Software Engineering for Students A Programming Approach

210 Chapter 15 ■ Object-oriented programming
>
>
Bomb to create two objects, storing them in an array list named game, and displaying
them. The display is shown in Figure 15.1.
Alien alien = new Alien(20, 20, 100);
Bomb bomb = new Bomb(80, 80, 10);
ArrayList game = new ArrayList();
game.add(alien);
game.add(bomb);
for (int s = 0; s < game.size(); s++) {
Object item = game.get(s);
Sprite sprite = (Sprite) item;
sprite.display(paper);
}
Polymorphism is in use here – the method display is called on two occasions with
different results according to which object is in use. You can see that the two calls of
display within the for loop:
sprite.display(paper);
give two different outputs. Two different outputs are displayed because the Java system
automatically selects the version of display associated with the class of the object.
When method display is first called, the variable sprite contains the object alien
and so the version of display in the class Alien is called. Then the corresponding
thing happens with bomb. This is the essence of polymorphism.
The class of an object is determined when the object is created using new classes,
and stays the same whatever happens to the object. Whatever you do to an object in a
program, it always retains the features it had when it was created. An object can be
assigned to a variable of another class and passed around the program as a parameter,
but it never loses its true identity.
Polymorphism allows us to write a single concise statement, such as:
sprite.display(paper);
instead of a series of if statements like this:
if (sprite instanceof Alien) {
Alien alien = (Alien) sprite;
alien.display(paper);
}
if (sprite instanceof Bomb) {
Bomb bomb = (Bomb) sprite;
bomb.display(paper);
}
>
>