Java，BorderLayout.CENTER，获取JPanel的宽高

Java, BorderLayout.CENTER, getting the width and height of the JPanel

提问by Hidde

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:

我正在使用 Swing 和 AWT（用于听众）来制作一个小程序。我在获取 JPanel（名为 Chess 的类）的大小时遇到​​问题。我的布局：

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.

正如您在代码中看到的那样，我在 CENTER 中有一个 JPanel，它几乎占据了整个屏幕。在底部我有另一个 JPanel (SOUTH)，它有一排按钮。

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?

我需要的是 CENTER 中 JPanel 的大小。当我调用从 JPanel 继承的 getWidth()、getHeight() 或 getBounds() 方法时，由于 BorderLayout，它们都返回 0。知道如何获得真实值吗？

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

PS：屏幕总是占据整个屏幕，并且永远不会调整大小，如果有帮助的话。

回答by Hovercraft Full Of Eels

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

您可能会在 JPanel 渲染之前调用 getWidth，因此它将为 0。解决方案是在渲染后获取大小，例如在保存的根容器上调用 pack() 或 setVisible(true) 之后这个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.

此外，我建议不要在任何事情上调用 setSize()，因为大多数标准布局管理器观察组件的首选大小，而不是大小，并且当您调用 pack() 告诉布局管理器做他们的事情时，设置的大小是通常被忽略。如果需要特定大小，您可能希望通过覆盖其 setPreferredSize 方法使位于中心的 JPanel 设置其自己的大小。然后让 JFrame 及其持有的容器在您调用 pack 时根据它们的布局管理器设置下注大小。

e.g.,

例如，

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);
         }
      }
   }
}