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.

Answer 1

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') ...

Answer 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.

Answer 3

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);
            }
        }