Design patterns

When engineers and architects decide to build a bridge, or a tower, or a building, they follow certain principles to ensure structural integrity. There are various possible designs for bridges (suspension or cantilever for example), but if the engineer doesn't use one of the standard designs, and doesn't have a brilliant new design, it is likely the bridge he/she designs will collapse.

Design patterns are an attempt to bring this same formal definition for correctly designed structures to software engineering. There are many different design patterns to solve different general problems. People who create design patterns first identify a common problem faced by developers in a wide variety of situations. They then suggest what might be considered the ideal solution for that problem, in terms of object-oriented design.

We already have plenty of experience with one of the most common design patterns, the iterator. Canonically, the iterator pattern is meant to provide a common...