Arrays and ArrayLists - Java: How to Program Early Objects - page 261

in order by suit and face. Line 10 invokes myDeckOfCards 's shuffle method to rearrange
the Card objects. Lines 13-20 deal all 52 Card s and print them in four columns of 13
Card s each. Line 16 deals one Card object by invoking myDeckOfCards 's dealCard meth-
od, then displays the Card left justified in a field of 19 characters. When a Card is output
as a String , the Card 's toString method (lines 17-20 of Fig. 7.9) is implicitly invoked.
Lines 18-19 start a new line after every four Card s.

1 // Fig. 7.11: DeckOfCardsTest.java

2 // Card shuffling and dealing.

3
4 public class DeckOfCardsTest

5 {

6 // execute application

7 public static void main(String[] args)

8 {

9 DeckOfCards myDeckOfCards = new DeckOfCards();

10 myDeckOfCards.shuffle(); // place Cards in random order

11

12 // print all 52 Cards in the order in which they are dealt

13 for ( int i = 1 ; i <= 52 ; i++)

14 {

15 // deal and display a Card

16

System.out.printf( "%-19s" , myDeckOfCards.dealCard());

17
18

if (i % 4 == 0 ) // output a newline after every fourth card

19

System.out.println();

20

}

21

}

22

} // end class DeckOfCardsTest

Six of Spades Eight of Spades Six of Clubs Nine of Hearts
Queen of Hearts Seven of Clubs Nine of Spades King of Hearts
Three of Diamonds Deuce of Clubs Ace of Hearts Ten of Spades
Four of Spades Ace of Clubs Seven of Diamonds Four of Hearts
Three of Clubs Deuce of Hearts Five of Spades Jack of Diamonds
King of Clubs Ten of Hearts Three of Hearts Six of Diamonds
Queen of Clubs Eight of Diamonds Deuce of Diamonds Ten of Diamonds
Three of Spades King of Diamonds Nine of Clubs Six of Hearts
Ace of Spades Four of Diamonds Seven of Hearts Eight of Clubs
Deuce of Spades Eight of Hearts Five of Hearts Queen of Spades
Jack of Hearts Seven of Spades Four of Clubs Nine of Diamonds
Ace of Diamonds Queen of Diamonds Five of Clubs King of Spades
Five of Diamonds Ten of Clubs Jack of Spades Jack of Clubs

Fig. 7.11 | Card shuffling and dealing.

Preventing NullPointerException s
In Fig. 7.10, we created a deck array of 52 Card references—each element of every refer-
ence-type array created with new is default initialized to null . Reference-type variables
which are fields of a class are also initialized to null by default. A NullPointerException
occurs when you try to call a method on a null reference. In industrial-strength code, en-
suring that references are not null before you use them to call methods prevents Null-
PointerException s.