Design Patterns #

What?! ... Well, that question would have made sense if we were going to take a road trip in a car. In that context, you would know what I am asking about: a spare is an extra tire you carry in your car in case you are driving and one of the tires goes flat. Let's take a moment and reflect on this:

• Problem: when driving a car, a tire might go flat.
• Solution: change the tire (requires to have an extra tire).
• The problem happens often enough that manufacturers, by design, equip your car with an extra tire, just in case.
• The extra tire is called a spare.

This is an example of a design pattern! A problem that occurs so often that it is given a name and a known solution. So when I say "where is your spare?" (before a road trip) you immediately know what I mean and why I ask for it.

Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (collectively known as the Gang of Four or GoF) wrote the book Design Patterns: Elements of Reusable Object-Oriented Software in 1994. These authors looked at over 300 projects that other software developers were working on (at the time):

• They recognized that many of the same problems kept appearing.
• They also noticed that solutions to the problems were roughly the same.
• Their book discusses these problems and solutions, identifying each as a pattern and giving it a name.

GoF categorized the patterns into three categories:

• Creational patterns - writing classes and creating objects.
• Structural patterns - aligning classes and objects.
• Behavioral patterns - interactions between objects.

We will look at samples of all three types.