Demo

package com.di.action;

import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;

public class ss {
public static void main(String args[]) throws NumberFormatException,
IOException {
/*
* 求平方
*/
StringBuffer sb = new StringBuffer();
File file = new File("ss");

System.out.print("请输入您要求的数字：");
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int len;
char c = 0;
len = Integer.parseInt(br.readLine());
System.out.println(len * len);

// 把大写转化为小写
System.out.print("请输入您要求的字母：");
BufferedReader br1 = new BufferedReader(
new InputStreamReader(System.in));
String str = br1.readLine();
// 第一种方法
// String s = con(str);
// 第二种方法
String s = StringChange(str);
System.out.println(s);

//   判断是否是偶数
  System.out.print("请输入您要判断的数字：");
  BufferedReader br2=new  BufferedReader(new InputStreamReader(System.in));
  String str2=br2.readLine();
  int len2=Integer.valueOf(str2);
  if(len2%2==0){
  System.out.println("输入的"  +  len2 +"是偶数");
  }else{
       System.out.println("输入的" +  len2 +"是基数");
  }
 
//  用while循环打印九九乘法表
   int i=1;
   int j=1;
   while(i<=9){
   System.out.println("i*j="+i*j);
   if(i<j)
     j++;
   System.out.println();
   }
 
 

  /*
   *  交换数字
   */
// int a=10;
// int b=19;
// int temp;
// temp = a;
// a=b;
// b=temp;
// System.out.println("a = "+ a);
// System.out.println("b = "+ b);

}

private static String con(String str) {
char[]  s=str.toCharArray();
for (int i = 0; i < s.length; i++) {
if(s[i]>=65&&s[i]<=90){
   s[i]=(char)(s[i]+32);
}else if(s[i]<=122&&s[i]>=97){
       s[i]=(char)(s[i]-32);
}
}
  return String.valueOf(s);
}


public static String StringChange(String s)
    {
        StringBuilder sb=new StringBuilder(s);
        String  temp = null;
        for (int i = 0; i < s.length(); i++)
        {
            if(Character.isLowerCase(s.charAt(i)))
            {
                temp=s.substring(i, i+1).toUpperCase();//把i这个位置小写的字母变成大写并放进temp
                sb.replace(i, i+1, temp);  //在字符串i这个位置替换temp
            }else if(Character.isUpperCase(s.charAt(i)))
            {
                temp=s.substring(i, i+1).toLowerCase();
                sb.replace(i, i+1, temp);
            }
        }
        return sb.toString();
    }  
}