大家好，请教大家一个与Graphics有关的问题

[songqicpp]

大家好，下面是我从core java书上摘的一段程序，相当与一个小画板，但是，当有别的窗口覆盖窗口后，或者是该窗口最小化后，原来在窗口上的画的线就会消失，我想请问一下，有什么办法以不让这些线条消失，希望大家指教。

/**
* @version 1.20 25 Jun 1998
* @author Cay Horstmann
*/

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

class SketchPanel extends JPanel
implements KeyListener
{ public SketchPanel()
{ addKeyListener(this);
}

public void keyPressed(KeyEvent evt)
{ int keyCode = evt.getKeyCode();
int d;
if (evt.isShiftDown())
d = 5;
else
d = 1;
if (keyCode == KeyEvent.VK_LEFT) add(-d, 0);
else if (keyCode == KeyEvent.VK_RIGHT) add(d, 0);
else if (keyCode == KeyEvent.VK_UP) add(0, -d);
else if (keyCode == KeyEvent.VK_DOWN) add(0, d);
}

public void keyReleased(KeyEvent evt)
{}

public void keyTyped(KeyEvent evt)
{ char keyChar = evt.getKeyChar();
int d;
if (Character.isUpperCase(keyChar))
{ d = 5;
keyChar = Character.toLowerCase(keyChar);
}
else
d = 1;
if (keyChar == 'h') add(-d, 0);
else if (keyChar == 'l') add(d, 0);
else if (keyChar == 'k') add(0, -d);
else if (keyChar == 'j') add(0, d);
}

public boolean isFocusTraversable() { return true; }

public void add(int dx, int dy)
{ end.x += dx;
end.y += dy;
Graphics g = getGraphics();
g.drawLine(start.x, start.y, end.x, end.y);
g.dispose();
start.x = end.x;
start.y = end.y;
}

private Point start = new Point(0, 0);
private Point end = new Point(0, 0);
}

class SketchFrame extends JFrame
{ public SketchFrame()
{ setTitle("Sketch");
setSize(300, 200);
addWindowListener(new WindowAdapter()
{ public void windowClosing(WindowEvent e)
{ System.exit(0);
}
} );

Container contentPane = getContentPane();
contentPane.add(new SketchPanel());
}
}

public class Sketch
{ public static void main(String[] args)
{ JFrame frame = new SketchFrame();
frame.show();
}
}


谢谢大家。

[Danger2000]

只要重载paintComponent(Graphics g)方法就可以了。
看看作者的在core java 6e中的程序：

/**
@version 1.30 2000-06-03
@author Cay Horstmann
*/

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

public class Sketch
{
public static void main(String[] args)
{
SketchFrame frame = new SketchFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
}

/**
A frame with a panel for sketching a figure
*/
class SketchFrame extends JFrame
{
public SketchFrame()
{
setTitle("Sketch");
setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);

// add panel to frame

SketchPanel panel = new SketchPanel();
Container contentPane = getContentPane();
contentPane.add(panel);
}

public static final int DEFAULT_WIDTH = 300;
public static final int DEFAULT_HEIGHT = 200;
}

/**
A panel for sketching with the keyboard.
*/
class SketchPanel extends JPanel
{
public SketchPanel()
{
last = new Point2D.Double(100, 100);
lines = new ArrayList();
KeyHandler listener = new KeyHandler();
addKeyListener(listener);
setFocusable(true);
}

/**
Add a new line segment to the sketch.
@param dx the movement in x direction
@param dy the movement in y direction
*/
public void add(int dx, int dy)
{
// compute new end point
Point2D end = new Point2D.Double(last.getX() + dx,
last.getY() + dy);

// add line segment
Line2D line = new Line2D.Double(last, end);
lines.add(line);
repaint();

// remember new end point
last = end;
}

public void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;

// draw all lines
for (int i = 0; i < lines.size(); i++)
g2.draw((Line2D)lines.get(i));
}

private Point2D last;
private ArrayList lines;

private static final int SMALL_INCREMENT = 1;
private static final int LARGE_INCREMENT = 5;

private class KeyHandler implements KeyListener
{
public void keyPressed(KeyEvent event)
{
int keyCode = event.getKeyCode();

// set distance
int d;
if (event.isShiftDown())
d = LARGE_INCREMENT;
else
d = SMALL_INCREMENT;

// add line segment
if (keyCode == KeyEvent.VK_LEFT) add(-d, 0);
else if (keyCode == KeyEvent.VK_RIGHT) add(d, 0);
else if (keyCode == KeyEvent.VK_UP) add(0, -d);
else if (keyCode == KeyEvent.VK_DOWN) add(0, d);
}

public void keyReleased(KeyEvent event) {}

public void keyTyped(KeyEvent event)
{
char keyChar = event.getKeyChar();

// set distance
int d;
if (Character.isUpperCase(keyChar))
{
d = LARGE_INCREMENT;
keyChar = Character.toLowerCase(keyChar);
}
else
d = SMALL_INCREMENT;

// add line segment
if (keyChar == 'h') add(-d, 0);
else if (keyChar == 'l') add(d, 0);
else if (keyChar == 'k') add(0, -d);
else if (keyChar == 'j') add(0, d);
}
}
}

[javer6]

方在paintComponent中即可
这是对于swing组件扩展的典型错误！

[panpan221]

新手学习,关注

[stanleyzou]

回复人：javer6(孤舟万里) ( ) 信誉：100 2004-04-24 22:57:00 得分:0
?
方在paintComponent中即可
这是对于swing组件扩展的典型错误！

------------------------------------------------------------------------

可否说详细一点？谢谢。

[javer6]

说的详细一点的话：
就是Jcomponent为绘制自身以及自身包含的组件，提供了三个方法：
paint(graphics g);
paitComponent( )
paintChildren(..)
paintBorer(..)。
你要是扩展这些swing组件添加自己一些画图方法，那么你必须在paintComponent中添加你的代码，你仔细看看paintComponent中得代码实现，你就知道了，在swing中画图并不是简单取得graphics进行draw那么简单的拉，它涉及到很多标志尾的设置，而这些标志尾的setter方法是private的，这意味着你只能在paintComponent中进行扩展画图。

至于如果你不extends自swing组件，那另当别论了！

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