Question

This code is part of a Tic Tac Toe program that I’m making with Java Swing. Why does it return NullPointerException when the for statement to add the buttons is added?

import java.awt.*;
import java.awt.event.ActionListener;
import javax.swing.*;

public class TicTacToeGui extends JFrame
{
    public final static int r = 3;
    public final static int c = 3;
    TicTacToeGui()
    {   
         JButton[][] button = new JButton[3][3];
         JPanel panel = new JPanel();
         panel.setLayout(new GridLayout(r, c));
         JLabel label = new JLabel("This is a tic tac toe game.");
         for(int i = 0; i < r; i++)
         {
            for(int j = 0; j < c; j++)
            {
                panel.add(button[i][j]);
            }
         }
         this.add(label);
         this.add(panel);
         this.setSize(400, 400);
         this.setVisible(true);
         this.setDefaultCloseOperation(EXIT_ON_CLOSE);
     }


    public static void main(String [] args)
    {
        new TicTacToeGui();
    }
}

Answer 1

You never initialize any JButton. When you declare

JButton[][] button = new JButton[3][3];

It just creates an empty 3×3 array of null, and you have to manually go through each spot in your array of arrays and initialize with

button[row][col] = new JButton("");