2.1: The Time Class

One common reason to define a new class is to encapsulate related data in an object that can be treated as a single unit. That way, we can use objects as parameters and return values, rather than passing and returning multiple values. This design principle is called data encapsulation.

We have already seen two types that encapsulate data in this way: Point and Rectangle. Another example, which we will implement ourselves, is Time, which represents a time of day. The data encapsulated in a Time object are an hour, a minute, and a number of seconds. Because every Time object contains these data, we define attributes to hold them.

Attributes are also called instance variables, because each instance has its own variables (as opposed to class variables, coming up in Section 12.3).

The first step is to decide what type each variable should be. It seems clear that hour and minute should be integers. Just to keep things interesting, let’s make second a double.

Instance variables are declared at the beginning of the class definition, outside of any method. By itself, this code fragment is a legal class definition:

public class Time {
    private int hour;
    private int minute;
    private double second;
}

The Time class is public, which means that it can be used in other classes. But the instance variables are private, which means they can only be accessed from inside the Time class. If you try to read or write them from another class, you will get a compiler error.

Private instance variables help keep classes isolated from each other so that changes in one class won’t require changes in other classes. It also simplifies what other programmers need to understand in order to use your classes. This kind of isolation is called information hiding.