Java - generate random string from letters and digits of size 10

1. Generate random string with ThreadLocalRandom

public static String randomString(int len) {
	String alphabet =
			"ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
			"abcdefghijklmnopqrstuvwxyz" +
			"0123456789";

	StringBuilder b = new StringBuilder();

	for (int i = 0; i < len; i++) {
		int randIdx = ThreadLocalRandom.current().nextInt(alphabet.length());
		char randChar = alphabet.charAt(randIdx);
		b.append(randChar);
	}

	return b.toString();
}

Example:

System.out.println(randomString(10)); // oBmUYZ3MIg
System.out.println(randomString(10)); // 8gGOjO5Ga2
System.out.println(randomString(10)); // RVrvgA9Okd

Generated random text will contains:

• upper letters: A-Z
• lower case letters: a-z
• digits: 0-9