How to generate random characters without repeating?

I want the function to generate random characters from A-Z without repeating if it's called again. For example, i called it in my main first and it generates an A character. If i called it the second time, it shouldn't generate A again. How to do it?

public static void random(){
    Random r = new Random();
    char c = (char)(r.nextInt(26) + 'A');
    System.out.print(c);
}

If this helps, what i'm planning is to put it on a hashmap as a key and it shouldn't have the same characters.