public static String random()

in src/main/java/org/apache/commons/lang3/RandomStringUtils.java [192:266]
64 lines of code
30 McCabe index (conditional complexity)

    public static String random(int count, int start, int end, final boolean letters, final boolean numbers,
                                final char[] chars, final Random random) {
        if (count == 0) {
            return StringUtils.EMPTY;
        }
        if (count < 0) {
            throw new IllegalArgumentException("Requested random string length " + count + " is less than 0.");
        }
        if (chars != null && chars.length == 0) {
            throw new IllegalArgumentException("The chars array must not be empty");
        }

        if (start == 0 && end == 0) {
            if (chars != null) {
                end = chars.length;
            } else if (!letters && !numbers) {
                end = Character.MAX_CODE_POINT;
            } else {
                end = 'z' + 1;
                start = ' ';
            }
        } else if (end <= start) {
            throw new IllegalArgumentException("Parameter end (" + end + ") must be greater than start (" + start + ")");
        }

        final int zeroDigitAscii = 48;
        final int firstLetterAscii = 65;

        if (chars == null && (numbers && end <= zeroDigitAscii
                || letters && end <= firstLetterAscii)) {
            throw new IllegalArgumentException("Parameter end (" + end + ") must be greater then (" + zeroDigitAscii + ") for generating digits " +
                    "or greater then (" + firstLetterAscii + ") for generating letters.");
        }

        final StringBuilder builder = new StringBuilder(count);
        final int gap = end - start;

        while (count-- != 0) {
            final int codePoint;
            if (chars == null) {
                codePoint = random.nextInt(gap) + start;

                switch (Character.getType(codePoint)) {
                case Character.UNASSIGNED:
                case Character.PRIVATE_USE:
                case Character.SURROGATE:
                    count++;
                    continue;
                }

            } else {
                codePoint = chars[random.nextInt(gap) + start];
            }

            final int numberOfChars = Character.charCount(codePoint);
            if (count == 0 && numberOfChars > 1) {
                count++;
                continue;
            }

            if (letters && Character.isLetter(codePoint)
                    || numbers && Character.isDigit(codePoint)
                    || !letters && !numbers) {
                builder.appendCodePoint(codePoint);

                if (numberOfChars == 2) {
                    count--;
                }

            } else {
                count++;
            }
        }
        return builder.toString();
    }