Applied Design Patterns with Java

Behavioral :: Strategy (315) {C ch 24}

Intent
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

As Known As
Policy

Motivation
Many algorithms exist for breaking a stream of text into lines. Hard-wiring all such algorithms into the classes that require them isn't desirable for several reasons:

• Clients that need linebreaking get more complex if they include the linebreaking code. That makes clients bigger and harder to maintain, especially if they support multiple linebreaking algorithms.
• Different algorithms will be appropriate at different times. Don't support multiple linebreaking algorithms if there is no use for them.
• It's difficult to add new algorithms and vary existing ones when linebreaking is an integral part of a client.

Avoid these problems by defining classes that encapsulate different linebreaking algorithms. An algorithm that's encapsulated in this way is called a Strategy. Suppose a Composition class is responsible for maintaining and updating the linebreaks of text displayed in a text viewer. Linebreaking strategies aren't implemented by the class Composition. Instead, they are implemented separately by subclasses of the abstract Compositor class, as shown here:

Compositor subclasses implement different Strategy patterns:

• SimpleCompositor implements a simple Strategy that determines linebreaks one at a time.
• TeXCompositor implements the TeX algorithm for finding linebreaks. This Strategy tries to optimize linebreaks globally, that is, one paragraph at a time.
• ArrayCompositor implements a Strategy that selects breaks so that each row has a fixed number of items. It's useful for breaking a collection of icons into rows, for example.

A Composition maintains a reference to a Compositor object. Whenever a Composition reformats its text, it forwards this responsibility to its Compositor object. The client of Composition specifies which Compositor should be used by installing the Compositor it desires into the Composition.

Catalog Behavioral Prev Next