在ActionEvent中更改JLabel的文本

Question

OK, so I'm creating a Tic Tac Toe GUI. I currently having a working frame of 3x3 buttons that will change to either an X or O. My issue is implementing a JLabel on the top of the buttons. The label keeps track of who's turn it is and will change from either "O's turn" or "X's turn" however I can't seem to get the JLabel to update (only spouts errors). Here's a portion of my code showing how Trying to implement this. The program runs fine but as soon as I add the (turn.setText("O's turn");) I get errors.

Any feedback as to why this may not be working would be appreciated.

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class TicTacToe extends JFrame {

    XOButton[] xob = new XOButton[9];
    private JLabel turn;

    public static int state;

    public TicTacToe() {
        super("TicTacToe");

        //add XOButtons to panel
        JPanel center = new JPanel();
        state = 0;

        JLabel turn = new JLabel("X's turn");

        center.setLayout(new GridLayout(3, 3, 2, 2));
        //action listener
        ButtonListener bl = new ButtonListener();
        for (int i = 0; i < 9; i++) {
            xob[i] = new XOButton();
            xob[i].addActionListener(bl);
            center.add(xob[i]);
        }

        //add panel to frame
        setLayout(new BorderLayout());
        add(center, BorderLayout.CENTER);
        add(turn, BorderLayout.NORTH);

    }



    //inner action listener class
    private class ButtonListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent ae) {
            for (int i = 0; i < 9; i++) {
                if(ae.getSource() == xob[i] && xob[i].getState() == XOButton.NONE && state == 0) {
                    xob[i].setState(XOButton.X);
                    state = 1;
                    //turn.setText("O's turn");

                } else if(ae.getSource() == xob[i] && xob[i].getState() == XOButton.NONE && state == 1) {
                    xob[i].setState(XOButton.O);
                    state = 0;
                }
            }
        }
    }
}

Answer 1

Guess: your error is a NullPointerException because turn is null. Solution: don't shadow the variable. In other words you're re-declaring the turn variable in the constructor leaving the field null.

eg,

public class TicTacToe extends JFrame{
XOButton[] xob = new XOButton[9];
private JLabel turn;

public static int state;

public TicTacToe() {
    super("TicTacToe");

    //add XOButtons to panel
    JPanel center = new JPanel();
    state = 0;

    JLabel turn = new JLabel("X's turn");  // **** you're re-declaring turn here!

Better:

public class TicTacToe extends JFrame{
XOButton[] xob = new XOButton[9];
private JLabel turn;

public static int state;

public TicTacToe() {
    super("TicTacToe");

    //add XOButtons to panel
    JPanel center = new JPanel();
    state = 0;

    turn = new JLabel("X's turn");  // **** Note the difference?

In the future, if you have similar problems, please post the entire error message and indicate which line throws the exception as it will help us immensely!