Java Reference
In-Depth Information
6.4 The toString Method
It seems tedious to write System.out.println(d.getValue()) every time we want to
print the value of the die d . Can we just write System.out.println(d) and expect Java to
automatically convert the die d to a string? Surprisingly, the answer is yes. The toString
method is defined for every Java object. For example, we can always send an object to the
print and println methods. The object is automatically converted to a string by calling
the toString method.
The default implementation of the toString method does not do much: it just returns
the name of the class and the address (or the reference, to be more precise) of the object
(we saw this behavior when we tried to print an array). To demonstrate this behavior again,
let us rewrite our code as follows.
Die d = new Die () ;
System. out . println (d) ;
for ( int i=0;i < 1000;
i++) {
d. rollDie() ;
System. out . println (d) ;
One possible output is: Die@42e816 one thousand and one times. In order to get mean-
ingful output, we need to add the toString method to the Die class. One possible imple-
mentation follows.
public String toString()
return dieValue+ "" ;
Note that the Die class contains the value of the die as an integer. We can use the
trick of adding an integer and an empty string to convert the integer to a string. We will
make one more modification to the Die class and make the number of sides on the die a
parameter. Note that the above toString method actually overrides an existing toString
method. Therefore, if we want the printing of a die to work properly, we cannot change the
name of the toString method or its return value type. Note that the toString method
does not print anything. It simply converts an object to a string.
class Die {
public static final int DEFAULT NUMBER OF SIDES=6 ;
private int dieValue ;
private int numberOfSides ;
public Die () {
dieValue = getRandomDieValue() ;
public Die( int numberOfSides) {
this . numberOfSides = numberOfSides ;
dieValue = getRandomDieValue() ;
public void rollDie()
dieValue = getRandomDieValue() ;
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