Solution to bdb4: Flawed house
See code at solutions/code/tutorialquestions/questionbdb4
The problem is rather similar to question 937d. The House class should
enforce the invariant:
floorArea == floorSize.getWidth() * floorSize.getHeight()
which can easily be invalidated despite the fact that class Rectangle has no
public fields.
The first way of destroying the invariant is:
House h = new House(new Rectangle(10, 20));
h.getFloorSize().setWidth(50);
The getFloorSize() method returns a reference to a Rectangle
object held by h, which is then changed without h's knowing.
The second way of destroying the invariant is:
Rectangle r = new Rectangle(10, 20);
House h = new House(r);
r.setWidth(50);
The problem is that the reference r points to the inner part of h,
which allows us to change the state of h without h's knowledge.
This may not seem like an important problem; however, in practice, similar situations often happen unintentionally.
A simple way of handling both problems is to modify House so that it keeps a private
internal copy of a Rectangle that is never made accessible to its clients. This
solution is shown in the fixed sub-package: examine it carefully. By completely
hiding the floorSize field from the outside world, we ensure that this field cannot
be changed without the enclosing House object's knowledge.
Other solutions are possible, e.g. we could make floorArea a computed attribute,
as suggested for the area attribute of Rectangle in question 937d.