java - Java Swing 谜题 : What doesn't the first iteration display on the JPanel?

Question

这是我差一点就能弄清楚的一个问题，但从未真正做到过。

下面列出的代码应该在第一次单击时绘制一个绿色圆圈。事实并非如此。后续单击将绘制连接当前单击点与前一个单击点的线(红色)。该代码在第一次单击时失败，但在所有后续单击中都有效。为什么第一次点击不显示？它运行了!

我做错了什么？

代码应该在任何当前的 JDE 上编译。

TIA

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

class Demo extends JFrame
            implements  ActionListener, ListSelectionListener, MouseListener {

  int clkCt = 0, // Count of the number of clicks we've done.
      oldX,      // Penultimate X value
      oldY,      // Penultimate X value
      scrH,      // Height of the drawing canvas.
      scrW;      // Width of the drawing canvas.

  JFrame f;      // Holder for the drawing canvas.

  JLabel ctL;    // Displays the number of clicks we've done.

  JPanel canvas; // The drawing canvas.

  public void demoLines() {

    Dimension d = Toolkit.getDefaultToolkit().getScreenSize();
    scrH = (int) ((double) d.height * 0.75);
    scrW = (int) ((double) d.width * 0.75);

    oldX = scrH / 2;
    oldY = oldX;

    // Create and set up the window.
    f = new JFrame("Multi Click Demo");
    f.getContentPane().setLayout(null);
    int h = scrH / 5;
    f.setBounds(h, h, scrW, scrH);

    // Create a panel
    canvas = new JPanel();
    canvas.setBackground(Color.black);
    canvas.setForeground(Color.red);
    canvas.setLayout(null);
    canvas.setBounds(0, 0, scrW, scrH);
    canvas.setPreferredSize(new Dimension(scrW, scrH));
    canvas.addMouseListener(this);
    f.getContentPane().add(canvas);

    // Create the exit button.
    JButton exit = new JButton("Exit");
    exit.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        goAway();
      }
    });
    exit.setBackground(Color.black);
    exit.setForeground(Color.red);
    exit.setBounds(0, 0, (scrW / 15), (scrH / 15));
    canvas.add(exit); //*/

    // Create the label for the click count.
    ctL = new JLabel("None Yet");
    ctL.setBackground(Color.black);
    ctL.setForeground(Color.red);
    ctL.setBounds((scrH / 15), (scrH * 13 / 15), (scrW / 15), (scrH / 15));
    canvas.add(ctL);

    f.getContentPane().add(canvas);
    f.setVisible(true);

    Graphics g = canvas.getGraphics();
    if (g == null) {
      System.out.println("No graphics for canvas!");
    } else {
      canvas.revalidate(); // This didn't help.
      paintComponent(g, (oldX + oldX / 2), (oldY + oldY / 2));
    }
  }

  void goAway() {
    f.setVisible(false);
    f.dispose();
  }

  public void mouseClicked(MouseEvent m) {
    // Where was the mouse clicked?
    int clkdBtn = m.getButton(),
        x = m.getX(),
        y = m.getY();
    Graphics g = canvas.getGraphics();
    paintComponent(g, x, y);
  }

  public void paintComponent(Graphics g,
                             int x,
                             int y) {

    // This always runs.
    ctL.setText(clkCt + "");

    if (clkCt == 0) {
      // This never displays!
      g.setColor(Color.green);
      int r = scrH * 4 / 5;
      g.drawOval((scrH / 10), (scrH / 10), r, r);
    }

   g.setColor(Color.red);
   g.drawLine(oldX, oldY, x, y);
   oldX = x;
    oldY = y;
    clkCt++;
  }


  public void actionPerformed(ActionEvent event) { }
  public void valueChanged(ListSelectionEvent event) { }

  public void mouseEntered(MouseEvent e) { }
  public void mouseExited(MouseEvent e) { }
  public void mousePressed(MouseEvent e) { }
  public void mouseReleased(MouseEvent e) { }

  public static void main(String[] s) {

    Demo m = new Demo();
    m.demoLines();
  }
}

Answer 1

What am I doing wrong?

您正在使用getGraphics用于定制绘画。当 repaint 时，任何以前的绘画都将丢失。叫做。相反，将所有自定义绘画功能移动到基于 JComponent 的新类中。或JPanel并覆盖paintComponent那里。记得调用super.paintComponent(g) .

参见Custom Painting Approaches用于从内部绘制多个组件的解决方案paintComponent 。您可以构建一个 List<Shape>自定义可绘制组件，从方法内迭代列表，并使用 drawLine或drawOval视情况而定。