Jakarta Commons学习笔记之RandomStringUtils常用方法和实现

1. 创建
    静态类
2. public static String random(int count, int start, int end, boolean letters, boolean numbers,                                   char[] chars, Random random)
    用途：产生根据参数描述的随机字符串。
    参数：count，产生的随机字符长度；start，随机字符选择集的起始位置；end，随机字符选择集的结束位置； letters，是否全为字符； numbers，是否全为数字；chars，字符选择集合；random，随机
    实现：
    public static String random(int count, int start, int end, boolean letters, boolean numbers,
                                char[] chars, Random random) {
        if (count == 0) {
            return "";
        } else if (count < 0) {
            throw new IllegalArgumentException("Requested random string length " + count + " is less than 0.");
        }
        if ((start == 0) && (end == 0)) {
            end = 'z' + 1;
            start = ' ';
            if (!letters && !numbers) {
                start = 0;
                end = Integer.MAX_VALUE;
            }
        }

        char[] buffer = new char[count];
        int gap = end - start;

        while (count-- != 0) {
            char ch;
            if (chars == null) {
                ch = (char) (random.nextInt(gap) + start);
            } else {
                ch = chars[random.nextInt(gap) + start];
            }
            if ((letters && Character.isLetter(ch))
                || (numbers && Character.isDigit(ch))
                || (!letters && !numbers))
            {
                if(ch >= 56320 && ch <= 57343) {
                    if(count == 0) {
                        count++;
                    } else {
                        // low surrogate, insert high surrogate after putting it in
                        buffer[count] = ch;
                        count--;
                        buffer[count] = (char) (55296 + random.nextInt(128));
                    }
                } else if(ch >= 55296 && ch <= 56191) {
                    if(count == 0) {
                        count++;
                    } else {
                        // high surrogate, insert low surrogate before putting it in
                        buffer[count] = (char) (56320 + random.nextInt(128));
                        count--;
                        buffer[count] = ch;
                    }
                } else if(ch >= 56192 && ch <= 56319) {
                    // private high surrogate, no effing clue, so skip it
                    count++;
                } else {
                    buffer[count] = ch;
                }
            } else {
                count++;
            }
        }
        return new String(buffer);
    }