设置按钮的位置

Question

此代码的输出有两个并排放置的按钮。我想改变按钮的位置，如位置是顶部和底部。如何设置按钮的位置。我需要修改哪部分代码？

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 
/** 
* Custom Graphics Example: Using key/button to move a line left or right. 
*/ 
@SuppressWarnings("serial") 
public class CGMoveALine extends JFrame { 
    // Name-constants for the various dimensions 
    public static final int CANVAS_WIDTH = 400; 
    public static final int CANVAS_HEIGHT = 140; 
    public static final Color LINE_COLOR = Color.BLACK; 
    public static final Color CANVAS_BACKGROUND = Color.CYAN; 

    // The line from (x1, y1) to (x2, y2), initially position at the center 
    private int x1 = CANVAS_WIDTH/2; 
    private int y1 = CANVAS_HEIGHT/8; 
    private int x2 = x1; 
    private int y2 = CANVAS_HEIGHT/8 * 7; 

    private DrawCanvas canvas; // the custom drawing canvas (extends JPanel) 

    **strong text**/** Constructor to set up the GUI */ 
    public CGMoveALine() { 
    // Set up a panel for the buttons 
    JPanel btnPanel = new JPanel(new FlowLayout()); 
    JButton btnLeft = new JButton("Move Left "); 
    btnPanel.add(btnLeft); 
    btnLeft.addActionListener(new ActionListener() { 
    public void actionPerformed(ActionEvent e) { 
     x1 -= 10; 
     x2 -= 10; 
     canvas.repaint(); 
     requestFocus(); // change the focus to JFrame to receive KeyEvent 
    } 
    }); 
    JButton btnRight = new JButton("Move Right"); 
    btnPanel.add(btnRight); 
    btnRight.addActionListener(new ActionListener() { 
    public void actionPerformed(ActionEvent e) { 
     x1 += 10; 
     x2 += 10; 
     canvas.repaint(); 
     requestFocus(); // change the focus to JFrame to receive KeyEvent 
    } 
    }); 

    // Set up a custom drawing JPanel 
    canvas = new DrawCanvas(); 
    canvas.setPreferredSize(new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT)); 

    // Add both panels to this JFrame 
    Container cp = getContentPane(); 
    cp.setLayout(new BorderLayout()); 
    cp.add(canvas, BorderLayout.CENTER); 
    cp.add(btnPanel, BorderLayout.SOUTH); 

    // "this" JFrame fires KeyEvent 
    addKeyListener(new KeyAdapter() { 
    @Override 
    public void keyPressed(KeyEvent evt) { 
     switch(evt.getKeyCode()) { 
      case KeyEvent.VK_LEFT: 
       x1 -= 10; 
       x2 -= 10; 
       repaint(); 
       break; 
      case KeyEvent.VK_RIGHT: 
       x1 += 10; 
       x2 += 10; 
       repaint(); 
       break; 
     } 
    } 
    }); 

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // Handle the CLOSE button 
    setTitle("Move a Line"); 
    pack();   // pack all the components in the JFrame 
    setVisible(true); // show it 
    requestFocus(); // set the focus to JFrame to receive KeyEvent 
    } 

    /** 
* DrawCanvas (inner class) is a JPanel used for custom drawing 
*/ 
    class DrawCanvas extends JPanel { 
     @Override 
     public void paintComponent(Graphics g) { 
     super.paintComponent(g); 
     setBackground(CANVAS_BACKGROUND); 
     g.setColor(LINE_COLOR); 
     g.drawLine(x1, y1, x2, y2); // draw the line 
     } 
    } 

    /** The entry main() method */ 
    public static void main(String[] args) { 
     // Run GUI codes on the Event-Dispatcher Thread for thread safety 
     SwingUtilities.invokeLater(new Runnable() { 
     @Override 
     public void run() { 
      new CGMoveALine(); // Let the constructor do the job 
     } 
     }); 
    } 
} 

我要让按钮像这样position

Answer 1

在本节：

**strong text**/** Constructor to set up the GUI */ 
    public CGMoveALine() { 
    // Set up a panel for the buttons 
    JPanel btnPanel = new JPanel(new FlowLayout()); 

变化按以下操作：

public CGMoveALine() { 
    // Set up a panel for the buttons 
    JPanel btnPanel = new JPanel(new GridLayout(2,1)); 

输出：

---

或者，如果你想要一个按钮，在顶部和一个在底部，你可以这样做：

public CGMoveALine() { 
    // Remove button panels 
    JButton btnLeft = new JButton("Move Left "); 
    btnLeft.addActionListener(new ActionListener() { 
    public void actionPerformed(ActionEvent e) { 
     x1 -= 10; 
     x2 -= 10; 
     canvas.repaint(); 
     requestFocus(); // change the focus to JFrame to receive KeyEvent 
    } 
    }); 
    JButton btnRight = new JButton("Move Right"); 
    btnRight.addActionListener(new ActionListener() { 
    public void actionPerformed(ActionEvent e) { 
     x1 += 10; 
     x2 += 10; 
     canvas.repaint(); 
     requestFocus(); // change the focus to JFrame to receive KeyEvent 
    } 
    }); 

    // Set up a custom drawing JPanel 
    canvas = new DrawCanvas(); 
    canvas.setPreferredSize(new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT)); 

    // Add both panels to this JFrame 
    Container cp = getContentPane(); 
    cp.setLayout(new BorderLayout()); 
    // Manually add a button to top and one to bottom (ie, NORTH and SOUTH) 
    cp.add(btnRight, BorderLayout.NORTH); 
    cp.add(canvas, BorderLayout.CENTER); 
    cp.add(btnLeft, BorderLayout.SOUTH); 

输出：

在侧面说明，为什么你有这个：**strong text**/** Constructor to set up the GUI */？此行不正确，将导致编译问题。

使用通常的评论技巧：//Constructor to set up the GUI而不是这可能吗？