Question

I am going to show you my code and for some reason when I addDomino to front, my program gives me a null value, I need help figuring out why.  

public void addDominoFront(String d){
        String temp1 = Domino[0];
        String temp2;
        Domino[0] = d;
        for(int i = 1; i< numDominoes; i++){
            temp2 = Domino[i];
            Domino[i] = temp1;
            temp1 = temp2;
        }
        numDominoes = numDominoes +1;
    }

The I have the following code and it will not work addtoFront adds the value to front, but a null value to back

package assign1;
import javax.swing.JOptionPane;

public class Assign1 {

    public static void main(String[] args) {
        DominoGroup OriginalSet;
        DominoGroup PlayerSet;
        DominoGroup ComputerSet;
        DominoGroup GameSet;
        Domino CompareDomino1;
        Domino CompareDomino2;
        String RemovedDomino;
        int PlayerNumber;
        int ComputerNumber;
        int GameNumber;
        String HoldDominoes;
        String DominoOptions;
        String Game;
        int Counter;
        String StringCounter;
        int PlayerDomino;
        String Side;
        String Swap;
        char SideChar;
        char DominoNum1;
        char DominoNum2;
        int Num1;
        int Num2;
        OriginalSet = new DominoGroup();
        PlayerSet = new DominoGroup();
        ComputerSet = new DominoGroup();
        GameSet = new DominoGroup();
        CompareDomino1 = new Domino();
        CompareDomino2 = new Domino();
        OriginalSet.makeDominoSet();
        OriginalSet.mix();
        for ( int i = 0; i<7; i++){
            RemovedDomino = OriginalSet.removeDomino(0);
            PlayerSet.addDominoBack(RemovedDomino);
            RemovedDomino = OriginalSet.removeDomino(0);
            ComputerSet.addDominoBack(RemovedDomino);
        }
        JOptionPane.showMessageDialog(null, "Welcome to Dominoes."+ "\n"
                                       + "You will lie the first domino.");
        PlayerNumber = PlayerSet.getNumDominoes();
        ComputerNumber = ComputerSet.getNumDominoes();
        while ((PlayerNumber !=0) && (ComputerNumber != 0)){
            Counter = 0;
            DominoOptions = "";
            for ( int i = 0; i< PlayerNumber; i++){
                Counter++;
                StringCounter = String.valueOf(Counter)+ ". ";
                HoldDominoes = PlayerSet.getDomino(i);
                DominoOptions = DominoOptions +StringCounter + HoldDominoes
                                + "\n";
            }
        Counter++;
        StringCounter = String.valueOf(Counter)+ ". Pass";
        DominoOptions = DominoOptions + StringCounter;
        Game = GameSet.toString();
        System.out.println(Game);
        PlayerDomino = Integer.parseInt(JOptionPane.showInputDialog(" Please "
                + "pick the domino to be laid"+ "\n" + Game + "\n"
                +DominoOptions));
        RemovedDomino = PlayerSet.removeDomino(PlayerDomino-1);
        GameNumber = GameSet.getNumDominoes();
        if (GameNumber == 0){
            GameSet.addDominoBack(RemovedDomino);
        }
        else GameSet.addDominoFront(RemovedDomino);
        PlayerNumber = PlayerSet.getNumDominoes();
    }
    }
    }

Answer 1

public void addDominoFront(String d){
    // lets suppose array was of size 4 and containing
    // 1, 2, 3, 4
    // now you are trying to insert d=0 at front

    String temp1 = Domino[0];
    // temp1 = 1

    String temp2;
    Domino[0] = d;
    // array becomes 0,2,3,4

    // now you loop from i=1 to i=3 (i < 4)..
    // and you shift the elements one by one..
    // after i=1
    // 0,1,3,4, temp1 = 2
    // after i=2
    // 0,1,2,4, temp1 = 3
    // after i=3
    // 0,1,2,3, temp1 = 4
    for(int i = 1; i< numDominoes; i++){
        temp2 = Domino[i];
        Domino[i] = temp1;
        temp1 = temp2;
    }

    // now you increment the number of dominoes to 5
    // which gives you an idea that the 5 elements have been put
    // while in reality, your loop just kept 4 elements only..
    // and at 5th position, the null value remained
    numDominoes = numDominoes +1;
}



// the correct code would be:
public void addDominoFront(String d){
    String temp1 = Domino[0];
    String temp2;
    Domino[0] = d;
    numDominoes = numDominoes +1;
    for(int i = 1; i< numDominoes; i++){
        temp2 = Domino[i];
        Domino[i] = temp1;
        temp1 = temp2;
    }
}

Hi. please find the code.. i have given code comments so that it is very easy for you to understand the flow. In case of any doubts, please ask in comments. If the answer helps you, please upvote. Thanks!