Composite pattern

The composite pattern allows complex tree-like structures to be built from simple components. Composite objects are simply container objects, where the content may actually be another composite object.

Traditionally, each component in a composite object must be either a leaf node (that cannot contain other objects) or a composite node. The key is that both composite and leaf nodes can be treated identically. The UML diagram is very simple:

This simple pattern, however, allows us to create very complex arrangements of elements, all of which satisfy the interface of the component object. As an example, here is one such complicated arrangement:

The composite pattern is commonly useful in file/folder-like trees. Regardless of whether a node in the tree is a normal file or a folder, it is still subject to operations such as moving, copying, or deleting the node. We can create a component interface that supports these operations, and then use a composite object to represent folders...