java产生随机字符串

　　private static Random randGen = null;
private static char[] numbersAndLetters = null;

public static final String randomString(int length) {
         if (length < 1) {
             return null;
         }
         if (randGen == null) {
                randGen = new Random();
                numbersAndLetters = ("0123456789abcdefghijklmnopqrstuvwxyz" +
                   "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                  //numbersAndLetters = ("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                 }
         char [] randBuffer = new char[length];
         for (int i=0; i<randBuffer.length; i++) {
             randBuffer[i] = numbersAndLetters[randGen.nextInt(71)];
          //randBuffer[i] = numbersAndLetters[randGen.nextInt(35)];
         }
         return new String(randBuffer);
}

//调用此方法randomString(int),int是字符串的长度，即可产生指定长度的随机字符串。