Array of Object of Arrays [duplicate]

Question

Basically I'm trying to create an array of objects, that has two arrays inside of it, however when I try and populate these I keep getting a null pointer, but when I populate the object without making an array of objects it seems to work.

And I keep getting --> Exception in thread "main" java.lang.NullPointerException

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;

public class CardTrick{
    int[] combo;
    int[] cardTeller;

    public CardTrick() 
    {
        combo = new int[3];
        for (int j = 0; j < 3; j++) {
            combo[j] = 0;
        }
        cardTeller = new int[2];  
        for (int j = 0; j < 2; j++) {
            cardTeller[j] = 0;
        }   
    }

    public CardTrick(CardTrick c) { // notice the parameter is an object

        combo = new int[3];
        for (int j = 0; j < 3; j++) {
            combo[j] = 0;
        }
        cardTeller = new int[2];  
        for (int j = 0; j < 2; j++) {
            cardTeller[j] = 0;
        }   

    }
    public void whatCards(){
        int num = 0;
        CardTrick[] list = new CardTrick[56];

        list[0].combo[1] = 1;
        list[0].combo[2] = 1;
        list[0].cardTeller[0] = 1;
        list[0].cardTeller[1] = 1;

        System.out.println(list[0].combo[1]);

    }

    public static void main(String[] args){
        CardTrick ct = new CardTrick();
        ct.whatCards();
    } 
}

Can you share the stack trace as well?

Akin Okegbile
– Akin Okegbile

2019-12-03 19:14:59 +00:00
Commented Dec 3, 2019 at 19:14 — Akin Okegbile
– Akin Okegbile, Commented Dec 3, 2019 at 19:14
@flakes I'll try to edit it

FailingCoder
– FailingCoder

2019-12-03 20:36:25 +00:00
Commented Dec 3, 2019 at 20:36 — FailingCoder
– FailingCoder, Commented Dec 3, 2019 at 20:36

Michał Krzywański · Accepted Answer · 2019-12-04 07:45:55Z

3

The problem is in these lines of code

CardTrick[] list = new CardTrick[56];

list[0].combo[1] = 1;

You created an array of objects, but your objects themselves aren't instantiated thus the NullPointerException is thrown.

You have to instantiate each cell of the array before accessing it

CardTrick[] list = new CardTrick[56];
list[0] = new CardTrick();

list[0].combo[1] = 1;

edited Dec 4, 2019 at 7:45

Michał Krzywański

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answered Dec 3, 2019 at 19:17

b.GHILAS

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3 Comments

FailingCoder Over a year ago

Perfect explanation. Don't forget you can also use Arrays.fill(list, new CardTrick())

brimborium Over a year ago

@KnowNoTrend Won't that just fill the entire array with exactly the same object (which is most probably not what you want in that case)?

FailingCoder Over a year ago

@brimborium they don't randomize the cards. they'll be the same anyway.

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