Java 我不知道'；“我不知道这个错误的原因”；BoxLayout"；在爪哇

当我试图编译下面的程序（直接取自教科书）时，出现以下错误。该程序在GUI中创建滑块组件。布局为BoxLayout。程序没有创建任何box对象，但调用类“box”上的“createRigidArea”方法。我想知道错误的原因，以及可以做些什么来修复它

SlideColorPanel.java:64: error: cannot find symbol
      controls.add(Box.createRigidArea (new Dimension (0, 20)));
                      ^
  symbol:   method createRigidArea(Dimension)
  location: class Box
SlideColorPanel.java:67: error: cannot find symbol
      controls.add(Box.createRigidArea (new Dimension (0, 20)));
                      ^
  symbol:   method createRigidArea(Dimension)
  location: class Box
2 errors






//********************************************************************
//  SlideColorPanel.java       
//********************************************************************

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


public class SlideColorPanel extends JPanel
{
   private JPanel controls, colorPanel;
   private JSlider rSlider, gSlider, bSlider;
   private JLabel rLabel, gLabel, bLabel;



   //-----------------------------------------------------------------
   //  Sets up the sliders and their labels, aligning them along
   //  their left edge using a box layout.
   //-----------------------------------------------------------------
   public SlideColorPanel()
   {
      rSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
      rSlider.setMajorTickSpacing(50);
      rSlider.setMinorTickSpacing(10);
      rSlider.setPaintTicks(true);
      rSlider.setPaintLabels(true);
      rSlider.setAlignmentX(Component.LEFT_ALIGNMENT);

      gSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
      gSlider.setMajorTickSpacing(50);
      gSlider.setMinorTickSpacing(10);
      gSlider.setPaintTicks(true);
      gSlider.setPaintLabels(true);
      gSlider.setAlignmentX(Component.LEFT_ALIGNMENT);

      bSlider = new JSlider(JSlider.HORIZONTAL, 0, 255, 0);
      bSlider.setMajorTickSpacing(50);
      bSlider.setMinorTickSpacing(10);
      bSlider.setPaintTicks(true);
      bSlider.setPaintLabels(true);
      bSlider.setAlignmentX(Component.LEFT_ALIGNMENT);
      
      SliderListener listener = new SliderListener();
      rSlider.addChangeListener(listener);
      gSlider.addChangeListener(listener);
      bSlider.addChangeListener(listener);

      rLabel = new JLabel("Red: 0");
      rLabel.setAlignmentX(Component.LEFT_ALIGNMENT);
      gLabel = new JLabel("Green: 0");
      gLabel.setAlignmentX(Component.LEFT_ALIGNMENT);
      bLabel = new JLabel("Blue: 0");
      bLabel.setAlignmentX(Component.LEFT_ALIGNMENT);

      controls = new JPanel();
      BoxLayout layout = new BoxLayout(controls, BoxLayout.Y_AXIS);
      controls.setLayout(layout);
      controls.add(rLabel);
      controls.add(rSlider);
      controls.add(Box.createRigidArea (new Dimension (0, 20)));      
      controls.add(gLabel);
      controls.add(gSlider);
      controls.add(Box.createRigidArea (new Dimension (0, 20)));
      controls.add(bLabel);
      controls.add(bSlider);

      colorPanel = new JPanel();
      colorPanel.setPreferredSize(new Dimension (100, 100));
      colorPanel.setBackground(new Color (0, 0, 0));

      add(controls);
      add(colorPanel);
   }

   //*****************************************************************
   //  Represents the listener for all three sliders.
   //*****************************************************************
   private class SliderListener implements ChangeListener
   {
      private int red, green, blue;

      //-----------------------------------------------------------------
      //  Gets the value of each slider, then updates the labels and
      //  the color panel.
      //-----------------------------------------------------------------
      public void stateChanged(ChangeEvent event)
      {
         red = rSlider.getValue();
         green = gSlider.getValue();
         blue = bSlider.getValue();

         rLabel.setText("Red: " + red);
         gLabel.setText("Green: " + green);
         bLabel.setText("Blue: " + blue);

         colorPanel.setBackground(new Color(red, green, blue));
      }
   }
}

---

对我来说很好。删除项目中的所有类文件。也许你有一个旧的类文件名“Box.class”。