Editing Composite pattern (section)

==Overview==
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The Composite<ref name="GoF">{{cite book|author=Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides|title=Design Patterns: Elements of Reusable Object-Oriented Software|year=1994|publisher=Addison Wesley|isbn=0-201-63361-2|pages=[https://archive.org/details/designpatternsel00gamm/page/163 163ff]|url=https://archive.org/details/designpatternsel00gamm/page/163}}</ref>
design pattern is one of the twenty-three well-known 
''[[Design Patterns|GoF design patterns]]'' 
that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

===Problems the Composite design pattern can solve===

* Represent a part-whole hierarchy so that clients can treat part and whole objects uniformly.
* Represent a part-whole hierarchy as tree structure.

When defining (1) <code>Part</code> objects and (2) <code>Whole</code> objects that act as containers for <code>Part</code> objects, clients must treat them separately, which complicates client code.<ref>{{cite web|title=The Composite design pattern - Problem, Solution, and Applicability|url=http://w3sdesign.com/?gr=s03&ugr=proble|website=w3sDesign.com|access-date=2017-08-12}}</ref>

===Solutions the Composite design pattern describes===

* Define a unified <code>Component</code> interface for part (<code>Leaf</code>) objects and whole (<code>Composite</code>) objects.
* Individual <code>Leaf</code> objects implement the <code>Component</code> interface directly, and <code>Composite</code> objects forward requests to their child components.

This enables clients to work through the <code>Component</code> interface to treat <code>Leaf</code> and <code>Composite</code> objects uniformly:
<code>Leaf</code> objects perform a request directly,
and <code>Composite</code> objects 
forward the request to their child components recursively downwards the tree structure.
This makes client classes easier to implement, change, test, and reuse.

See also the UML class and object diagram below.