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代码如下，，，，package drawUs;import javax.swing.*;import javax.swing.event.ListSelectionEimport javax.swing.event.ListSelectionLimport java.awt.*;public class DrawUs {public static void main(String [] args){DrawUs us=new DrawUs();us.go();}JLpublic void go(){JFrame f=new JFrame();f.setSize(800,1000);f.setVisible(true);f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);Panel panel=new Panel();String [] a={&wangbao&,&didi&,&chenxi&,&yanggang&,&tuzi&,&mengmeng&,&487&};list=new JList(a);list.setVisibleRowCount(7);list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);list.addListSelectionListener(new Panel());f.getContentPane().add(BorderLayout.EAST,list);f.getContentPane().add(BorderLayout.CENTER,panel);}class Panel extends JPanel implements ListSelectionListener{public void valueChanged(ListSelectionEvent b){if(!b.getValueIsAdjusting()){repaint();}}public void paintComponent(Graphics g){String selection=&a&;selection=(String)list.getSelectedValue();Iswitch(selection){case &wangbao&: Image image1=new ImageIcon(&wang.jpg&).getImage();simage=image1.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);case &didi&:Image image2=new ImageIcon(&didi.jpg&).getImage();simage=image2.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);case &mengmeng&:Image image3=new ImageIcon(&mengmeng.jpg&).getImage();simage=image3.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);case &tuzi&:Image image4=new ImageIcon(&tuzi.jpg&).getImage();simage=image4.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);case &487&:Image image5=new ImageIcon(&487.jpg&).getImage();simage=image5.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);case &yanggang&:Image image6=new ImageIcon(&yanggang.jpg&).getImage();simage=image6.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);case &chenxi&:Image image7=new ImageIcon(&chenxi.jpg&).getImage();simage=image7.getScaledInstance(this.getWidth(), this.getHeight(),Image.SCALE_DEFAULT);default:Image image8=new ImageIcon(&beijing.jpg&).getImage();simage=image8.getScaledInstance(20, 20,Image.SCALE_DEFAULT);}g.drawImage(simage, 1, 1, this);}}}然而并不能显示，，只有一个空白的窗口，，调整大小之后就报错：Exception in thread &AWT-EventQueue-0& java.lang.NullPointerExceptionat drawUs.DrawUs$Panel.paintComponent(DrawUs.java:38)at javax.swing.JComponent.paint(Unknown Source)at javax.swing.JComponent.paintChildren(Unknown Source)at javax.swing.JComponent.paint(Unknown Source)at javax.swing.JComponent.paintChildren(Unknown Source)at javax.swing.JComponent.paint(Unknown Source)at javax.swing.JLayeredPane.paint(Unknown Source)at javax.swing.JComponent.paintChildren(Unknown Source)at javax.swing.JComponent.paintToOffscreen(Unknown Source)at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)at javax.swing.RepaintManager.paint(Unknown Source)at javax.swing.JComponent.paint(Unknown Source)at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source)at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source)at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)at java.awt.Container.paint(Unknown Source)at java.awt.Window.paint(Unknown Source)at javax.swing.RepaintManager$4.run(Unknown Source)at javax.swing.RepaintManager$4.run(Unknown Source)at java.security.AccessController.doPrivileged(Native Method)at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)at javax.swing.RepaintManager.access$1200(Unknown Source)at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)at java.awt.event.InvocationEvent.dispatch(Unknown Source)at java.awt.EventQueue.dispatchEventImpl(Unknown Source)at java.awt.EventQueue.access$500(Unknown Source)at java.awt.EventQueue$3.run(Unknown Source)at java.awt.EventQueue$3.run(Unknown Source)at java.security.AccessController.doPrivileged(Native Method)at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)at java.awt.EventQueue.dispatchEvent(Unknown Source)at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)at java.awt.EventDispatchThread.pumpEvents(Unknown Source)at java.awt.EventDispatchThread.pumpEvents(Unknown Source)at java.awt.EventDispatchThread.run(Unknown Source)Exception in thread &AWT-EventQueue-0& java.lang.NullPointerExceptionat drawUs.DrawUs$Panel.paintComponent(DrawUs.java:38)at javax.swing.JComponent.paint(Unknown Source)
想买澳洲奶粉？这里有一条包税专线
不要看我的等级，，，我只是个签到党，，，
不是报了空指针异常么
NullPointerException - 空指针引用异常ClassCastException - 类型强制转换异常。IllegalArgumentException - 传递非法参数异常。ArithmeticException - 算术运算异常ArrayStoreException - 向数组中存放与声明类型不兼容对象异常IndexOutOfBoundsException - 下标越界异常NegativeArraySizeException - 创建一个大小为负数的数组错误异常NumberFormatException - 数字格式异常SecurityException - 安全异常UnsupportedOperationException - 不支持的操作异常这些是常见的RunTimeException（运行时异常），也就是说，这些异常不需要强制声明会抛出，或者强制捕捉的异常。
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