Annex B: Design Patterns

B6.Factory Method

Description

When an algorithm needs to create an object whose its concrete class is variable according to the context, this owner proposes the delegation with a subclass of the instanciation of this object.

Structure and Components

Structure and Components of the Factory Method design pattern

• Creator defines a method in which an object must be created and delegates this instantiation to one of its subclasses by the definition of a method to be specialised (method newProduct ),
• Creator1 and Creator2 establish in a concrete way instanyiation of the required object,
• Product defines the abstract interface of the object which is handled by Creator ,
• Product1 and Product2 establish the behaviours of the objects to be used.

Collaborations

• Creator delegates to its subclasses the instantiation of the necessary object.

Strengths and Weaknesses

we do not have to specify the object to be instantiated directly and allows a subclass to choose the desired concrete object,

in certain cases, the specialisation of the base-class is carried out only to specify the instantiation (heavier operation for example than to pass as a parameter the requested object). It is thus preferable to use this pattern when it is also necessary for other reasons to specialise the base-class.

Use in CBR*Tools

This pattern occurs in various algorithms of CBR*Tools and in particular in the internal protocols of the index for the creation of the objects holding the result of a search (of IndexResult class) so as to instantiate the adequate objects.

Brigitte Trousse
Last modified: Fri Sep 28 13:51:57 MEST 2001