How do I create a string of random characters with a length determined by a user?

Question

I keep getting a "StringIndexOutOfBoundsException" everytime I run the program. But I need the string to be flexible because the length of the string of characters is determined by a user, therefore the string can't be fixed. I then need to display the complete string in a textfield.

private static final char CL = getRandomCapitalLetter(),  ML = getRandomMinusculeLetter(),
         NC = getRandomNumericCharacter(), SC = getRandomSpecialCharacter();

I'm storing my generators in an array to later be used to build a string. Each method outputs one character.

// Generate string with all character types
public static String generateAll(){
    // Use StringBuilder to build a string of characters
    StringBuilder myCharacters = new StringBuilder();
    char[] methods = {CL, ML, NC, SC};
    int index = (int)(Math.random() * 4);
    
    for (int i = 0; i < getLength(); i++){
        myCharacters.setCharAt(i, methods[index]); // This is the line where problem arises
    }
    String charString = myCharacters.toString();
    return charString;
}

Here's the exception:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0

at java.lang.AbstractStringBuilder.setCharAt(Unknown Source)

at java.lang.StringBuilder.setCharAt(Unknown Source)