java - 线程 "AWT-EventQueue-1"java.lang.NullPointerException 中的异常

Question

嘿伙计们，我得到了错误

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
        at P6.itemStateChanged(P6.java:215)
        at javax.swing.AbstractButton.fireItemStateChanged(AbstractButton.java:2
023)
        at javax.swing.AbstractButton$Handler.itemStateChanged(AbstractButton.ja
va:2325)
        at javax.swing.DefaultButtonModel.fireItemStateChanged(DefaultButtonMode
l.java:440)
        at javax.swing.JToggleButton$ToggleButtonModel.setSelected(JToggleButton
.java:255)
        at javax.swing.ButtonGroup.setSelected(ButtonGroup.java:147)
        at javax.swing.JToggleButton$ToggleButtonModel.setSelected(JToggleButton
.java:237)
        at javax.swing.JToggleButton$ToggleButtonModel.setPressed(JToggleButton.
java:272)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonL
istener.java:236)
        at java.awt.Component.processMouseEvent(Component.java:6267)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6032)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577
)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)

        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThre
ad.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.
java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)

        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)

        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

在

import java.awt.Color;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import javax.swing.ButtonGroup;
import javax.swing.JApplet;
import javax.swing.JComboBox;
import javax.swing.JRadioButton;

public class P6 extends JApplet implements MouseListener, MouseMotionListener, ItemListener
{
   private final int RED = 1;
   private final int ORANGE = 2;
   private final int YELLOW = 3;
   private final int GREEN = 4;
   private final int BLUE = 5;
   private final int CYAN = 6;
   private final int PINK = 7;
   private final int MAGENTA =8;
   private final int BLACK = 9;
   private final int ARC = 1;
   private final int LINE = 2;
   private final int OVAL = 3;
   private final int RECT = 4;
   private final int POLY = 5;

   private String[] colorNames = { "Red", "Orange", "Yellow", "Green", "Blue",
   "Cyan", "Pink", "Magenta", "Black"};
   private JComboBox colorComboBox;
   private JRadioButton jrbArc;
   private JRadioButton jrbLine;
   private JRadioButton jrbOval;
   private JRadioButton jrbRect;
   private JRadioButton jrbPolygon;
   private int color = 0;
   private int shape = 0;
   private int x1;
   private int y1;
   private int x2;
   private int y2;

   public void init()
   {
      Container c = getContentPane();
      c.setLayout( new FlowLayout() );
      c.setBackground( Color.orange );
      JComboBox colorComboBox = new JComboBox(colorNames);
      colorComboBox.addItemListener(this);

      JRadioButton jrbArc = new JRadioButton("Arc");
      jrbArc.addItemListener(this);
      JRadioButton jrbLine = new JRadioButton("Line");
      jrbLine.addItemListener(this);
      JRadioButton jrbOval = new JRadioButton("Oval");
      jrbOval.addItemListener(this);
      JRadioButton jrbRect = new JRadioButton("Rectangle");
      jrbRect.addItemListener(this);
          JRadioButton jrbPolygon = new JRadioButton("Polygon");
      jrbPolygon.addItemListener(this);

      ButtonGroup btg = new ButtonGroup();
      btg.add(jrbArc);
      c.add(jrbArc);
      btg.add(jrbLine);
      c.add(jrbLine);
      btg.add(jrbOval);
      c.add(jrbOval);
      btg.add(jrbRect);
      c.add(jrbRect);
      btg.add(jrbPolygon);
      c.add(jrbPolygon);
      c.add(colorComboBox);

      addMouseMotionListener(this);
      addMouseListener(this);

   }

   public void paint(Graphics g)
   {
      super.paint(g);
      g.setColor(Color.white);
      g.fillRect(20, 50, 500, 500);
      g.setFont( new Font("Serif", Font.ITALIC+Font.BOLD, 18));
      g.setColor(Color.black);
      g.drawString("Start Here!", 10, 50);
      g.drawString("End Here!", 510, 560);

      switch (this.color)
      {
         case 1:
           g.setColor(Color.red);
           break;
         case 2:
           g.setColor(Color.orange);
           break;
         case 3:
           g.setColor(Color.yellow);
           break;
         case 4:
           g.setColor(Color.green);
           break;
         case 5:
           g.setColor(Color.blue);
           break;
         case 6:
           g.setColor(Color.cyan);
           break;           
         case 7:
           g.setColor(Color.pink);
           break;
         case 8:
           g.setColor(Color.magenta);
           break;
         case 9:
           g.setColor(Color.black);
           break;
      }

      switch (shape)
          {
        case 1:
          g.fillArc(this.x1, this.y1, this.x2 - this.x1, this.y2 - this.y1, 0, 90);
          g.fillArc(this.x1, this.y1, this.x2 - this.x1, this.y2 - this.y1, 180,90);
          break;
        case 2:
          g.drawLine(this.x1, this.y1, this.x2, this.y2);
          break;
        case 3:
          g.fillOval(this.x1, this.y1, this.x2 - this.x1, this.y2 - this.y1);
          break;
        case 4:
          g.fillRect(this.x1, this.y1, this.x2 - this.x1, this.y2 - this.y1);
          break;
        case 5:
          int[] arr1 = { this.x1, this.x2, this.x1, this.x2, this.x1};
          int[] arr2 = { this.y1, this.y1, this.y2 ,this.y2, this.y1};
          g.fillPolygon(arr1, arr2, 5);
          break;
      }
   }

   public void mousePressed( MouseEvent e)
   {
      this.x1 = e.getX();
      this.y1 = e.getY();
   }

   public void mouseDragged( MouseEvent e)
   {
      this.x2 = e.getX();
      this.y2 = e.getY();
   }

   public void mouseReleased( MouseEvent e)
   {
      this.x2 = e.getX();
      this.y2 = e.getY();
   }

  public void mouseClicked( MouseEvent e)
  {
  }

  public void mouseEntered( MouseEvent e)
  {
  }

  public void mouseExited( MouseEvent e)
  {
  }

  public void mouseMoved( MouseEvent e)
  {
    showStatus( " (" + e.getX() + ", " + e.getY() +")");
  }

  public void itemStateChanged(ItemEvent i)
  {
     if(this.jrbArc.isSelected())
       this.shape = ARC;
     else if( this.jrbLine.isSelected())
       this.shape = LINE;
     else if( this.jrbOval.isSelected())
       this.shape = OVAL;
     else if( this.jrbRect.isSelected())
       this.shape = RECT;
     else if( this.jrbPolygon.isSelected())
       this.shape = POLY;

     this.color = ( this.colorComboBox.getSelectedIndex() + 1 );
     repaint();
   }
}

我一直在努力解决这个问题，但没有成功。有什么帮助吗？

第 215 行是:

if(this.jrbArc.isSelected())

Answer 1

代替

JRadioButton jrbArc = new JRadioButton("Arc");

你需要

jrbArc = new JRadioButton("Arc");

其他控件也有类似的内容。