Software Engineering for Students A Programming Approach

15.2 Encapsulation 203
Often (though not in the Alien example shown above) there will need to be additional
methods that the class needs in order to carry out its tasks. These are private
methods that need not and therefore should not be accessible from outside
the class.
A class represents a real fusion of data and actions. A class extends the built-in data
types provided by the language, so that the programmer can invent data suitable for the
problem being solved. The programmer specifies how the data can be manipulated and
thus creates truly abstract data.
The advantages of encapsulation are:
■ to make useful classes generally available
■ to use a class without the need to know how it works
■ to have the flexibility to modify a class without affecting its users.
Properties
We have seen that it is very bad practice to make public any of the instance variables
of a class. Some languages, for example, C# and Visual Basic, provide a mechanism that
simulates accessing variables directly. This feature, called properties, enables users to
have convenient but controlled access to the data associated with an object. In general
there are two distinct kinds of access to data:
1. reading a value – called get access
2. writing the value – called set access.
For example, suppose we want to allow a user of an alien object to refer to (get) the
x coordinate of the alien and display its value in a text field. The value is held in the
private variable named x at the top of the class. Using a property named xCoord, we
can write:
textField.setText(Integer.toString(alien.xCoord));
Suppose also that we want the user to be able to change (set) the value of the x coordinate.
Using the property facility we can write:
alien.xCoord = 56;
The way to provide these facilities is to write a property. Here is the revised class that
includes the property code:
public class AlienWithProperties {
private int x;
private int y;
private int size;
private ImageIcon alienImage;
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