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iluwatar#590 add explanation for Memento
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‎memento/README.md‎

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## Intent
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Without violating encapsulation, capture and externalize an
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object's internal state so that the object can be restored to this state later.
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Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored
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to this state later.
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## Explanation
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Real world example
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> We are working on astrology application where we need to analyze star properties over time. We are creating snapshots of star state using Memento pattern.
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In plain words
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> Memento pattern captures object internal state making it easy to store and restore objects in any point of time.
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Wikipedia says
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> The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).
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**Programmatic Example**
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Let's first define the types of stars we are capable to handle.
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```java
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public enum StarType {
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SUN("sun"), RED_GIANT("red giant"), WHITE_DWARF("white dwarf"), SUPERNOVA("supernova"), DEAD(
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"dead star"), UNDEFINED("");
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private String title;
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StarType(String title) {
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this.title = title;
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}
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@Override
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public String toString() {
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return title;
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}
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}
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```
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Next let's jump straight to the essentials. Here's the star class along with the mementos that we need manipulate.
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```java
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public interface StarMemento {
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}
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public class Star {
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private StarType type;
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private int ageYears;
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private int massTons;
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public Star(StarType startType, int startAge, int startMass) {
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this.type = startType;
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this.ageYears = startAge;
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this.massTons = startMass;
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}
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public void timePasses() {
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ageYears *= 2;
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massTons *= 8;
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switch (type) {
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case RED_GIANT:
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type = StarType.WHITE_DWARF;
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break;
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case SUN:
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type = StarType.RED_GIANT;
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break;
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case SUPERNOVA:
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type = StarType.DEAD;
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break;
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case WHITE_DWARF:
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type = StarType.SUPERNOVA;
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break;
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case DEAD:
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ageYears *= 2;
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massTons = 0;
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break;
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default:
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break;
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}
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}
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StarMemento getMemento() {
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StarMementoInternal state = new StarMementoInternal();
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state.setAgeYears(ageYears);
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state.setMassTons(massTons);
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state.setType(type);
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return state;
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}
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void setMemento(StarMemento memento) {
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StarMementoInternal state = (StarMementoInternal) memento;
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this.type = state.getType();
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this.ageYears = state.getAgeYears();
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this.massTons = state.getMassTons();
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}
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@Override
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public String toString() {
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return String.format("%s age: %d years mass: %d tons", type.toString(), ageYears, massTons);
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}
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private static class StarMementoInternal implements StarMemento {
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private StarType type;
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private int ageYears;
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private int massTons;
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public StarType getType() {
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return type;
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}
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public void setType(StarType type) {
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this.type = type;
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}
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public int getAgeYears() {
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return ageYears;
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}
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public void setAgeYears(int ageYears) {
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this.ageYears = ageYears;
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}
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public int getMassTons() {
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return massTons;
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}
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public void setMassTons(int massTons) {
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this.massTons = massTons;
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}
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}
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}
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```
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And finally here's how we use the mementos to store and restore star states.
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```java
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Stack<StarMemento> states = new Stack<>();
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Star star = new Star(StarType.SUN, 10000000, 500000);
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LOGGER.info(star.toString());
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states.add(star.getMemento());
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star.timePasses();
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LOGGER.info(star.toString());
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states.add(star.getMemento());
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star.timePasses();
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LOGGER.info(star.toString());
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states.add(star.getMemento());
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star.timePasses();
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LOGGER.info(star.toString());
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states.add(star.getMemento());
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star.timePasses();
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LOGGER.info(star.toString());
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while (states.size() > 0) {
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star.setMemento(states.pop());
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LOGGER.info(star.toString());
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}
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// sun age: 10000000 years mass: 500000 tons
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// red giant age: 20000000 years mass: 4000000 tons
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// white dwarf age: 40000000 years mass: 32000000 tons
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// supernova age: 80000000 years mass: 256000000 tons
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// dead star age: 160000000 years mass: 2048000000 tons
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// supernova age: 80000000 years mass: 256000000 tons
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// white dwarf age: 40000000 years mass: 32000000 tons
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// red giant age: 20000000 years mass: 4000000 tons
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// sun age: 10000000 years mass: 500000 tons
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```
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## Class diagram
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![alt text](./etc/memento.png "Memento")

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