import java.awt.event.*;
import java.awt.*;
import java.applet.Applet; 
public class Relative extends Applet implements ActionListener
{
  Label s ;
  TextField input;
  Button btn;
  public void init()
     {
       s = new Label("请输入一个字母！");
       input = new TextField(4);
       btn = new Button("确定");
       add(s); 
       add(input);
       add(btn);
       btn.addActionListener(this);

     }
  public void paint (Graphics g)
     {
         int n=0;
         char c = ' ';
         n=Integer.parseInt(input.getText());
     
         c = input.getText();
         if(c>97)
         {
           n=97+(122-n);
           c=(char)n;
           g.drawString("对应的字母为："+c,200,200);
         }
       else if(n>65&&n<90)
              {
               n = 65+(90-n);
              c = (char)n;
               g.drawString("对应的字母为："+c,200,200);
              }
     }
  public void actionPerformed(ActionEvent e)
            {
             if (e.getSource()==btn)
             repaint();
            }
}

    //n=Integer.parseInt(input.getText());
    //c = input.getText();
    String s = input.getText();  // read a String, not character
    if (! "".equals(s)) {
      c = s.charAt(0);  // take the first character
    }

    if (c >= 97) { //>= 97
      c = (char) (97 + (122 - c));  // character would be casted to int for integer arithmetic
      g.drawString("对应的字母为：" + c, 200, 200); // resize the applet window or try 50,50 if you can't see the text
    } else if (c >= 65 && c <= 90) { //>= 65 && <=90
      c = (char) (65 + (90 - c));
      g.drawString("对应的字母为：" + c, 200, 200);
    }

茂珊 wrote:
下次写上注释就是了。。。
不过 c = input.getText(); 不能默认将读入的第一个字符 赋给 c 吗？
还是Java 中就不能读入 字符阿？

package org.jeason.test1;
import java.io.*;

public class testfunction {
  
  public static void main(String[] args){
      
        System.out.println("请输入一个字母...");
        try{
          System.out.println(new testfunction().charChangeFunction(System.in.read()));
        } catch (Exception ex){
          ex.printStackTrace();
        }
  }
  
  String charChangeFunction(int charInt) throws Exception{
    String result = "输入了非字母";
    char[] letterArr = new char[26];
    for (int i = 0; i < letterArr.length; i++)
      letterArr[i] =(char)(65 + i);//大写字符
      
    int postion  = this.charIndexInArray(charInt,letterArr);
    
    if (postion != -1){
      try {
        result = this.changeCharResult(charInt,postion,letterArr);
      } catch (Exception ex){
        throw new Exception(ex.getMessage());
      }
    }
    return result;
  }
  
  int charIndexInArray(int inputInt,char[] arrChar){
    int result = -1;
    if(arrChar != null){
      
      for(int i = 0;i < arrChar.length; i++){
        if (inputInt <= (int)('Z')){
          if (inputInt == (int)arrChar[i]){
            result = i;
            break;
          }
        } else {
          if (inputInt - 32 == (int)arrChar[i]){
            result = i;
            break;
          }
        }
      }
    }
    
    return result;
  }
  
  String changeCharResult(int inputInt,int postionInt,char[]arrChar)throws Exception{
    if(arrChar != null && inputInt > -1 && postionInt >-1){
      int count = arrChar.length;
      int curIndex = count - postionInt -1;
      char reChar = arrChar[curIndex];
      if (inputInt >= (int)('a'))
        reChar = (char)(((int)reChar) + 32);
      return  String.valueOf(reChar);
    } else
      throw new Exception("输入了非字母");
  }
}

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

public class Applet1 extends Applet implements ActionListener
{
    Label s;
    TextField input;
    Button btn;
    
    public void init()
    {
        s = new Label("pls input an alphabet:");
        input = new TextField(4);
        btn = new Button("confirm");
        add(s);
        add(input);
        add(btn);
        btn.addActionListener(this);
    }

    public void paint(Graphics g)
    {
        int n = 0;
        char c = ' ';

        String s = input.getText(); // read a String, not character

        if (!"".equals(s))
        {
            c = s.charAt(0); // take the first character
            n = (int) c;
        }

        if (n >= 97 && n <= 122)
        {
            n = 122 - (n - 97);
            c = (char) n;
            g.drawString("response alphabet：" + c, 200, 200);
        }
        else if (c >= 65 && c <= 90)
        {
            n = 90 - (n - 65);
            c = (char) n;
            g.drawString("response alphabet：" + c, 200, 200);
        }
    }

    public void actionPerformed(ActionEvent e)
    {
        if (e.getSource() == btn)
        {
            repaint();
        }
    }
}