Classes - Building Java Programs: A Back to Basics Approach - page 532

The fields would have been auto-initialized to 0 anyway, but many programmers
prefer to explicitly initialize field values for clarity.
Another useful operation for TimeSpan objects is the ability to print them on the
console. We'll add this ability by including a toString method that returns a
String such as "2h 35m" for 2 hours and 35 minutes.
Here is the code for the complete TimeSpan class that enforces its invariant:
1 // Represents a time span of hours and minutes elapsed.
2 // Class invariant: hours >= 0 && minutes >= 0 && minutes < 60
3
4 public class TimeSpan {
5 private int hours;
6 private int minutes;
7
8 // Constructs a time span with the given interval.
9 // pre: hours >= 0 && minutes >= 0
10 public TimeSpan( int hours, int minutes) {
11 this .hours = 0;
12 this .minutes = 0;
13 add(hours, minutes);
14 }
15
16 // Adds the given interval to this time span.
17 // pre: hours >= 0 && minutes >= 0
18 public void add( int hours, int minutes) {
19 if (hours < 0 || minutes < 0) {
20 throw new IllegalArgumentException();
21 }
22
23 this .hours += hours;
24 this .minutes += minutes;
25
26 // converts each 60 minutes into one hour
27 this .hours += this.minutes / 60;
28 this .minutes = this.minutes % 60;
29 }
30
31 // returns a String for this time span, such as "6h 15m"
32 public String toString() {
33 return hours + "h " + minutes + "m";
34 }
35 }