NullPointerException on object methods

Question

Hello everybody! I just started learning Java yesterday, and am looking forward to contributing to the forum!
Here is a code I was working on today- it's the start of a simple game of blackjack.
import java.util.Scanner;
class BlackJack {
     public static void main(String[] args) {
          Deck deck = new Deck();
          System.out.print("Enter your name > ");
          Scanner in = new Scanner(System.in);
          String name = in.nextLine();
          Hand phand = new Hand(name);
          phand.addCards(2,deck);
          Hand dhand = new Hand("Dealer");
          dhand.addCards(2,deck);
          System.out.println("Dealer: "+dhand.readHand(true));
          System.out.println("You:    "+phand.readHand());
          System.out.println(phand.sum);
}Then here is the Deck class:
import java.util.Random;
public class Deck {
     private int[] cards;
     public int getCard() {
          Random generator=new Random();
          int randomInt, out;
          do {randomInt=generator.nextInt(52);}
          while (cards[randomInt]==0);
          out=cards[randomInt];
          cards[randomInt]=0;
          return out;
     public void shuffle() {
          int[] newCards={1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5,6,6,6,6,7,7,7,7,8,8,8,8,9,9,9,9,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10};
          cards=newCards;
     public Deck() {
          int[] cards = {1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5,6,6,6,6,7,7,7,7,8,8,8,8,9,9,9,9,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10};
}And here is the Hand class:
public class Hand {
     private int[] cards;
     private int sum;
     public String name;
     public void addCards(int numCards, Deck deck) {
          for (int i=0;i<numCards;i++) {
               int newCard=deck.getCard();
               cards[cards.length]=newCard;
               sum+=newCard;
     public int getSum() {
          return sum;
     public String readHand(boolean isHidden) {
          String out="";
          if (isHidden) {
               out=cards[0]+" x";
          else {
               for (int i=0;i<cards.length;i++) {
                    out+=cards[i]+" ";
          return out;
     public String readHand() {
          return readHand(false);
     public Hand(String newName) {
          cards=new int[10];
          sum=0;
          name=newName;
}Now I'm getting this runtime error:
Exception in thread "main" java.lang.NullPointerException
        at Deck.getCard(Deck.java:8)
        at Hand.addCards(Hand.java:7)
        at BlackJack.main(BlackJack.java:9)I found online that the NullPointerException means that the object I'm calling is null, but I can't figure out why. I assume it's a simple thing that I overlooked (or haven't learned yet), but the curious thing is when I commented out the lines which caused the exception, and made the sum from the hand class public, I could use it fine. Any help is greatly appreciated!
-seveneightn9ne

Answer 1

Now compiling Deck results in these errors:
Deck.java:7: cannot find symbol
symbol  : variable cards
location: class Deck
        cards = new int[] {1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5,6,6,6,6,7,7,7,7,8,8,8,8,9,9,9,9,10,10,10,
10,10,10,10,10,10,10,10,10,10,10,10,10};
        ^
Deck.java:13: cannot find symbol
symbol  : variable cards
location: class Deck
                while (cards[randomInt]==0);
                       ^
Deck.java:14: cannot find symbol
symbol  : variable cards
location: class Deck
                out=cards[randomInt];
                    ^
Deck.java:15: cannot find symbol
symbol  : variable cards
location: class Deck
                cards[randomInt]=0;
                ^
Deck.java:20: cannot find symbol
symbol  : variable cards
location: class Deck
                cards=newCards;
                ^
5 errorsHere is what Deck currently looks like:
import java.util.Random;
public class Deck {
     public Deck() {
     cards = new int[] {1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5,6,6,6,6,7,7,7,7,8,8,8,8,9,9,9,9,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10};
     public int getCard() {
          Random generator=new Random();
          int randomInt, out;
          do {randomInt=generator.nextInt(52);}
          while (cards[randomInt]==0);
          out=cards[randomInt];
          cards[randomInt]=0;
          return out;
     public void shuffle() {
          int[] newCards={1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4,5,5,5,5,6,6,6,6,7,7,7,7,8,8,8,8,9,9,9,9,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10};
          cards=newCards;
}Is it correct that cards is not declared outside the constructor, or is that the cause of the error?