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') ...