我使用 JFrame 用 Java 制作了一个简单的绘图程序（这是我第一次使用）。用户单击并拖动以绘制形状，但这并不重要。我有JMenuBar很多选项，比如形状类型、新建和退出。当用户单击new按钮时，它应该清除屏幕。当用户点击 Ctrl+N 时，这工作得很好。但是，当单击按钮时，它根本不起作用。
我System.out.println在 newItem 的 actionEvent 中进行了调试，当单击该项目时它打印得很好，但它实际上并没有擦除屏幕。知道什么会导致这种情况吗？
我删掉了程序的大部分内容，并留下了看到问题所必需的部分。您仍然可以按住并拖动来绘制一个形状（直到您释放鼠标按钮该形状才会出现），然后按 Ctrl+N 清除屏幕，但单击新建不会这样做。
import java.awt.*;
import java.awt.event.*;
import java.lang.*;
import javax.swing.*;
public class E3G04 extends JFrame implements WindowListener, ActionListener, MouseListener, MouseMotionListener
{
//Variables are declared as volatile to ensure that they're always called from system RAM
static volatile String type = "rectangle";
static volatile Boolean fill = true;
static Color lineColor = Color.BLACK;
static Color fillColor = Color.RED;
static int size = 1;
CanvasEX cx = new CanvasEX();
static boolean running = true;
JMenuBar mb = new JMenuBar();
JMenu fileMenu = new JMenu("File");
JMenuItem newItem = new JMenuItem("New");
JMenuItem quitItem = new JMenuItem ("Quit");
protected E3G04()
{
mb.add(fileMenu);
fileMenu.add(newItem);
fileMenu.add(quitItem);
newItem.setMnemonic('N');
quitItem.setMnemonic('Q');
newItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_N, ActionEvent.CTRL_MASK));
quitItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_Q, ActionEvent.CTRL_MASK));
newItem.addActionListener(this);
quitItem.addActionListener(this);
cx.setSize(800,600);
cx.addMouseListener(this);
cx.addMouseMotionListener(this);
setJMenuBar(mb);
setBounds(100,100,800,600);
setLayout(new BorderLayout());
add("Center",cx);
addWindowListener(this);
setResizable(true);
setVisible(true);
}
public static void main(String [] args)
{
new E3G04();
}
public void stop()
{
newItem.removeActionListener(this);
quitItem.removeActionListener(this);
dispose();
System.exit(0);
}
public void actionPerformed(ActionEvent e)
{
Object o = e.getSource();
if (o == newItem)
{
cx.erase = true;
cx.repaint();
}
if (o == quitItem)
{
running = false;
stop();
}
}
public void mousePressed(MouseEvent m)
{
cx.start = m.getPoint();
cx.end = cx.start;
cx.cur = cx.start;
}
public void mouseDragged(MouseEvent m)
{
cx.cur = m.getPoint();
}
public void mouseReleased(MouseEvent m)
{
cx.end = cx.cur;
cx.repaint();
}
public void itemStateChanged(ItemEvent e)
{
Object o = e.getSource();
}
public void windowClosing(WindowEvent e)
{
running = false;
stop();
}
public void mouseClicked(MouseEvent m){}
public void mouseExited(MouseEvent m){}
public void mouseEntered(MouseEvent m){}
public void mouseMoved(MouseEvent m){}
public void windowClosed(WindowEvent e){}
public void windowOpened(WindowEvent e){}
public void windowActivated(WindowEvent e){}
public void windowDeactivated(WindowEvent e){}
public void windowIconified(WindowEvent e){}
public void windowDeiconified(WindowEvent e){}
}
class CanvasEX extends Canvas
{
Point start = new Point(100,100);
Point cur = new Point(100,100);
Point end = new Point(100,100);
Image offscreen;
boolean erase = false;
public void update(Graphics g)
{
//This is adds the new stuff to the screen or erases the screen if erase is true
Graphics buffer;
if (offscreen == null)
{
offscreen = createImage(getWidth(), getHeight());
}
buffer = offscreen.getGraphics();
if (erase)
{
buffer.setColor(getBackground());
buffer.fillRect(0,0,800, 600);
buffer.dispose();
erase = false;
}
paint(buffer);
g.drawImage(offscreen, 0, 0, this);
}
public void paint(Graphics g)
{
Graphics buffer = g;
if (erase)
{
g.dispose();
erase = false;
}
g.setColor(E3G04.lineColor);
if (end.x > start.x && end.y > start.y)
g.fillRect(start.x,start.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
if (end.x > start.x && end.y < start.y)
g.fillRect(start.x,end.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
if (end.x < start.x && end.y > start.y)
g.fillRect(end.x, start.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
if (end.x < start.x && end.y < start.y)
g.fillRect(end.x, end.y, Math.abs(end.x-start.x),Math.abs(end.y-start.y));
g.setColor(E3G04.fillColor);
if (end.x > start.x && end.y > start.y)
g.fillRect(start.x + E3G04.size,start.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);
if (end.x > start.x && end.y < start.y)
g.fillRect(start.x + E3G04.size,end.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);
if (end.x < start.x && end.y > start.y)
g.fillRect(end.x + E3G04.size, start.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);
if (end.x < start.x && end.y < start.y)
g.fillRect(end.x + E3G04.size, end.y + E3G04.size, Math.abs(end.x-start.x) - 2 * E3G04.size,Math.abs(end.y-start.y) - 2 * E3G04.size);
}
}