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= Composite Pattern

The http://en.wikipedia.org/wiki/Composite_pattern[Composite Pattern] allows you to treat single instances of an object the same way as a group of objects. The pattern is often used with hierarchies of objects. Typically, one or more methods should be callable in the same way for either __leaf__ or __composite__ nodes within the hierarchy. In such a case, composite nodes typically invoke the same named method for each of their children nodes.

== Example

Consider this usage of the composite pattern where we want to call `toString()` on either `Leaf` or `Composite` objects.

image::assets/img/CompositeClasses.gif[]

In Java, the `Component` class is essential as it provides the type used for both leaf and composite nodes. In Groovy, because of duck-typing, we don't need it for that purpose, however, it can still server as a useful place to place common behaviour between the leaf and composite nodes.

For our purposes, we will assemble the following hierarchy of components.

image::assets/img/CompositeComponents.gif[]

Here is the code:

[source,groovy]
----
include::{projectdir}/src/spec/test/DesignPatternsTest.groovy[tags=composite_code,indent=0]
----

Here is the resulting output:

----
root
-leaf A
-comp B
--leaf B1
--leaf B2
-leaf C
----