Introduction to Gang of Four Design Patterns

A design pattern is a named, reusable approach to a recurring software-design problem. It is not finished code and it is not a rule that must be followed. A useful pattern explains the context in which a problem occurs, the forces that make the problem difficult, a general arrangement of collaborating objects, and the consequences of choosing that arrangement.

This course uses the 23 patterns cataloged by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides as a vocabulary for object-oriented design. The goal is not to memorize class diagrams. The goal is to recognize design pressure, compare alternatives, and explain why a pattern is appropriate or unnecessary in a specific program.

By the end of the seven modules, you should be able to:

• distinguish creational, structural, and behavioral design problems;
• identify the responsibilities and collaborations that define a pattern;
• compare inheritance with object composition and choose between them;
• evaluate benefits such as flexibility against costs such as additional classes, indirection, and runtime complexity;
• implement selected patterns in Java or C++ without coupling the design to one framework; and
• explain when a simpler design is preferable to applying a pattern.

Intent

The design problem the pattern addresses and the result it is meant to produce.

Context and forces

The conditions that make the problem recur, including requirements that pull the design in competing directions.

Structure and participants

The classes or objects involved, their responsibilities, and the relationships among them.

Collaborations

How the participants exchange requests or data to perform the behavior.

Consequences

The flexibility gained, the complexity introduced, and the assumptions that future maintainers must understand.

A pattern name is valuable because it compresses this design knowledge into a shared term. Saying that a component acts as an Adapter or that algorithms are selected with Strategy gives a team a starting point, but the name does not replace an explanation of the local design decision.

Patterns are most useful after a concrete problem has been identified. Adding factories, observers, or decorators before the requirements demand them can produce unnecessary abstractions. Throughout this course, ask four questions before selecting a pattern:

1. What is likely to vary?
2. Which responsibility is difficult to test, replace, or extend?
3. What coupling will the pattern remove?
4. What new complexity will the pattern introduce?

The course project provides one continuing context for answering these questions. You will model traffic moving through an intersection, then use patterns where they clarify object creation, composition, communication, or state-dependent behavior.

1. Confirm the course prerequisites and toolchain.
2. Review the expected learning process and deliverables.
3. Study the traffic-flow project scope and design constraints.
4. Identify the first candidate objects and classes.

Optional reference: Design Patterns: Elements of Reusable Object-Oriented Software. This is an affiliate link; the course can be completed without purchasing the book.