Memento Pattern or Token

-Sayanton Vhaduri DibboDepartment of Computer Science & EngineeringUniversity of Dhaka, BangladeshMemento Patternor Token

TopicsFriday, May 13, 20162IntentMotivationApplicabilityParticipantsExampleReal life scenerioCodingClass DiagramAdvantage & Disadvantage

IntentFriday, May 13, 2016

3

A Behavioral Pattern that can be used to promote undo or rollback to objectstatus.

Without violating encapsulation, capture and externalize an objects internal state so that the object can be returned to this statelater

MotivationFriday, May 13, 20164

Often it is needed to backtrack when a particular path proves unproductive.

Examples: graph algorithms, text navigation etc.

The exploration of some complex data structure is done by many technical processes.

ApplicabilityFriday, May 13, 20165

To Keep a snapshot of object to restore laterWhen using direct interface to restore state will violate design rule

ParticipantsFriday, May 13, 20166Mementostores internal state of the Originator objectInner class of originator & private.The Memento must have two interfaces

OriginatorCreates a memento objectUse the memento to save & restore

CaretakerResponsible for keeping the mementoThe memento is black box to the caretaker, and the caretaker must not operate on it

Internal behavior of code is unknown

ExampleFriday, May 13, 20167

MementoCaretakerOriginatorStores internal materials of the house(like the store room of home)Responsible for storing & maintaining household good(like a night guard)Who stores in house(like the owner of the house)

Use Of MementoFriday, May 13, 20168

GamesDatabase Transaction

Mom Alice

Well. I want to add.Editor (any type of files)Calculator

Real life ScenerioFriday, May 13, 20169

Im getting fatty!Which pattern can restore me to my slim previous state???

SolutionFriday, May 13, 201610

Yes!MementoIs the solution*Diet_info class: originatorDieter fieldNo of diet dayWeight of dieter*inside memento class to save previous state.*save(): creates & return object*Restore(): return previous state

Code Friday, May 13, 201611public class DietInfo {

String personName;int dayNumber;int weight;

public DietInfo(String personName, int dayNumber, int weight) {this.personName = personName;this.dayNumber = dayNumber;this.weight = weight;}

public String toString() {return "Name: " + personName + ", day number: " + dayNumber + ", weight: " + weight;}

public void setDayNumberAndWeight(int dayNumber, int weight) {this.dayNumber = dayNumber;this.weight = weight;}

Originator class

CodeFriday, May 13, 201612public Memento save() {return new Memento(personName, dayNumber, weight);}

public void restore(Object objMemento) {Memento memento = (Memento) objMemento;personName = memento.mementoPersonName;dayNumber = memento.mementoDayNumber;weight = memento.mementoWeight;}

private class Memento {String mementoPersonName;int mementoDayNumber;int mementoWeight;

public Memento(String personName, int dayNumber, int weight) {mementoPersonName = personName;mementoDayNumber = dayNumber;mementoWeight = weight;}}}creates and returns a Memento objectrestore the state of the Diet Infosave the state of DietInfo

CodeFriday, May 13, 201613// note that DietInfo.Memento isn't visible to the caretaker so we need to cast the memento to Object//

public class DietInfoCaretaker {

Object objMemento;

public void saveState(DietInfo dietInfo) {objMemento = dietInfo.save();}

public void restoreState(DietInfo dietInfo) {dietInfo.restore(objMemento);}

}caretaker classstores the stateOf dietinfo class

CodeFriday, May 13, 201614public class MementoDemo {

public static void main(String[] args) { // caretakerDietInfoCaretaker dietInfoCaretaker = new DietInfoCaretaker();

// originatorDietInfo dietInfo = new DietInfo("Fred", 1, 100);System.out.println(dietInfo);

dietInfo.setDayNumberAndWeight(2, 99);System.out.println(dietInfo);

System.out.println("Saving state.");dietInfoCaretaker.saveState(dietInfo);

dietInfo.setDayNumberAndWeight(3, 98);System.out.println(dietInfo);

dietInfo.setDayNumberAndWeight(4, 97);System.out.println(dietInfo);

System.out.println("Restoring saved state.");dietInfoCaretaker.restoreState(dietInfo);System.out.println(dietInfo);

}

}

Class DiagramFriday, May 13, 201615

Diet info(Originator)

+Person name:string+day no:int+weight: int

+DietInfo():void+setDayNumberAndWeight():void

+save() :string+Restore() :void- Memento() :void

Memento

-mementoPersonName: string-mementoDayNumber: int-mementoWeight :int

+memento(): voidDiet info caretaker(caretaker)

+Object objMemento

+saveState() :void+restoreState :void

Creates a memento objectstores internal state Object stored by caretaker

Advantages & disadvantagesFriday, May 13, 201616 Benefits:

Provides a way of recording the internal state of an object in a separate object without violating law of design pattern.

Eliminates the need for multiple creation of the same object for the sole purpose of saving its state.

Simplifies the Originator by giving responsibility of Memento storage among the Caretakers.

Advantages & disadvantages(cont)Friday, May 13, 201617 Drawbacks:

Saving and restoring state may be time consuming.

Memento object must provide two types of interfaces: a narrow interface for Caretaker and a wide interface for the Originator.

enables the other object to arbitrarily change the state of object.

Related PatternsFriday, May 13, 201618Command :

Commands can use mementos to maintain state for undoable operations.

But usually command pattern does not provide this undo operation.

Iterator :

Mementos can be used for iteration to keep the state of every elements saved in.

Friday, May 13, 201619

Thank You!