Create a random string by a given length and characters

StringGenerator.java

public class StringGenerator {
  
    public static String createRandomString(int length, String characters) {
        StringBuilder stringBuilder = new StringBuilder(length);
        for (int i = 0; i < length; i++) {
            int index = (int) (characters.length() * Math.random());
            stringBuilder.append(characters.charAt(index));
        }
        return stringBuilder.toString();
    } 
}