Object-Oriented Programming Foundations

A well-designed hierarchy places shared behaviour high and specialised behaviour low. Vehicle defines move(); Car and Bicycle refine it. The Liskov substitution principle asks that every subclass remain usable wherever its parent is expected.

When a subclass must disable or contradict its parent's behaviour, the hierarchy is usually wrong. Composition — giving an object a component rather than a parent — is often the better tool.

In Java, a class may extend only one parent but implement many interfaces. In Python, multiple inheritance is permitted, guided by the method resolution order.