Answer Posted / njoy

Q2(A)
In the Java programming language, an interface is a
reference type, similar to a class, that can contain only
constants, method signatures, and nested types. There are
no method bodies. Interfaces cannot be instantiated—they
can only be implemented by classes or extended by other
interfaces. Extension is discussed later in this lesson.

java comes with the Swing widgets. The only hard part is
learning the different Layout methods. We have to have a
LayoutManager in a top-level container -- such as a JFrame.
The Layout will divide up all the space to present the
buttons, listboxes, checkboxes and so forth. Really, the
Sun Swing tutorials are the best route to learning GUI.

Defining an interface is similar to creating a new class:
public interface OperateCar {

// constant declarations, if any

// method signatures
int turn(Direction direction, // An enum with values
RIGHT, LEFT
double radius, double startSpeed, double
endSpeed);
int changeLanes(Direction direction, double startSpeed,
double endSpeed);
int signalTurn(Direction direction, boolean signalOn);
int getRadarFront(double distanceToCar, double
speedOfCar);
int getRadarRear(double distanceToCar, double
speedOfCar);
......
// more method signatures
}
Note that the method signatures have no braces and are
terminated with a semicolon.
To use an interface, you write a class that implements the
interface. When an instantiable class implements an
interface, it provides a method body for each of the
methods declared in the interface. For example,
public class OperateBMW760i implements OperateCar {

// the OperateCar method signatures, with
implementation --
// for example:
int signalTurn(Direction direction, boolean signalOn) {
//code to turn BMW's LEFT turn indicator lights on
//code to turn BMW's LEFT turn indicator lights off
//code to turn BMW's RIGHT turn indicator lights on
//code to turn BMW's RIGHT turn indicator lights off
}

// other members, as needed -- for example, helper
classes
// not visible to clients of the interface

}
In the robotic car example above, it is the automobile
manufacturers who will implement the interface. Chevrolet's
implementation will be substantially different from that of
Toyota, of course, but both manufacturers will adhere to
the same interface. The guidance manufacturers, who are the
clients of the interface, will build systems that use GPS
data on a car's location, digital street maps, and traffic
data to drive the car. In so doing, the guidance systems
will invoke the interface methods: turn, change lanes,
brake, accelerate, and so forth.

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