Memento

About 2 min

Also known as

Snapshot
Token

Intent

Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

Explanation

Real-world example

We are working on an astrology application where we need to analyze star properties over time. We are creating snapshots of star states using the Memento pattern.

In plain words

Memento pattern captures object internal state making it easy to store and restore objects in any point of time.

Wikipedia says

The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).

Programmatic Example

Let's first define the types of stars we are capable to handle.

public enum StarType {
  SUN("sun"),
  RED_GIANT("red giant"),
  WHITE_DWARF("white dwarf"),
  SUPERNOVA("supernova"),
  DEAD("dead star");
  // ...
}

Next, let's jump straight to the essentials. Here's the Star class along with the mementos that we need to manipulate. Especially pay attention to getMemento and setMemento methods.

public interface StarMemento {
}

public class Star {

    private StarType type;
    private int ageYears;
    private int massTons;

    public Star(StarType startType, int startAge, int startMass) {
        this.type = startType;
        this.ageYears = startAge;
        this.massTons = startMass;
    }

    public void timePasses() {
        ageYears *= 2;
        massTons *= 8;
        switch (type) {
            case RED_GIANT -> type = StarType.WHITE_DWARF;
            case SUN -> type = StarType.RED_GIANT;
            case SUPERNOVA -> type = StarType.DEAD;
            case WHITE_DWARF -> type = StarType.SUPERNOVA;
            case DEAD -> {
                ageYears *= 2;
                massTons = 0;
            }
            default -> {
            }
        }
    }

    StarMemento getMemento() {
        var state = new StarMementoInternal();
        state.setAgeYears(ageYears);
        state.setMassTons(massTons);
        state.setType(type);
        return state;
    }

    void setMemento(StarMemento memento) {
        var state = (StarMementoInternal) memento;
        this.type = state.getType();
        this.ageYears = state.getAgeYears();
        this.massTons = state.getMassTons();
    }

    @Override
    public String toString() {
        return String.format("%s age: %d years mass: %d tons", type.toString(), ageYears, massTons);
    }

    private static class StarMementoInternal implements StarMemento {

        private StarType type;
        private int ageYears;
        private int massTons;

        // setters and getters ->
        // ...
    }
}

And finally here's how we use the mementos to store and restore star states.

    var states = new Stack<>();
    var star = new Star(StarType.SUN, 10000000, 500000);
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    states.add(star.getMemento());
    star.timePasses();
    LOGGER.info(star.toString());
    while (states.size() > 0) {
      star.setMemento(states.pop());
      LOGGER.info(star.toString());
    }

Program output:

sun age: 10000000 years mass: 500000 tons
red giant age: 20000000 years mass: 4000000 tons
white dwarf age: 40000000 years mass: 32000000 tons
supernova age: 80000000 years mass: 256000000 tons
dead star age: 160000000 years mass: 2048000000 tons
supernova age: 80000000 years mass: 256000000 tons
white dwarf age: 40000000 years mass: 32000000 tons
red giant age: 20000000 years mass: 4000000 tons
sun age: 10000000 years mass: 500000 tons

Class diagram

Applicability

Use the Memento pattern when

A snapshot of an object's state must be saved so that it can be restored to that state later, and
A direct interface to obtaining the state would expose implementation details and break the object's encapsulation

Known uses

Java Util Package: Various classes in the Java Util Package, such as java.util.Date and java.util.Calendar, can be reverted to previous states using similar principles, though not implemented directly as the Memento Pattern.
Undo mechanisms in software: Text editors and graphic editors often use this pattern to implement undo actions.

Consequences

Benefits:

Preserves encapsulation boundaries.
Simplifies the originator by removing the need to manage version history or undo functionality directly.

Trade-offs:

Can be expensive in terms of memory if a large number of states are saved.
Care must be taken to manage the lifecycle of mementos to avoid memory leaks.

Commandopen in new window: Often used together; commands store state for undoing operations in mementos.
Prototypeopen in new window: Mementos may use prototyping to store the state.