Composite

Gamma et al. 95

Intent

  • Compose objects into tree structures to represent part-whole hierarchies.

  • Let clients treat individual objects and compositions of objects uniformly.

Motivation

  • Occasionally, some applications (e.g. graphic editor) need to deal with primitive objects and container objects, i.e., objects containing other objects.

  • Clients want to treat identically primitive and container objects.

Motivation - example

composite example

Motivation - example

composite instances

Motivation - example

sd composite motivation

Applicability

Use the Composite pattern when:

  • you want to represent part-whole hierarchies of objects.

  • you want clients to be able to ignore the difference between compositions of objects and individual objects.

    • Clients will treat all objects in the composite structure uniformly.

Structure

composite structure

Consequences

  • Clients treat equally composite or atomic structures.

  • Clients are simplified.

  • It is easy to add new components.

  • The solution is sometimes too general (composite accepts any component).

Implementation Tradeoffs

  • Explicit reference to the parent: sometimes the Component may need to know the Composite that contains it (if any).

  • Sharing components: sometimes a Component may be possessed by multiple Composites − can influence the behavior or the destruction of Composites.

Implementation Tradeoffs (Cont.)

  • Where to define operations to add/remove components?:

In the Component class?

  • Possible error if not called on a Composite.

cd composite tradeoff a
In the Composite class?

  • Need to type check beforehand.

cd composite tradeoff b

Implementation Tradeoffs (Cont.)

  • The Composite may contain a cache to store information on the sum of components it contains.

od composite cache
Figure 1. Composite as property cache

Author and Date

  • Design Patterns: Elements of Reusable Object-Oriented Software. Erich Gamma, Richard Helm,Ralph Johnson, and John Vlissides. Addison Wesley. October 1994.

More Examples

java.awt.Container

cd container

Files and Folders

cd file

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