Java arraylist all items are the same

Question

Why are all the cards in the list the same ? I tried hand.add(i,card); but output is still the same.

Main.java

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class Main {

    public static void main(String[] args) {
        List<Card> hand = new ArrayList();
        Card card = new Card((short) 7,"red");
        int b = 0;
        Random rn = new Random();
        for (int i=0;i<7;i++){
            card.setValue((short) (rn.nextInt((14 - 7) + 1) + 7));
            b = rn.nextInt(4);
            String[] colors = {"green","red","gold","brown"};
            card.setColor(colors[b]);
            System.out.println("Adding card to hand: " + card.getColor() + card.getValue() + " to: " +i);
            hand.add(card);
        }
        System.out.println("Your cards: ");
        for (Card k: hand) {
            System.out.println(k.show());
        }


    }
}

Card.java

public class Card {
    public short getValue() {
        return value;
    }

    public void setValue(short value) {
        this.value = value;
    }

    short value;

    public String getColor() {
        return color;
    }

    public void setColor(String farba) {
        this.color = farba;
    }

    String color;

    public Card(short value, String color) {
        this.value = value;
        this.color = color;
    }

    public String show(){
        return color + value;
    }
}

Output:

/usr/lib/jvm/java-1.8.0-openjdk-amd64/bin/java
Adding card to hand: red13 to: 0
Adding card to hand: green10 to: 1
Adding card to hand: gold8 to: 2
Adding card to hand: brown10 to: 3
Adding card to hand: gold10 to: 4
Adding card to hand: gold8 to: 5
Adding card to hand: gold7 to: 6
Your cards: 
gold7
gold7
gold7
gold7
gold7
gold7
gold7

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean nisl.Curabitur ac arcu ornare, aliquet eros eu, pretium massa.

Vasu · Accepted Answer · 2016-11-19 09:30:59Z

3

You are using the same Card object for all items in the list, but you need different objects (each Card object holds it's own color and value), so you have to create the new Card inside the for loop as shown below:

    int b = 0;
    Random rn = new Random();
    for (int i=0;i<7;i++){

        b = rn.nextInt(4);
        String[] colors = {"green","red","gold","brown"};

        Card card = new Card((short) (rn.nextInt((14 - 7) + 1) + 7),colors[b]);

        System.out.println("Adding card to hand: " + 
             card.getColor() + card.getValue() + " to: " +i);

        hand.add(card);
    }

edited Nov 19, 2016 at 9:30

answered Nov 19, 2016 at 9:25

Vasu

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Comments

henrybbosa · Accepted Answer · 2016-11-19 09:25:28Z

Because You are using one card object that you create before the loop. change it to:

public static void main(String[] args) {
        List<Card> hand = new ArrayList();

        int b = 0;
        Random rn = new Random();
        for (int i=0;i<7;i++){

            Card card = new Card((short) 7,"red"); //changed

            card.setValue((short) (rn.nextInt((14 - 7) + 1) + 7));
            b = rn.nextInt(4);
            String[] colors = {"green","red","gold","brown"};
            card.setColor(colors[b]);
            System.out.println("Adding card to hand: " + card.getColor() + card.getValue() + " to: " +i);
            hand.add(card);
        }
        System.out.println("Your cards: ");
        for (Card k: hand) {
            System.out.println(k.show());
        }


    }

so that after every iteration a new card object is created

ablurb · Accepted Answer · 2016-11-19 09:39:18Z

as it was answered before, your issue occurred because you were adding the same card to the list all the time. What you did was to change the state of the object and add the reference to the same object to the list every time, so with each change, all the references to the same object will show the same new value.

What you need to do in this case, is to make sure that you are creating a new card each time, and that the reference to the new card will never change within the for loop, to avoid unexpected overwritings.

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class HelloWorld {

    public static void main(String[] args) {
        List<Card> hand = new ArrayList();
        int b = 0;
        Random rn = new Random();
        for (int i=0;i<7;i++){
            final Card card = new Card((short) 7,"red");
            card.setValue((short) (rn.nextInt((14 - 7) + 1) + 7));
            b = rn.nextInt(4);
            String[] colors = {"green","red","gold","brown"};
            card.setColor(colors[b]);
            System.out.println("Adding card to hand: " + card.getColor() + card.getValue() + " to: " +i);
            hand.add(card);
        }
        System.out.println("Your cards: ");
        for (Card k: hand) {
            System.out.println(k.show());
        }
    }
}

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