Java，BorderLayout.CENTER，获取JPanel的宽度和高度

Question

I am using Swing and AWT (for the listeners) to make a small program. I have a problem concerning getting the size of my JPanel (the class named Chess). My Layout:

public class Main extends JFrame implements MouseListener, ActionListener{

    Chess chessPanel = new Chess ();
    JButton newGameButton = new JButton ("New Game");
    JButton loadGameButton = new JButton ("Load Game");
    JButton saveGameButton = new JButton ("Save Game");
    JButton exitButton = new JButton ("Exit");

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

    Main () {
        super ("Chess");
        Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
        setSize(dim);
        setLocation(0,0);
        setUndecorated(true);

        chessPanel.addMouseListener(this);
        add(chessPanel, BorderLayout.CENTER);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout());


        newGameButton.addActionListener(this);
        loadGameButton.addActionListener(this);
        saveGameButton.addActionListener(this);
        exitButton.addActionListener(this);

        buttonPanel.add(newGameButton);
        buttonPanel.add(loadGameButton);
        buttonPanel.add(saveGameButton);
        buttonPanel.add(exitButton);

        add(buttonPanel, BorderLayout.SOUTH);

        setVisible(true);
    }

    // ... Code ...
}

As you can see by the code, I have one JPanel in the CENTER, which takes nearly all the screen. In the bottom I have another JPanel (SOUTH), which has a row of buttons.

What I need is the size that the JPanel in the CENTER takes. When I call the getWidth(), getHeight() or getBounds() methods inherited from JPanel, they all return 0, because of the BorderLayout. Any idea how to get the real values?

PS: The screen always takes up the entire screen, and will never be resized, if that helps.

Answer 1

You're likely calling getWidth before the JPanel has been rendered, and so it will be 0. The solution is to get the size after rendering, for instance after pack() or setVisible(true) has been called on the root container that holds this JPanel.

Also, I recommend against calling setSize() on anything since most of the standard layout managers observe the preferred size of a component, not the size, and when you call pack() telling the layout managers to do their thing, the set sizes are usually ignored. You may want to make your JPanel that is in the center set its own size by overriding its setPreferredSize method if it needs to be a certain size. Then let the JFrame and its held containers set the bet fit size based on the their layout managers when you call pack.

eg,

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

public class Main extends JFrame {

   Chess chessPanel = new Chess();
   JButton newGameButton = new JButton("New Game");
   JButton loadGameButton = new JButton("Load Game");
   JButton saveGameButton = new JButton("Save Game");
   JButton exitButton = new JButton("Exit");

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

   Main() {
      super("Chess");
      add(chessPanel, BorderLayout.CENTER);

      JPanel buttonPanel = new JPanel();
      buttonPanel.setLayout(new FlowLayout());

      buttonPanel.add(newGameButton);
      buttonPanel.add(loadGameButton);
      buttonPanel.add(saveGameButton);
      buttonPanel.add(exitButton);

      System.out.printf("chessPanel Size before rendering: %s%n", chessPanel.getSize());
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      add(buttonPanel, BorderLayout.SOUTH);
      pack();
      System.out.printf("chessPanel Size after rendering: %s%n", chessPanel.getSize());
      setLocationRelativeTo(null);
      setVisible(true);
   }

   // ... Code ...
}

@SuppressWarnings("serial")
class Chess extends JPanel {
   private static final int CHESS_WIDTH = 600;
   private static final int CHESS_HEIGHT = CHESS_WIDTH;
   private static final int MAX_ROW = 8;
   private static final int MAX_COL = 8;
   private static final Color LIGHT_COLOR = new Color(240, 190, 40);
   private static final Color DARK_COLOR = new Color(180, 50, 0);

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(CHESS_WIDTH, CHESS_HEIGHT);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      int panelWidth = getWidth();
      int panelHeight = getHeight();
      int sqrWidth = panelWidth / MAX_ROW;
      int sqrHeight = panelHeight / MAX_COL;
      for (int row = 0; row < MAX_ROW; row++) {
         for (int col = 0; col < MAX_COL; col++) {
            Color c = (row % 2 == col % 2) ? LIGHT_COLOR : DARK_COLOR;
            g.setColor(c);
            int x = (row * panelWidth) / MAX_ROW;
            int y = (col * panelHeight) / MAX_COL;
            g.fillRect(x, y, sqrWidth, sqrHeight);
         }
      }
   }
}