The Strategy design pattern
The Strategy pattern is a behavioral design pattern that allows us to change object behaviors at runtime. We can also use this pattern to compose complex object trees and rely on it to follow the Open/Closed Principle (OCP) without much effort. As a follow up on that last point, the Strategy pattern plays a significant role in the composition over inheritance way of thinking. In this chapter, we focus on the behavioral part of the Strategy pattern. In the next chapter, we cover how to use the Strategy pattern to compose systems dynamically.
Goal
The Strategy pattern's goal is to extract an algorithm (strategy) away from the host class needing it (context). That allows the consumer to decide on the strategy (algorithm) to use at runtime.
For example, we could design a system that fetches data from two different types of databases. Then we could apply the same logic over that data and use the same user interface to display it. To achieve this...