Java Swing 帮助

Question

I'm trying to create a grid of JButtons , with index values to the left and right of the button grid.

Here is my code, but I'm getting a NullPointerException . What is the problem?

public class Test {
public static void main(String args[]) {
    JFrame myapp = new JFrame("test");
    myapp.setLayout(new FlowLayout());

    myapp.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    myapp.setSize(400,400);
    myapp.setVisible(true);

    myapp.getContentPane().add(Board());
}

private static JPanel Board() {
    JButton[][] buttons = new JButton [10][10];

    JPanel board = new JPanel(new GridLayout(11,11));

    String[] rowlabels = {" ","A","B","C","D","E","F","G","H","I","J"};
    String[] columnlabels = {" ","1","2","3","4","5","6","7","8","9","10"};
    JTextField k;

    for (int i = 0; i < 11; i++) {
        for (int j = 0; j < 11; j++) {
            if ((j != 0) && (i != 0)) {
                //myboard[i-1][j-1].addActionListener(new ButtonHandler());
                board.add(buttons[i-1][j-1]);
            }
            if (i == 0) {
                if (j != 0) {
                    // used to display row of numbers
                    k = new JTextField(columnlabels[j]);
                    k.setEditable(false);
                    k.setHorizontalAlignment((int) JFrame.CENTER_ALIGNMENT);
                } else {
                    // used to display column of numbers
                    k = new JTextField();
                    k.setEditable(false);
                }
                board.add(k);
            } else if (j == 0) {
                k = new JTextField(rowlabels[i]);
                k.setEditable(false);
                k.setHorizontalAlignment((int) JFrame.CENTER_ALIGNMENT);
                board.add(k);
            }
        }
    }

    return board;
}
}

I'm probably doing something obvious, but Swing is kind of new to me, so any help would be greatly appreciated.

Answer 1

Swing GUIs should be constructed on the EDT. That part is left as an exercise for the OP.

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

public class Test {
    public static void main(String args[]) {
        JFrame myapp = new JFrame("test");
        myapp.setLayout(new FlowLayout());

        myapp.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        myapp.getContentPane().add(Board());

        // very important!
        myapp.pack();

        // do these last.
        myapp.setSize(400,400);
        myapp.setVisible(true);

    }

    private static JPanel Board() {
        JButton[][] buttons = new JButton [10][10];
        for(int x=0; x<buttons.length; x++) {
            for (int y=0; y<buttons[0].length; y++) {
                buttons[x][y] = new JButton();
            }
        }

        JPanel board = new JPanel(new GridLayout(11,11));

        String[] rowlabels = {" ","A","B","C","D","E","F","G","H","I","J"};
        String[] columnlabels = {" ","1","2","3","4","5","6","7","8","9","10"};
        JTextField k;

        for (int i = 0; i < 11; i++) {
            for (int j = 0; j < 11; j++) {
                if ((j != 0) && (i != 0)) {
                    //myboard[i-1][j-1].addActionListener(new ButtonHandler());
                    board.add(buttons[i-1][j-1]);
                }
                if (i == 0) {
                    if (j != 0) {
                        // used to display row of numbers
                        k = new JTextField(columnlabels[j]);
                        k.setEditable(false);
                        k.setHorizontalAlignment((int) JFrame.CENTER_ALIGNMENT);
                    } else {
                        // used to display column of numbers
                        k = new JTextField();
                        k.setEditable(false);
                    }
                    board.add(k);
                } else if (j == 0) {
                    k = new JTextField(rowlabels[i]);
                    k.setEditable(false);
                    k.setHorizontalAlignment((int) JFrame.CENTER_ALIGNMENT);
                    board.add(k);
                }
            }
        }

        return board;
    }
}

---

Given the nature of the GUI, I guess you are after something more like this.

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

public class Test {
    public static void main(String args[]) {
        JFrame myapp = new JFrame("test");
        myapp.setLayout(new FlowLayout());

        myapp.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        myapp.getContentPane().add(Board());

        // very important!
        myapp.pack();

        // do this last.
        myapp.setVisible(true);
    }

    private static JPanel Board() {
        JButton[][] buttons = new JButton [10][10];
        for(int x=0; x<buttons.length; x++) {
            for (int y=0; y<buttons[0].length; y++) {
                JButton temp = new JButton();
                temp.setPreferredSize(new Dimension(30,30));
                buttons[x][y] = temp;
            }
        }

        JPanel board = new JPanel(new GridLayout(11,11));

        String[] rowlabels = {" ","A","B","C","D","E","F","G","H","I","J"};
        String[] columnlabels = {" ","1","2","3","4","5","6","7","8","9","10"};
        JTextField k;

        for (int i = 0; i < 11; i++) {
            for (int j = 0; j < 11; j++) {
                if ((j != 0) && (i != 0)) {
                    //myboard[i-1][j-1].addActionListener(new ButtonHandler());
                    board.add(buttons[i-1][j-1]);
                }
                if (i == 0) {
                    if (j != 0) {
                        // used to display row of numbers
                        k = new JTextField(columnlabels[j]);
                        k.setEditable(false);
                        k.setHorizontalAlignment((int) JFrame.CENTER_ALIGNMENT);
                    } else {
                        // used to display column of numbers
                        k = new JTextField();
                        k.setEditable(false);
                    }
                    board.add(k);
                } else if (j == 0) {
                    k = new JTextField(rowlabels[i]);
                    k.setEditable(false);
                    k.setHorizontalAlignment((int) JFrame.CENTER_ALIGNMENT);
                    board.add(k);
                }
            }
        }

        return board;
    }
}

Answer 2

JButton[][] buttons = new JButton [10][10];

only creates an array of nulls, you have to initialize the buttons. and adding null to panel causes a NPE.

Answer 3

Just tried your code and noticed that you get NPE because you are just creating a buttons array but do not create buttons in it so your buttons array contains null values and when trying to add it in board it throws NPE.

Try something like this after creating array:

 private static JPanel Board() {
    JButton[][] buttons = new JButton [10][10];
    for (int i = 0; i < 10; i++) {
        for (int j = 0; j < 10; j++) {
            buttons[i][j] = new JButton();
        }
    }
    JPanel board = new JPanel(new GridLayout(11,11));

    String[] rowlabels = {" ","A","B","C","D","E","F","G","H","I","J"};
    String[] columnlabels = {" ","1","2","3","4","5","6","7","8","9","10"};
    JTextField k;

    // ---- your remaining code.

 }