Recall class Point from question 0f05. If you construct two Point objects
with identical coordinates, should they be regarded as equal? This depends on the context in which
you use Point, but very likely the answer is yes: instances of Point
are immutable value objects, so that if two Points look identical, they should be regarded
as the same. (Remember that we call such objects value objects because they have some of the
attractive properties of primitive values: in particular, aliasing of value objects is not problematic. However,
value objects are still objects, and comparing two identical value objects with == may return
false.)
As you know from the lectures, if we construct two identical Point objects and compare them
with the == operator, the operator will yield false, because the points being compared
are not the same object.
What happens if you construct two points, p and q with identical coordinates, and compare them by testing
whether p.equals(q) holds? Try it. You should find that the result is also false. Why is this?
Implement Point equality testing by overriding the equals method from Object. Remember the
recipe for constructing a basic equals method:
- Check that the incoming object has the right type, using
instanceof - If it does, cast the incoming object to this type
- Compare fields
Test that your implementation works by confirming that a selection of identical points are regarded as being equal, and that points with differing coordinates are not.
Now create two instances of ColouredPoint that have identical coordinates, but different colours.
Does your implementation of equals regard these objects as being equal, and why does it behave in
this way? If the ColouredPoint objects are regarded as being equal then you should fix this by overriding
equals appropriately in ColouredPoint.
Finally, consider comparing a (non-coloured) Point with a ColouredPoint. Construct a
Point p and a ColouredPoint cp such that p.equals(cp)
holds, but cp.equals(p) does not hold. In other words, demonstrate that the equals relation you
have defined is not symmetric in the presence of subclasses. We will return to this issue in question aa68.