复合图案

Question

I have a question regarding composite pattern.

Does the base class "Component" act like a pointer to point leaf object in "Composite" class?

Edit: Let me ask my question in following words. "What is the relation between Composite and Component class?"

Here is the uml class diagram of the pattern.

Answer 1

Relation between composite and component:

1) Leaf and Composite usually implement one interface or one abstract class. In your diagram they extend Component. So, the relation on your diagram is inheritance .

2) Composite contains instances of Component. Component, as it occasionally can be Composite, can also contain instances of Component. This is called recursive composition . In general, the relation is called aggregation .

Answer 2

Component

• is the abstraction for all components, including composite ones
• declares the interface for objects in the composition
• (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate

Leaf

• represents leaf objects in the composition
• implements all Component methods

Composite

• represents a composite Component (component having children)
• implements methods to manipulate children
• implements all Component methods, generally by delegating them to its children

http://en.wikipedia.org/wiki/Composite_pattern

Answer 3

All container and containee classes declare an “is a” relationship to the interface( Component ).

All container classes declare a one-to-many “has a” relationship to the interface.

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