4 - Design Patterns Basics

Summary

“There is one timeless way of building.”

Christopher Alexander

Essentials

Whereas code reuse played an important role in Part I of this text, design reuse plays an equally important role in Part II. The effort put into thinking abstractly about software structure and behavior pays off in high-level designs that prove useful independent of the application and implementation language. Patterns comprise reusable elements of successful designs.

The software community typically uses the terms “design patterns” and “object-oriented design patterns” interchangeably. This stems from the expressiveness of OOP languages in describing the relationships and interactions between ADTs. Patterns can improve a code's structure and readability and reduce its development costs by encouraging reuse.

Software design patterns comprise four elements (Gamma et al. 1995):

1. The pattern name: a handle that describes a design problem, its solution, and consequences in a word or two.

2. The problem: a description of when to apply the pattern and within what context.

3. The solution: the elements that constitute the design, the relationships between these elements, their responsibilities, and their collaborations.

4. The consequences: the results and trade-offs of applying the pattern.

Although there have been suggestions to include additional information in identifying a pattern, for example, sample code and known uses to validate the pattern as a proven solution, authors generally agree that elements 2-4 enumerate the three essential factors in each pattern.