Memento Pattern
Author: Deron Eriksson
Description: This Java tutorial describes the memento pattern, a behavioral object pattern.
Tutorial created using: Windows Vista || JDK 1.6.0_11 || Eclipse JEE Ganymede SR1 (Eclipse 3.4.1)

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The memento pattern is a behavioral design pattern. The memento pattern is used to store an object's state so that this state can be restored at a later point. The saved state data in the memento object is not accessible outside of the object to be saved and restored. This protects the integrity of the saved state data.

In this pattern, an Originator class represents the object whose state we would like to save. A Memento class represents an object to store the state of the Originator. The Memento class is typically a private inner class of the Originator. As a result, the Originator has access to the fields of the memento, but outside classes do not have access to these fields. This means that state information can be transferred between the Memento and the Originator within the Originator class, but outside classes do not have access to the state data stored in the Memento.

The memento pattern also utilizes a Caretaker class. This is the object that is responsible for storing and restoring the Originator's state via a Memento object. Since the Memento is a private inner class, the Memento class type is not visible to the Caretaker. As a result, the Memento object needs to be stored as an Object within the Caretaker.

Now, let's look at an example. The DietInfo class is our Originator class. We'd like to be able to save and restore its state. It contains 3 fields: a dieter name field, the day number of the diet, and the weight of the dieter on the specified day of the diet.

This class contains a private inner class called Memento. This is our Memento class that is used to save the state of DietInfo. Memento has 3 fields representing the dieter name, the day number, and the weight of the dieter.

Notice the save() method of DietInfo. This creates and returns a Memento object. This returned Memento object gets stored by the caretaker. Note that DietInfo.Memento is not visible, so the caretaker can't reference DietInfo.Memento. Instead, it stores the reference as an Object.

The restore() method of DietInfo is used to restore the state of the DietInfo. The caretaker passes in the Memento (as an Object). The memento is cast to a Memento object and then the DietInfo object's state is restored by copying over the values from the memento.

package com.cakes;

// originator - object whose state we want to save
public 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;

	public 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;

	// memento - object that stores the saved state of the originator
	private class Memento {
		String mementoPersonName;
		int mementoDayNumber;
		int mementoWeight;

		public Memento(String personName, int dayNumber, int weight) {
			mementoPersonName = personName;
			mementoDayNumber = dayNumber;
			mementoWeight = weight;

DietInfoCaretaker is the caretaker class that is used to store the state (ie, the memento) of a DietInfo object (ie, the originator). The memento is stored as an object since DietInfo.Memento is not visible to the caretaker. This protects the integrity of the data stored in the Memento object. The caretaker's saveState() method saves the state of the DietInfo object. The caretaker's restoreState() method restores the state of the DietInfo object.

package com.cakes;

// caretaker - saves and restores a DietInfo object's state via a memento
// 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 =;

	public void restoreState(DietInfo dietInfo) {


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