这是我已经接近弄清楚但从未真正做到的一个。
下面列出的代码应该在看到第一次点击后立即绘制一个绿色圆圈。它没有。随后的单击绘制将当前单击点与前一个单击点连接起来的线，以红色显示。该代码在第一次单击时失败，并且适用于所有后续单击。为什么第一次点击不显示？它运行！
我究竟做错了什么？
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();
}
}
