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Update README.md

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Ilkka Seppälä 2020-08-29 21:51:32 +03:00
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memento/README.md
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@ -9,24 +9,30 @@ tags:
--- ---
## Also known as ## Also known as
Token Token
## Intent ## Intent
Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored
to this state later. Without violating encapsulation, capture and externalize an object's internal state so that the
object can be restored to this state later.
## Explanation ## Explanation
Real world example Real world example
> 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. > 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.
In plain words In plain words
> Memento pattern captures object internal state making it easy to store and restore objects in any point of time. > Memento pattern captures object internal state making it easy to store and restore objects in any
> point of time.
Wikipedia says 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). > 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** **Programmatic Example**
@ -38,23 +44,13 @@ public enum StarType {
RED_GIANT("red giant"), RED_GIANT("red giant"),
WHITE_DWARF("white dwarf"), WHITE_DWARF("white dwarf"),
SUPERNOVA("supernova"), SUPERNOVA("supernova"),
DEAD("dead star"), DEAD("dead star");
UNDEFINED(""); ...
private final String title;
StarType(String title) {
this.title = title;
}
@Override
public String toString() {
return title;
}
} }
``` ```
Next let's jump straight to the essentials. Here's the star class along with the mementos that we need manipulate. Next, let's jump straight to the essentials. Here's the `Star` class along with the mementos that we
need manipulate. Especially pay attention to `getMemento` and `setMemento` methods.
```java ```java
public interface StarMemento { public interface StarMemento {
@ -123,29 +119,8 @@ public class Star {
private int ageYears; private int ageYears;
private int massTons; private int massTons;
public StarType getType() { // setters and getters ->
return type; ...
}
public void setType(StarType type) {
this.type = type;
}
public int getAgeYears() {
return ageYears;
}
public void setAgeYears(int ageYears) {
this.ageYears = ageYears;
}
public int getMassTons() {
return massTons;
}
public void setMassTons(int massTons) {
this.massTons = massTons;
}
} }
} }
``` ```
@ -172,27 +147,33 @@ And finally here's how we use the mementos to store and restore star states.
star.setMemento(states.pop()); star.setMemento(states.pop());
LOGGER.info(star.toString()); LOGGER.info(star.toString());
} }
// 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
``` ```
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 ## Class diagram
![alt text](./etc/memento.png "Memento") ![alt text](./etc/memento.png "Memento")
## Applicability ## Applicability
Use the Memento pattern when 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 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 * A direct interface to obtaining the state would expose implementation details and break the
object's encapsulation
## Real world examples ## Real world examples

3
memento/src/main/java/com/iluwatar/memento/StarType.java
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@ -31,8 +31,7 @@ public enum StarType {
RED_GIANT("red giant"), RED_GIANT("red giant"),
WHITE_DWARF("white dwarf"), WHITE_DWARF("white dwarf"),
SUPERNOVA("supernova"), SUPERNOVA("supernova"),
DEAD("dead star"), DEAD("dead star");
UNDEFINED("");
private final String title; private final String title;