Java Reference
In-Depth Information
DeckOfCards
Constructor
The class's constructor instantiates the
deck
array (line 20) with
NUMBER_OF_CARDS
(52) ele-
ments. The elements of
deck
are
null
by default, so the constructor uses a
for
statement
(lines 24-26) to fill the
deck
with
Card
s. The loop initializes control variable
count
to
0
and
loops while
count
is less than
deck.length
, causing
count
to take on each integer value from
0 to 51 (the indices of the
deck
array). Each
Card
is instantiated and initialized with two
String
s—one from the
faces
array (which contains the
String
s
"Ace"
through
"King"
)
and one from the
suits
array (which contains the
String
s
"Hearts"
,
"Diamonds"
,
"Clubs"
and
"Spades"
). The calculation
count % 13
always results in a value from 0 to 12 (the 13 in-
dices of the
faces
array in lines 16-17), and the calculation
count / 13
always results in a
value from 0 to 3 (the four indices of the
suits
array in line 18). When the
deck
array is
initialized, it contains the
Card
s with faces
"Ace"
through
"King"
in order for each suit (all
13
"Hearts"
, then all the
"Diamonds"
, then the
"Clubs"
, then the
"Spades"
). We use arrays
of
String
s to represent the faces and suits in this example. In Exercise 7.34, we ask you to
modify this example to use arrays of
enum
constants to represent the faces and suits.
DeckOfCards
Method
shuffle
Method
shuffle
(lines 30-46) shuffles the
Card
s in the deck. The method loops through
all 52
Card
s (array indices 0 to 51). For each
Card
, a number between 0 and 51 is picked
randomly to select another
Card
. Next, the current
Card
object and the randomly selected
Card
object are swapped in the array. This exchange is performed by the three assignments
in lines 42-44. The extra variable
temp
temporarily stores one of the two
Card
objects be-
ing swapped. The swap cannot be performed with only the two statements
deck[first] = deck[second];
deck[second] = deck[first];
If
deck[first]
is the
"Ace"
of
"Spades"
and
deck[second]
is the
"Queen"
of
"Hearts"
,
after the first assignment, both array elements contain the
"Queen"
of
"Hearts"
and the
"Ace"
of
"Spades"
is lost—so, the extra variable
temp
is needed. After the
for
loop termi-
nates, the
Card
objects are randomly ordered. A total of only 52 swaps are made in a single
pass of the entire array, and the array of
Card
objects is shuffled!
[
Note:
It's recommended that you use a so-called
unbiased
shuffling algorithm for real
card games. Such an algorithm ensures that all possible shuffled card sequences are equally
likely to occur. Exercise 7.35 asks you to research the popular unbiased Fisher-Yates shuf-
fling algorithm and use it to reimplement the
DeckOfCards
method
shuffle
.]
DeckOfCards
Method
dealCard
Method
dealCard
(lines 49-56) deals one
Card
in the array. Recall that
currentCard
in-
dicates the index of the next
Card
to be dealt (i.e., the
Card
at the
top
of the deck). Thus,
line 52 compares
currentCard
to the length of the
deck
array. If the
deck
is not empty
(i.e.,
currentCard
is less than 52), line 53 returns the “top”
Card
and postincrements
cur-
rentCard
to prepare for the next call to
dealCard
—otherwise,
null
is returned. Recall
from Chapter 3 that
null
represents a “reference to nothing.”
Shuffling and Dealing Cards
Figure 7.11 demonstrates class
DeckOfCards
. Line 9 creates a
DeckOfCards
object named
myDeckOfCards
. The
DeckOfCards
constructor creates the deck with the 52
Card
objects
