自定义mousevent方法处的NullPointerException

Question

My gui class for my sudoku app throws a null exception "Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException" at line 1431

    private String click(int x, int y) {
        String dialog = null;
        if (5==5/*sudoku1.grid[x][y].isEditable()==true*/)     {
            dialog = JOptionPane.showInputDialog("Enter Input:");
            int input = Integer.parseInt(dialog);                
1431:       sudoku1.grid[x][y].setValue(input);


        }
        else    {
            //JOptionPane.showMessageDialog(...);
        }
    return dialog;
    }

At start of the class object sudoku1 is declared:

    public class ClassicGUI extends javax.swing.JFrame {
private boolean finished;
Sudoku sudoku1;
private final int size = 9;

and in main method run() created:

    public void run() {
    new ClassicGUI().setVisible(true);
    System.out.println("Testing GUI...");
    sudoku1 = new Sudoku(size);
    sudoku1.makeGrid(size);
    }

grid object is a 2dimensional array of type Cell:

  public void makeGrid(int size) {
      this.lines=size;
      grid = new Cell[lines][lines];
      for(int i=0;i<size;i++)
                for(int j=0;j<size;j++)    {
                    grid[i][j] = new Cell();
                    grid[i][j].setValue(0);
                }
      }

So appropriate methods for access to grid[x][y] are stated. Also objects in the array are initialized in the makeGrid() method. Why the Null Exception Error???

PS The first if is commented because i had the same exception there :-S... x and y are coordinates ranging from 0-8 and are arguments to click method as shown below

private void jLabel28MouseClicked(java.awt.event.MouseEvent evt)   {                                      
    jLabel28.setText(click(3, 0));
}

Answer 1

Most likely, sudoku1.grid[x][y] is evaluating to null. Then when you call .setValue(input) on null, you get the NPE.

Try changing your code to have .setValue(input) on its own line. I'm not clear from your example exactly what data type sudoku1.grid[x][y] is supposed to evalueate to.

Edit: I suppose that sudoku1.grid could itself be null. I assume this is a public member of your Sudoku class (you did not provide us with the class definition?).

Bottom line is you are getting NullPointerException because you are calling a method on an object that is null. If you are not sure how to use the debugger in your environment, another way to solve this is to isolate the problem by breaking down that line of code. It does a few different things and you should try to reduce it to doing one thing at a time on each line, if you want to locate the problem.