Software Engineering for Students A Programming Approach

208 Chapter 15 ■ Object-oriented programming
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exploit the common features of the classes. We do this by writing a class Sprite that
embodies the commonality. This name is chosen because, in computer games programs,
a sprite is the term for a graphical object. Here it is:
class Sprite {
}
protected int x, y;
protected size;
public void moveLeft(int amount) {
x = x - amount;
}
public void moveRight(int amount) {
x = x + amount;
}
You can see that the variables and methods within this class are relevant to all the
game objects. You will also notice that the variables declared at the head of the class
that were described as public, are now described as protected. This means that they
are accessible from any subclasses, as we shall see in a moment.
We can now write class Alien so as to exploit the class Sprite as follows:
class Alien extends Sprite {
}
private ImageIcon alienImage;
public Alien(int newX, int newY, int newSize) {
x = newX;
y = newY;
size = newSize;
alienImage = new ImageIcon("c:/alien.jpg");
}
public void display(JPanel panel) {
Graphics paper = panel.getGraphics();
alienImage.paintIcon(panel, paper, x, y);
}
and you can see that this is now shorter than it was. The operative word in this code is
extends. This is the Java keyword stating that class Alien inherits the features of class
Sprite. All the public variables and methods become part of class Alien. The terminology
is that Alien is a subclass of Sprite, Sprite is the superclass of Alien,
Alien extends Sprite, Sprite is the base class of Alien.
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