获取四位随机字符串：
public class ToolUtil {
public static void main(String[] args) {
Random random = new Random();
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 4; i++) {
// 获取ascii码中的字符 数字48-57 小写65-90 大写97-122
int range = random.nextInt(75)+48;
range = range<97?(range<65?(range>57?114-range:range):(range>90?180-range:range)):range;
sb.append((char)range);
}
System.out.println("随机码为：" + sb);
}
}
