Key Ideas
[Unit 4]
There are five key ideas in this unit:
- The first is that we can extend any normal class to invisibly gain the code within our own and use the class
as the basis of ours:
public class Point {
private double x = 0.0;
private double y = 0.0;
public void setX (double xIn) {
x = xIn;
}
public double getX() {
return x;
}
public void setY (double yIn) {
y = yIn;
}
public double getY() {
return y;
}
}
public class Space extends Point {
private double orbitHeight = 0.0;
public Space() {
// Launch pad location...
setX(28.4556);
setY(-80.5278);
}
public void setOrbitHeight (double orbitHeightIn) {
orbitHeight = orbitHeightIn;
}
public double getOrbitHeight() {
return orbitHeight;
}
}
- The second is that we can alternatively sign up to a set of promises about the kind of
class we'll be. This is called an Interface. Interfaces frequently also have standard-based static final
variables:
public interface Point {
public static final double GREENWICH_X = 51.4788;
public static final double GREENWICH_Y = 0.0;
public void setX (double xIn);
public double getX();
public void setY (double yIn);
public double getY();
}
public class Space implements Point {
private double x = 0.0;
private double y = 0.0;
private double orbitHeight = 0.0;
public Space() {
// Launch pad location...
x = Point.GREENWICH_X + 28.4556;
y = Point.GREENWICH_Y + -80.5278;
}
public void setX (double xIn) {
x = xIn;
}
public double getX() {
return x;
}
public void setY (double yIn) {
y = yIn;
}
public double getY() {
return y;
}
public void setOrbitHeight (double orbitHeightIn) {
orbitHeight = orbitHeightIn;
}
public double getOrbitHeight() {
return orbitHeight;
}
}
- Thirdly, you can do a bit of both with abstract classes:
public abstract class Point {
public static final double GREENWICH_X = 51.4788;
public static final double GREENWICH_Y = 0.0;
protected double x = 0.0;
protected double y = 0.0;
public void setX (double xIn) {
x = xIn;
}
public abstract double getX();
public void setY (double yIn) {
y = yIn;
}
public abstract double getY();
}
public class Space extends Point {
private double orbitHeight = 0.0;
public Space() {
// Launch pad location...
x = Point.GREENWICH_X + 28.4556;
y = Point.GREENWICH_Y + -80.5278;
}
public double getX() {
return x;
}
public double getY() {
return y;
}
public void setOrbitHeight (double orbitHeightIn) {
orbitHeight = orbitHeightIn;
}
public double getOrbitHeight() {
return orbitHeight;
}
}
- Fourth, all this is most useful when used with the fact that methods will accept a
subclass object in place of a superclass or interface. Once an object is passed into
a method, however, it can only be used as if it was that superclass or interface:
Space iss = new Space();
iss.setOrbitHeight(370000);
iss.setX(142.3624934744112);
iss.setY(-38.05295779081117);
print(iss);
public void print (Point pt) {
System.out.println("X = " + pt.getX());
System.out.println("Y = " + pt.getY());
// System.out.println ("Height = " + pt.getOrbitHeight()); // This wouldn't work.
}
- Fifth, UML is easier to understand than code. Which of the above is the following?