源代码如下：
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
public class TranslateTest extends Applet
{
Image img;
private Point pressed=new Point(),lastTranslate=new Point();

public void init()
{
Image img=getImage(getCodeBase(),"pure.gif");
try
{
MediaTracker mt=new MediaTracker(this);
mt.addImage(img,0);
mt.waitForID(0);
}
catch(InterruptedException e)
{
e.printStackTrace();
}
addMouseListener(new MouseAdapter()
{
public void mousePressed(MouseEvent e)
{
Point ioc=e.getPoint();
pressed.x=ioc.x-lastTranslate.x;
pressed.y=ioc.y-lastTranslate.y;
}
});
addMouseMotionListener(new MouseMotionAdapter()
{
public void mouseDragged(MouseEvent e)
{
Point ioc=e.getPoint();
Point translate=new Point(ioc.x-pressed.x,ioc.y-pressed.y);
Graphics g=getGraphics();
try
{
g.clearRect(0,0,getSize().width,getSize().height);
g.translate(translate.x,translate.y);
showStatus("Translate Graphics:"+translate);
g.drawImage(img,0,0,TranslateTest.this);
}
finally
{
g.dispose();
}
lastTranslate=translate;
}
});
}
public void paint(Graphics g)
{
g.drawImage(img,0,0,this);
}
} 

错误提示如下：
D:jdk1.2test>javac TranslateTest.java
TranslateTest.java:43: Attempt to use a non-final variable img from a different
method. From enclosing blocks, only final local variables are available.
                                        g.drawImage(img,0,0,TranslateTest.this);

                                                    ^
1 error
那位高手能解释一下这个错误发生的原因，谢谢

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
public class TranslateTest extends Applet
{
    static Image img;
    private Point pressed=new Point(),lastTranslate=new Point();

    public void init()
    {
Image img=getImage(getCodeBase(),"pure.gif");
        try
        {
            MediaTracker mt=new MediaTracker(this);
            mt.addImage(img,0);

            mt.waitForID(0);
        }
        catch(InterruptedException e)
        {
            e.printStackTrace();
        }
        addMouseListener(new MouseAdapter()
        {
            public void mousePressed(MouseEvent e)
            {
                Point ioc=e.getPoint();
                pressed.x=ioc.x-lastTranslate.x;
                pressed.y=ioc.y-lastTranslate.y;
            }
        });
        addMouseMotionListener(new MouseMotionAdapter()
        {
            public void mouseDragged(MouseEvent e)
            {
                Point ioc=e.getPoint();
                Point translate=new Point(ioc.x-pressed.x,ioc.y-pressed.y);
                Graphics g=getGraphics();
                try
                {
                    g.clearRect(0,0,getSize().width,getSize().height);
                    g.translate(translate.x,translate.y);
                    showStatus("Translate Graphics:"+translate);
                    g.drawImage(TranslateTest.this.img,0,0,TranslateTest.this);
                }
                finally
                {
                    g.dispose();
                }
                lastTranslate=translate;
            }
        });
    }
    public void paint(Graphics g)
    {
if(img==null)
img = getImage(getCodeBase(),"pure.gif");
        g.drawImage(img,0,0,this);
    }
}

编译通过

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;

public class TranslateTest extends Applet
{
    Image img;
    private Point pressed=new Point(),lastTranslate=new Point();

    public void init()
    {
        img=getImage(getCodeBase(),"pure.gif");
        try
        {
            MediaTracker mt=new MediaTracker(this);
            mt.addImage(img,0);
            mt.waitForID(0);
        }
        catch(InterruptedException e)
        {
            e.printStackTrace();
        }
        addMouseListener(new MouseAdapter()
        {
            public void mousePressed(MouseEvent e)
            {
                Point ioc=e.getPoint();
                pressed.x=ioc.x-lastTranslate.x;
                pressed.y=ioc.y-lastTranslate.y;
            }
        });
        addMouseMotionListener(new MouseMotionAdapter()
        {
            public void mouseDragged(MouseEvent e)
            {
                Point ioc=e.getPoint();
                Point translate=new Point(ioc.x-pressed.x,ioc.y-pressed.y);
                Graphics g=getGraphics();
                try
                {
                    g.clearRect(0,0,getSize().width,getSize().height);
                    g.translate(translate.x,translate.y);
                    showStatus("Translate Graphics:"+translate);
                    g.drawImage(img,0,0,TranslateTest.this);
                }
                finally
                {
                    g.dispose();
                }
                lastTranslate=translate;
            }
        });
    }
    public void paint(Graphics g)
    {
        g.drawImage(img,0,0,this);
    }
} 

呵呵，你多写了几个字符：
Image img=getImage(getCodeBase(),"pure.gif");
我想应该是 img=getImage(getCodeBase(),"pure.gif");
吧。
错误的意思是img如果在内嵌类里使用，必须定义为final。也就是说应该类里，而不是函数init里。