Further abstraction techniques - Objects First with Java: A Practical Introduction Using BlueJ - page 361

simulation by providing further implementations of the SimulatorView interface. The new
class GraphView , which produces the line graph, is an example of this.

Once we have more than one view implementation, we can easily replace the current view
with another or, as we have in our example, even display two views at the same time. In the
Simulator class, the concrete subclasses GridView and GraphView are only mentioned once
when each view was constructed. Thereafter, they are stored in a collection holding elements of
the SimulatorView supertype, and only the interface type is used to communicate with them.

The implementations of the GridView and GraphView classes are fairly complex, and we do not
expect you to fully understand them at this stage. The pattern of providing two implementations for a
single interface, however, is important here, and you should make sure that you understand this aspect.

Exercise 10.61 Review the source code of the Simulator class. Find all occurrences
of the view classes and interfaces, and trace all variables declared using any of these types.
Explain exactly how the views are used in the Simulator class.

Exercise 10.62 Implement a new class TextView that implements SimulatorView .
TextView provides a textual view of the simulation. After every simulation step, it prints out
one line in the form

Foxes: 121 Rabbits: 266

Use TextView instead of GridView for some tests. (Do not delete the GridView classes.
We want to have the ability to change between different views!)

Exercise 10.63 Can you manage to have all three views active at the same time?

10.8 The Class class

In Chapter 8, we described the paradoxically named Object class. It shouldn't surprise you,
therefore, that there is also a Class class! This is where talking about classes and objects can
become very confusing.

We used the Class type in defining the SimulatorView interface in the previous section. The
Class class has nothing specifically to do with interfaces; it is a general feature of Java, but we
just happen to be meeting it for the first time here. The idea is that each type has a Class object
associated with it.

The Object class defines the method getClass to return the Class associated with an object.
Another way to get the Class object for a type is to write “ .class ” after the type name: for
instance, Fox.class or int.class —notice that even the primitive types have Class objects
associated with them.

Class objects are particularly useful if we want to know whether the type of two objects is the
same. We use this feature in the original SimulatorView class to associate each animal type
with a color in the field. SimulatorView has the following field to map one to the other:

private Map<Class, Color> colors;