1
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science
14 – The Memento Pattern; Java Serialization
CMPSCI 220 (291A)
Programming Methodology
Spring 2009
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science 2
The Memento Pattern
Goals:
Save important state of a system’s key object …
While maintaining the key object’s encapsulation
Benefits:
Separating saved state from the key object maintains
cohesion and the key object’s encapsulation
Provides easy-to-implement recovery capability
Considerations:
Saving and restoring state can be time consuming
Java Serialization is a good implementation approach
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science 3
Memento Pattern
Client
//when important state is reached
Object saved =
(Object)ko.getCurrentState();
//when a restore is required
ko.restoreState(saved);
KeyObject ____
objectState _____
Object getCurrentState() {
//gather state
return(objectState);
}
restoreState(Object savedState); {
//restore state
}
// do other Key Object stuff
KeyObjectMemento
savedObjectState
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science 4
Memento Pattern in the Project
BreakoutGame/BOEngine
//when save is requested
GameSnapshot snapshot =
GameSnapshot.makeSnapshot(
(game, gameState); // **
//when restoring (a new game)
… = new BreakoutGame(
gameEngine, snapshot); // ***
BreakoutGame/GameState
GameState snapshot
void saveGame() {
//gather state: code at **
}
BreakoutGame(Game Engine e,
GameSnapshot s); {
//restore state: code at ***
}
// do other Key Object stuff
GameSnapshot
savedBall
savedBlocks
…
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science 5
Making a Memento in Java
Often want to make a copy of an object
In Java, this is called cloning
Class Object has a default clone method
Signature: protected Object clone()
Creates a duplicate whose fields have the same values as
the original
This is a shallow copy (more in a moment)
Must implement the Cloneable marker interface in
order to clone an object
U
UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science 6
Shallow versus Deep Copies
