Why does my program keep dealing duplicate cards? (JAVA)

so for a homework task I have to make a program that deals N(command line input) amount of poker decks (5 cards). I have a for-loop that checks if a card has been used, but nonetheless duplicate cards get printed. Any help would be greatly appreciated.

Code that checks for duplicates

do {///check for dupes
                    bUsed = false;
                    randS = (int) (Math.random() * 4);
                    randV = (int) (Math.random() * 13);
                   
                    for (int k = 0; k < 52; k++) {
                    value[randV]);
                        if ((used[k] == (suit[randS] + value[randV])) && (used[k] != null)) {

                            bUsed = true;
                        }
                    }


                } while (bUsed);///end check

Full code

public class Deal {
public static void main(String[] args) {
    String[] suit = {"Hearts", "Diamonds", "Spades", "Clubs"};
    String[] value = {"A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K"};

    String[] used = new String[52];
    boolean bUsed = false;
    int usedC = 0;

    int N = Integer.parseInt(args[0]);
    // int N = 4;

    if (N <= 10) {
        /////////check for max
        for (int i = 0; i < N; i++) {/////////print N amount of decks
            System.out.println("Deck " + (i + 1));
            for (int j = 0; j < 5; j++) {/////////print 5 cards

                int randV = 0;
                int randS = 0;

                do {///check for dupes
                    bUsed = false;
                    randS = (int) (Math.random() * 4);
                    randV = (int) (Math.random() * 13);
                    
                    for (int k = 0; k < 52; k++) {
                       
                        if ((used[k] == (suit[randS] + value[randV])) && (used[k] != null)) {

                            bUsed = true;
                        }
                    }


                } while (bUsed);///end check

                used[usedC] = suit[randS] + value[randV];
                System.out.println(suit[randS] + " " + value[randV]);

                usedC++;
            }////end print 5 cards

            System.out.println();
        }///end print amount of decks
    } else {
        System.out.println("Too many decks requested");
    }
}

}

Example of output

java Deal.java 5
Deck 1 Clubs 6 Clubs 10 Clubs J Hearts Q Spades J

Deck 2 Hearts 4 Hearts 7 Hearts A Clubs J Diamonds 3

Deck 3 Spades K Diamonds 3 Diamonds Q Diamonds 4 Clubs 2

Deck 4 Spades 7 Diamonds 7 Diamonds 3 Clubs A Diamonds 9

Deck 5 Clubs 2 Spades 4 Diamonds 4 Diamonds 3 Spades 7

>Solution :

You are comparing Strings with == instead of .equals().

This is the problematic line:

if ((used[k] == (suit[randS] + value[randV])) && (used[k] != null))

First of all you need to flip the null-check and the value-check, to prevent a NullPointerException, since .equals() is a method call (for the operator == this would not matter). Then use the method for comparison, like:

if ((used[k] != null) && used[k].equals(suit[randS] + value[randV]))

Besides that you would make your life a lot easier, if you used a List<String> for used instead of an array, because there you could then simply use the .contains method instead of your own loop – but I don’t know if that is allowed for your homework.

To make your program a little bit faster, you can add a break; statement at the end inside the if-block, as you can break out the for-loop as soon as you find a duplicate.