Classes - Building Java Programs: A Back to Basics Approach - page 534

no longer need to worry about storing minutes of 60 or greater. All minutes added are
properly grouped into the common total.
The constructor and toString method also require minor modifications to
account for our new representation. Here is the complete class, implemented with
total minutes instead of hours and minutes. This version is shorter and simpler than
the original:
1 // Represents a time span of elapsed hours and minutes.
2 // Second implementation using a single field for total minutes.
3 // Class invariant: totalMinutes >= 0
4
5 public class TimeSpan {
6 private int totalMinutes;
7
8 // Constructs a time span with the given interval.
9 // pre: hours >= 0 && minutes >= 0
10 public TimeSpan( int hours, int minutes) {
11 totalMinutes = 0;
12 add(hours, minutes);
13 }
14
15 // Adds the given interval to this time span.
16 // pre: hours >= 0 && minutes >= 0
17 public void add( int hours, int minutes) {
18 if (hours < 0 || minutes < 0) {
19 throw new IllegalArgumentException();
20 }
21 totalMinutes += 60 * hours + minutes;
22 }
23
24 // returns a String for this time span, such as "6h 15m"
25 public String toString() {
26 return (totalMinutes / 60) + "h " +
27 (totalMinutes % 60) + "m";
28 }
29 }
As another example, we could revisit our encapsulated Point class and change its
internal structure without having to modify the client code. For example, sometimes
it is useful to express two-dimensional points in polar coordinates in terms of a radius
r and an angle theta . In this representation, the ( x, y ) coordinates of a point are not
stored directly but can be computed as ( r cos theta, r sin theta ). When the Point
class is encapsulated, we can modify it to use r and theta fields internally, then mod-
ify getX , getY , and other methods so that they still return appropriate values. The