Java Reference
In-Depth Information
Each of the constructors is used to create a new object. During this creation, the variable
dieValue is initialized to a random value. Conversely, the rollDie method is called after
an object is already created in order to update the existing value of the variable dieValue
to a random number.
Sometimes, the different constructors of a class have a lot of code in common and it is
tedious to write the same code multiple times. After all, the hallmark of a good program
is that the same code should not repeat. Fortunately, Java provides a mechanism for one
constructor to call a different constructor. In particular, the empty constructor of the Die
class can be rewritten as follows.
public Die () {
The call this(DEFAULT NUMBER OF SIDES) means call the constructor of this class that
takes as input a single integer and pass the number DEFAULT NUMBER OF SIDES as a param-
eter. Having the empty constructor call other constructors with a default value is a common
trick when writing constructors. Note that the empty constructor cannot directly call the
other constructor by its method name and needs to use the syntax this( ... ) .The this
keyword is used because in Java it means the current object (or the hidden parameter).
The call calls the second constructor and passes the current object as a hidden parameter.
Consider next the new equals method of the Die class.
public boolean equals(Die otherDie) {
return ( this . dieValue==otherDie . dieValue) ;
If d1 and d2 are die objects, then d1.equals(d2) will return true exactly when the
values of the two dice are the same. The formal parameter otherDie corresponds to the
actual parameter d2 . One can think of the this object as being a hidden formal parameter
that corresponds to the actual parameter d1 . Therefore, the method simply checks if d1
and d2 have the same value for the variable dieValue . Of course, the reference to the this
object is unnecessary when there is a single variable dieValue that is in scope and the code
can be rewritten as follows.
public boolean equals(Die otherDie) {
return (dieValue==otherDie . dieValue) ;
6.7 Array of Objects and Multi-Class Solutions
Now that we are satisfied with the Die class, let us examine an implementation of the
DiceCup class. In the real world, a dice cup will contain a collection of dice. We will model
the real world by creating an array of die objects in the DiceCup class.
public class DiceCup
private Die [ ] dice ;
private int numberOfDice ;
public static int DEFAULT NUMBER OF DICE=5 ;
public DiceCup () {
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