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Freek Lijten: SOLID - The L is for Liskov Substitution Principle

Note: This article was originally published at PHPDeveloper on 4 September 2012.
PHPDeveloper

In this new post to his site Freek Lijten picks back up his series on the SOLID design principles with a look at the "L" in the acronym - the Liskov Substitution Principle.

The Liskov Substitution Principle (LSP) was coined by Barbara Liskov as early as 1987. The principle is very tightly connected to the earlier discussed Open Closed Principle. A good way of adhering to the OCP is understanding and implementing code that uses the Liskov Substitution Principle. In this article we will discover why and how.

He gives a more understandable explanation of what the rule tries to suggest - a standardized interface that each of his "Bike" instances follows to ensure the contract of a consistent API. Then each of the child classes can reliably assume that there are methods they can use because they'll always inherit them.