public static String random(final int count, final char... chars) {
        if (chars == null) {
            return random(count, 0, 0, false, false, null, RANDOM);
        }
        return random(count, 0, chars.length, false, false, chars, RANDOM);
    }
<p>Creates a random string whose length is the number of characters
specified.</p>

<p>Characters will be chosen from the set of characters specified.</p>

@param count the length of random string to create
@param chars the character array containing the set of characters to use,
             may be null
@return the random string
@throws IllegalArgumentException if {@code count} &lt; 0.
android/android_framework/base/src/main/java/lang3/RandomStringUtils.java