More about objects in Java - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 111

4.5.1 Interfacing classes
The term interface is a very suitable name for these kinds of abstract classes
because they can provide a systematic approach to adding access to a class. That
is, they can provide a common interface .
Forexample, say that we have classes Relay and Valve that are completely
independent, perhaps written by two different programmers. The class Test
could communicate easily with both of these classes if they were modified to
implement the same interface. Let's define an interface called Switchable ,
which holds a single method called getState() ,asin
public interface Switchable {
public boolean getState ();
}
We want both the Relay and Valve classes to implement Switchable and
provide a getState() method that returns a value true or false that indicates
whether a relay or a valve is in the on or off state.
In the code below we show the class Test that references instances of
Relay and Valve as Switchable types. Test can then invoke their respective
getState() methods to communicate with them.
class Test {
public static void main (String[] args) {
Switchable[] switches = new Switchable[2];
switches[0] = new Relay ();
switches[1] = new Valve ();
for (int i = 0; i < 2; i++) {
if (switches[i].getState ()) doSomething (i);
}
}
}
class Relay implements Switchable {
boolean setting = false;
// Implement the interface method getState()
boolean getState () {
return setting;
}
.. other code . .
}
class Valve implements Switchable {
boolean valveOpen = false;