Object-Oriented Programming 2023/24

Exercise 8

• Project
• Tasks

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1. Try the example with circle and ellipse from the lecture. Make sure you understand why deriving a circle from an ellipse leads to violating the Liskov substitution principle and design by contract. Change the example so that both circle and ellipse are derived from a common supertype. Note this prevents the increaseEllipse() method from being applied to circles.
2. Recall task 6 from Exercise 6 (exceptions in overriding). Note how this is related to the Liskov substitution principle: extending a set of exceptions that a method draws in practice weakens postconditions because by this the method narrows a set of cases in which it ends favorably and by returning an expected result. This would constitute a problem for the client code, but, fortunately, unlike with a common Liskov substitution principle violation, the compiler directly warns against this mistake.

If necessary, go back to the tasks from previous exercises and discuss any uncertainties.

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