In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
that provides the ability to restore an object to its previous state (undo
Undo
Undo is a command in many computer programs. It erases the last change done to the document reverting it to an older state. In some more advanced programs such as graphic processing, undo will negate the last command done to the file being edited....
via rollback).
The memento pattern is implemented with two objects: the originator and a caretaker. The originator is some object that has an internal state. The caretaker is going to do something to the originator, but wants to be able to undo the change. The caretaker first asks the originator for a memento object. Then it does whatever operation (or sequence of operations) it was going to do. To roll back to the state before the operations, it returns the memento object to the originator. The memento object itself is an opaque object
Magic cookie
A magic cookie or just cookie for short, is a token or short packet of data passed between communicating programs, where the data is typically not meaningful to the recipient program. The contents are opaque and not usually interpreted until the recipient passes the cookie data back to the sender...
(one which the caretaker cannot, or should not, change). When using this pattern, care should be taken if the originator may change other objects or resources - the memento pattern operates on a single object.
A pseudorandom number generator , also known as a deterministic random bit generator , is an algorithm for generating a sequence of numbers that approximates the properties of random numbers...
(it will always produce the same sequence thereafter when initialized with the seed state
Random seed
A random seed is a number used to initialize a pseudorandom number generator.The choice of a good random seed is crucial in the field of computer security...
A finite-state machine or finite-state automaton , or simply a state machine, is a mathematical model used to design computer programs and digital logic circuits. It is conceived as an abstract machine that can be in one of a finite number of states...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
program illustrates the "undo" usage of the Memento Pattern.
import java.util.List;
import java.util.ArrayList;
class Originator {
private String state;
// The class could also contain additional data that is not part of the
// state saved in the memento.
public void set(String state) {
System.out.println("Originator: Setting state to " + state);
this.state = state;
}
public Memento saveToMemento {
System.out.println("Originator: Saving to Memento.");
return new Memento(state);
}
public void restoreFromMemento(Memento memento) {
state = memento.getSavedState;
System.out.println("Originator: State after restoring from Memento: " + state);
}
public static class Memento {
private final String state;
private Memento(String stateToSave) {
state = stateToSave;
}
class Caretaker {
public static void main(String[] args) {
List savedStates = new ArrayList;
Originator originator = new Originator;
originator.set("State1");
originator.set("State2");
savedStates.add(originator.saveToMemento);
originator.set("State3");
// We can request multiple mementos, and choose which one to roll back to.
savedStates.add(originator.saveToMemento);
originator.set("State4");
The output is:
Originator: Setting state to State1
Originator: Setting state to State2
Originator: Saving to Memento.
Originator: Setting state to State3
Originator: Saving to Memento.
Originator: Setting state to State4
Originator: State after restoring from Memento: State3
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
