments

Sergey Kalinichenko Over a year ago

@MarkoTopolnik You're right, I mentioned it to illustrate the reason why OP's assessment that the list is probably "empty" is not correct.

Marko Topolnik Over a year ago

I understand... but it had the potential to confuse OP as well (make him think that an IOOBE would actually happen with his code and an empty list).

Sergey Kalinichenko Over a year ago

@user1221330 You are welcome! You may want to accept an answer that solves the problem by clicking the check mark outline next to it. This would indicate to others that you are not actively looking for an improved solution, and earn you a brand-new badge on Stack Overflow.

Marko Topolnik · Accepted Answer · 2012-10-29 12:39:57Z

2

If the list were empty, you wouldn't get the error. The arraylist in fact contains elements, but the elements are null and that's how you end up with your NullPointerException. If you demonstrate code that fills the list, I may be able to help you further.

On a separate note: don't iterate through the list by indexing: use the enhanced for syntax:

for (String element : wallMaterialName) ...

This will at least eliminate your worries about iterating over a non-existent element. The way you use the same index for two parallel lists shows that your design has potential to improve in that respect as well. You should have a single list of custom objects that contain both the strings from the first list and the floats from the second.

edited Oct 29, 2012 at 12:39

answered Oct 29, 2012 at 12:30

Marko Topolnik

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2 Comments

Thilo Over a year ago

In general, I agree with the foreach loop, but OP is simultaneously looping over two lists, so an index does come in handy.

Marko Topolnik Over a year ago

@Thilo True, but my advice for that would again be to redesign instead of modelling the domain with parallel lists.

PermGenError · Accepted Answer · 2012-10-29 12:32:50Z

0

your arraylist wallMaterialName has a null value at some index i. to know at what index you have null value do the below test.

for(int i=0; i<al.size(); i++){
            if(al.get(i)==null){
                System.out.println(i);
            }
        }

answered Oct 29, 2012 at 12:32

PermGenError

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