Making the Liskov Substitution Principle Happy and Sad
The Liskov Substitution Principle states, among other constraints, that a subtype is not substitutable for its super type if it strengthens its operations’ preconditions, or weakens its operations’ postconditions. We found that students in two subsequent courses had trouble remembering these rules. Their major stumbling block appeared to be recalling which condition (pre- or post-) could be strengthened and which could be weakened. We developed a simple visual reminder to help: A method is happy if it is substitutable—A smile is wider at the top than at the bottom, suggesting weaker/looser/wider pre-conditions, and stronger/tighter/narrower post conditions.; A method is sad if it isn’t substitutable—a frown is narrower at the top, suggesting stronger/tighter/narrower preconditions, and wider at the bottom, suggesting weaker/looser/wider postconditions. Though the technique is far from perfect, we found that it allowed students to move on to the more interesting design questions around the LSP.