小编use*_*330的帖子

我有以下代码:

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();
      //...
      }
  //...
  }
}

当我运行代码时,会出现"java.lang.NullPointerException".错误来自此代码:

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

我不知道为什么arraylist是空的,因为我检查它包含一些数据.谁知道如何解决这个问题？谢谢.