How to generate a random alphanumeric string of a custom length in java?

Question

What is the most simple way? Minimizing any imports.

This one is good:

String str = Long.toHexString(Double.doubleToLongBits(Math.random()));

But it's not perfect, for example it complicates with custom length.

Also an option: How to make this String unique?

• nanofarad: Why don't you use string-handling functions? Just generate a random character in your desired range, in a loop, and keep concatenating it onto the string you're building. Also, why do you want to minimize imports?
• Daniel M.: There are no prizes for minimizing imports. It doesn't affect the time it takes to run, or even really the time to compile.
• San: @hexafraction, yes, you are right, your method is rational. DanielM. thanks, I will notice that. But question is likely applied puzzle.

Answer 1

Create a String of the characters which can be included in the string:

String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

Generate an integer using the Random class and use it to get a random character from the String.

Random random = new Random();
alphabet.charAt(random.nextInt(alphabet.length()));

Do this n times, where n is your custom length, and append the character to a String.

StringBuilder builder = new StringBuilder(n);
for (int i = 0; i < n; i++) {
 builder.append(/* The generated character */);
}

Together, this could look like:

private static final String ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
public String generateString(int length) {
 Random random = new Random();
 StringBuilder builder = new StringBuilder(length);
 for (int i = 0; i < length; i++) {
 builder.append(ALPHABET.charAt(random.nextInt(ALPHABET.length())));
 }
 return builder.toString();
}

Answer 2

RandomStringUtils from commons-lang. If you don't wish to import, check out its source.