随机数算法，带英文和数字的

public static String getRandomEnglishNumber(int length) {
if (length <= 0) {
return "";
}
StringBuffer str = new StringBuffer();
Random r = new Random();
for (int i = 0; i < length; i++) {
int type = r.nextInt(3);
if (type == 0) {
str.append((char)(48 + r.nextInt(10)));
} else if (type == 1) {
str.append((char)(97 + r.nextInt(26)));
} else if (type == 2) {
str.append((char)(65 + r.nextInt(26)));
}
}

return str.toString();
}