Why Is My Program Causing Multiple Null Pointer Exceptions?

Question

This program is a work-in-progress tic-tac-toe game, where I've only just made the buttons; at least, theoretically. I looked up "null pointer exception" before posting this, but still don't understand how it applies here. Help?

//this line is line four
public class TicTacToe extends JFrame {
  public static void main(String[] args) {
    new TicTacToe();
  }
  public TicTacToe(){
    super.setTitle("Tic Tac Toe");
    super.setSize(800, 800);
    super.setDefaultCloseOperation(EXIT_ON_CLOSE);
    buildPanel();
    super.setVisible(true);
  }
  public void buildPanel(){
    GridLayout g = new GridLayout(3,3);
    JPanel p = new JPanel();
    p.setLayout(g);
    JButton [] buttons = new JButton[9];
    //I'm adding text temporarily to help me see where each button's position
    buttons[0] = new JButton("One");
    p.add(buttons[0]);
    buttons[1] = new JButton("Two");
    p.add(buttons[1]);
    buttons[2] = new JButton("Three");
    p.add(buttons[2]);
    buttons[3] = new JButton("Four");
    p.add(buttons[3]);
    buttons[4] = new JButton("Five");
    p.add(buttons[4]);
    buttons[5] = new JButton("Six");
    p.add(buttons[5]);
    buttons[6] = new JButton("Seven");
    p.add(buttons[6]);
    buttons[7] = new JButton("Eight");
    p.add(buttons[8]);
    buttons[8] = new JButton("Nine");
    p.add(buttons[8]);

    add(p);

    ActionListener Callback = event -> {
      String label = event.getActionCommand();

    };
    for (int i=0;i<buttons.length;i++){
      String string="O";
      if (i%2==0){
        string = "X";
      }
      buttons[i].setText(string);
      buttons[i].addActionListener(Callback);
    }
  }
}

The Result:

 Exception in thread "main" java.lang.NullPointerException
    at java.desktop/java.awt.Container.addImpl(Container.java:1117)
    at java.desktop/java.awt.Container.add(Container.java:436)
    at TicTacToe.buildPanel(TicTacToe.java:37)
    at TicTacToe.<init>(TicTacToe.java:13)
    at TicTacToe.main(TicTacToe.java:7)

More importantly -- the heuristic for debugging a NullPointerException is almost always the same: You should critically read your exception's stacktrace to find the line of code at fault, the line that throws the exception, and then inspect that line carefully, find out which variable is null, and then trace back into your code to see why. You will run into these again and again, trust me. In the future, please search on the subject before posting, since this is too common a problem to post yet another NPE question. — Hovercraft Full Of Eels, Feb 23 '20 at 20:36

dan1st · Accepted Answer · 2020-02-23T20:37:24.760

1

This is the problem:

buttons[7] = new JButton("Eight");
p.add(buttons[8]);

It seems to be a typo, I think it should be

buttons[7] = new JButton("Eight");
p.add(buttons[7]);

In your case, buttons[8] is not yet initialized (it would be initialized in the line after the exception) and will therefore result in a NullPointerException.

edited Feb 23 '20 at 20:37

answered Feb 23 '20 at 20:31

dan1st

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This is why we are here - to help others - even with such simple problems :) – dan1st Feb 24 '20 at 05:44

Why Is My Program Causing Multiple Null Pointer Exceptions?

1 Answers1