Implementing Classes Examples

version 1. basics

• Point.java simple Point definition
• PointMain.java a main that uses it (annotated)
• AnotherPointMain.java another main annotated. notice:
  • awkwardness of initialization
  • awkwardness of printing
  • how many Points do we have?
  • what does the assignment statement do?
• ObjectsAndMethods.java copying a reference to a method: just like an assignment statement (annotated)
• NewObjectsInFuncs.java within the function, we can't change which Point main's p references
• ReturningReferenceToNewObject.java create new Point in the function. Copy its location back to main() (annotated)

version 2. adding a constructor

• Point.java new Point class with a constructor (annotated)
• PointMain.java a main that uses it (annotated)

version 3. adding toString()

• Point.java (annotated)
• PointMain.java (annotated)

version 4. adding move()

We add an instance method called move which changes the Point's x and y coordinate.

• Point.java
• PointMain.java

version 5. private

Now the only way we can change a Point's x and y is through move, but we've introduced some errors.

• Point.java
• PointMain.java

version 6. addition of accessor methods

Because x and y are private, we can't access them directly, but now we can obtain read-only access to them through the accessor methods getX() and getY(). Notice that both x and y are private, but getX() and getY() are public.

• Point.java
• PointMain.java

version 7. mutator methods

x and y can now be changed, but only indirectly through the public methods setX() and setY().

• Point.java
• PointMain.java

version 8. arrays of points

There's nothing different about our Point class here. We just look at the syntax for creating and using arrays of points.

• Point.java
• PointMainWithArrays.java (annotated). A clearer diagram showing the same.
• PointMainWithArraysAgain.java. Notice that Arrays.toString() works. Arrays.toString() calls Point's toString() repeatedly.

version 9. comparing the contents of two Points

• PointMainEqualsBroken.java Broken. We see another example of the == operator comparing locations instead of the content of the objects stored at these locations.
• PointMainEqualsBrokenAgain.java We remember that if we use equals() it should work, but it doesn't, because our class doesn't have equals().
• PointMainEqualsWorking.java. Works, because we compare the contents and not just the locations.
• Point.java. New version of our class with equals() built-in.

version 10. making a copy of a Point

• PointMainCopyBroken.java two references to the same Point, not two Points.
• PointMainCopyWorking.java works because we've created a 2nd Point, and copied the contents of the first to the second.
• Point.java New version of the Point class which builds in this feature through a clone() method.
• AnotherPointMainCopyWorking.java working main that calls our clone().

version 11. overloading the constructor

• OverloadedAdd.java. Nothing to do with our Point class. It's just an example of overloading methods.
• Point.java. New version of the Point class. This one has an overloaded constructor.
• PointMain.java. main() that uses the new class calling both versions

version 12. composition

Building a class comprised of other classes.

• LabeledPoint.java a Point that contains a label, i.e., an instance of a String.
• LabeledPointMain.java a main( ) that uses it.
• LineSegment.java a LineSegment is made up of two Points.
• LineSegmentMain.java a main( ) that uses it.