Applied Design Patterns with Java

Behavioral :: Memento (283) {C ch 21}

Example - Java :: Patterns\Behavioral\Memento

Pattern Concept: to allow saving data about an object, so that it can be restored later. The Memento pattern provides a means to save and restore an object's state without making each object doing the work, and yet preserving Encapsulation. The Memento pattern has privileged access to the object's that need to have their information saved. Cooper's Memento pattern example uses a Java specific facility - package access. This is the access control for a class member that is unlabeled as public, protected, or private. This unlabled access control allows another class in the SAME PACKAGE to access the class member. Some Java authorities refer to this access control mode as 'friendly access'. Note that this Memento solution is not portable to other OOP languages.

Example - UML : MemDraw
Here is Cooper's Class Diagram for the application using this logic, followed by the Rose equivalent:

The example Java program is called 'MemDraw'.

The UML diagram is above, and the list of Java files is below:

Issues and consequences of the Memento pattern include:

• The Memento pattern provides a way to preserve the state of an object while still preserving Encapsulation. This is not possible in all OOP languages, but C++ and Java both have unique means to achieve this pattern. The key is that the saved / restored data remains "effectively" private. The Memento pattern simplifies the problem by delegating the responsibility for doing the save / restore work to the The Memento class.
• The Memento pattern may consume significant resources in both time and storage, if the amount of object data that needs to be saved / restored is large. One solution is to design a governing strategy that limits the number of objects for which a Memento object can manage saving / restoring. For very stable systems with objects that change state predictably, an incremental save / restore strategy might be useful for the Memento.

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