Unit 10: Interfaces and Behavioral Contracts

Object-oriented design becomes more flexible when one component can depend on a capability rather than the exact class providing it.

Java interfaces give that capability a type.

This unit focuses on:

• interface contracts;
• multiple implementations of one capability;
• variables and parameters typed to an interface;
• substituting implementations without rewriting callers;
• testing whether an implementation actually preserves the promised behavior.

The goal is not to create interfaces everywhere. It is to use one when a stable capability boundary makes collaboration easier to reason about.