Frame鼠标点击异常

Question

import java.awt.*;
import java.awt.event.*;public class DrawLine extends Frame
{
 static int starX;
 static int starY;
 static int endX;
 static int endY;
 static Graphics graphics; public DrawLine()
 {
  this.setSize(400,300);
  this.setLocation(300,200);
  this.addMouseListener(new MyMouseAdapter());
  this.addMouseMotionListener(new MyMouseMotionAdapter());
  this.addWindowListener(new WindowClosing());
  this.setVisible(true);
 } public static void main(String args[])
 {
  DrawLine drawline=new DrawLine();
 }
}class WindowClosing extends WindowAdapter
{
 public void windowClosing(WindowEvent e)
 {
  e.getWindow().dispose();
  System.exit(0);
 }
}class MyMouseAdapter extends MouseAdapter
{
 public void mousePressed(MouseEvent e)
 {
  DrawLine.starX=e.getX();
  DrawLine.starY=e.getY();
 }
 public void mouseReleased(MouseEvent e)
 {
  DrawLine.endX=e.getX();
  DrawLine.endY=e.getY();
  DrawLine.graphics.drawLine(DrawLine.starX,DrawLine.starY,DrawLine.endX,DrawLine.endY);
 }
} class MyMouseMotionAdapter extends MouseMotionAdapter
{
}
这程序可以运行,但运行后我点击运行的界面时就会出现异常,有哪位高手指点一下~

Answer 1

此回复为自动发出,仅用于显示而已，并无任何其他特殊作用
楼主【aheqq】截止到2008-07-12 02:37:26的历史汇总数据(不包括此帖)：
发帖的总数量:4                        发帖的总分数:80                       每贴平均分数:20                       
回帖的总数量:3                        得分贴总数量:0                        回帖的得分率:0%                       
结贴的总数量:0                        结贴的总分数:0                        
无满意结贴数:0                        无满意结贴分:0                        
未结的帖子数:4                        未结的总分数:80                       
结贴的百分比:0.00  %               结分的百分比:0.00  %                  
无满意结贴率:---------------------无满意结分率:---------------------
如何结贴请参考这里：http://topic.csdn.net/u/20080501/09/ef7ba1b3-6466-49f6-9d92-36fe6d471dd1.html

Answer 2

哈哈，你抛出的异常写出来呀
看了才知道是那个地方有debug啊

Answer 3

你那个里面的DrawLine.graphics是空的，自己再好好研究研究吧
让你见识一下真正的画线，hohoimport java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Cursor;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.util.ArrayList;
import java.util.List;import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;public class DrawPane extends JComponent
{
    private static final long serialVersionUID = -8963598853892068851L;    private int startX = 0;    private int startY = 0;    private int endX = 0;    private int endY = 0;    private List<Draw> drawList = new ArrayList<Draw>();    private Color color = Color.BLUE;    private Font font = new Font("Courier New", Font.BOLD, 14);    private boolean drawing = false;    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                final DrawPane draw = new DrawPane();
                JFrame frame = new JFrame();
                frame.setTitle("Draw Demo");
                frame.add(draw, BorderLayout.CENTER);
                frame.setSize(500, 300);
                frame.setLocationRelativeTo(null);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.getContentPane().setBackground(Color.WHITE);
                frame.setVisible(true);                new Thread()
                {
                    int i = 0;                    public void run()
                    {
                        while(i < 10)
                        {
                            try
                            {
                                sleep(1000);
                                i++;
                            }
                            catch(InterruptedException e)
                            {
                                e.printStackTrace();
                            }
                        }                        draw.setColor(Color.GREEN);
                        draw.setFont(new Font("Courier New", Font.BOLD, 20));
                    }
                }.start();
            }
        });
    }    public DrawPane()
    {
        super();
        init();
    }    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);        for(Draw draw: drawList)
        {
            draw.draw(g);
        }        if(drawing)
        {
            g.drawString(endX + "," + endY, endX, endY);
            g.drawLine(startX, startY, endX, endY);
        }
    }    private void init()
    {
        addMouseListener(new MouseL());
        addMouseMotionListener(new MouseMotionL());
        addComponentListener(new ComponentL());
        setCursor(new Cursor(Cursor.CROSSHAIR_CURSOR));
    }    public Color getColor()
    {
        return color;
    }    public void setColor(Color color)
    {
        this.color = color;
    }    public Font getFont()
    {
        return font;
    }    public void setFont(Font font)
    {
        this.font = font;
    }    private class MouseL extends MouseAdapter
    {
        public void mousePressed(MouseEvent e)
        {
            startX = e.getX();
            startY = e.getY();
            drawList.add(new Text(font, color, startX, startY));
            repaint();
        }        public void mouseReleased(MouseEvent e)
        {
            drawing = false;
            endX = e.getX();
            endY = e.getY();
            drawList.add(new Line(font, color, startX, startY, endX, endY));
            drawList.add(new Text(font, color, endX, endY));
            repaint();
            startX = 0;
            startY = 0;
            endX = 0;
            endY = 0;
        }
    }    private class MouseMotionL extends MouseMotionAdapter
    {
        public void mouseDragged(final MouseEvent e)
        {
            drawing = true;
            endX = e.getX();
            endY = e.getY();
            repaint();
        }
    }    private class ComponentL extends ComponentAdapter
    {
        public void componentResized(final ComponentEvent e)
        {
            repaint();
        }
    }
    
    private class Line implements Draw
    {
        private int orgX = 0;        private int orgY = 0;        private int endX = 0;        private int endY = 0;        private Font font;        private Color color;        public Line(Font font, Color color, int orgX, int orgY, int endX, int endY)
        {
            this.orgX = orgX;
            this.orgY = orgY;
            this.endX = endX;
            this.endY = endY;
            this.color = color;
            this.font = font;
        }        public void draw(Graphics g)
        {
            g.setFont(font);
            g.setColor(color);
            g.drawLine(orgX, orgY, endX, endY);
        }
    }    private class Text implements Draw
    {
        private int orgX = 0;        private int orgY = 0;        private Font font;        private Color color;        public Text(Font font, Color color, int orgX, int orgY)
        {
            this.orgX = orgX;
            this.orgY = orgY;
            this.color = color;
            this.font = font;
        }        public void draw(Graphics g)
        {
            Graphics2D g2d = (Graphics2D)g;
            g2d.setStroke(new BasicStroke(2));
            g.setFont(font);
            g.setColor(color);
            g.drawString(orgX + "," + orgY, orgX, orgY);
        }
    }    private interface Draw
    {
        public void draw(Graphics g);
    }
}

Answer 4

报的是空指针错误，问题出在这个方法里面
public void mouseReleased(MouseEvent e) 
{ 
  DrawLine.endX=e.getX(); 
  DrawLine.endY=e.getY(); 
  DrawLine.graphics.drawLine(DrawLine.starX,DrawLine.starY,DrawLine.endX,DrawLine.endY); 
 
} 

Answer 5

哈哈错误被我找到了 。
真正的错误在这一行：DrawLine.graphics.drawLine(DrawLine.starX,DrawLine.starY,DrawLine.endX,DrawLine.endY); 
因为你声明了graphics但是你没有初始化这个东西，当然是NullPointerException咯。
解决方法是 使用
public void paint( Graphics g){
  g.drawLine(DrawLine.starX,DrawLine.starY,DrawLine.endX,DrawLine.endY); 
}
然后在你的事件中使用
repaint();
HOHO很久没有看到用JB开发界面的，鼓励下你。