MEMENTO behavioral pattern

1
MEMENTO behavioral pattern

• LAVANYA GUNDAMARAJU

2
DEFINITION

• A Memento is an object that stores the snapshot
  of the internal state of another object.
• A memento is an alternate representation of
  another object, often in a format suitable for
  transmission across an external interface.

3
Why???

• Undoable operations
• Commands can use mementos to maintain state for
  undoable operations.
• Reminder operations
• A memento is a reminder of how things were.
• State specific operations
• A memento can be used with iterator to search
  collections for specific states.

4
What???

• Capture and externalize
• Memento's intent is to capture and
  externalize an object's state so that the object
  can be restored to that state at a later time.
  Its used in conjunction with Command and
  Iterator Patterns.
• Internal state  should be available but
  not visible to other objects

5
How???

• Public method
• Issues
• -Vulnerability The entire system vulnerable to
  change by external program code
• -Security Data inside an object will no more
  be private and encapsulated from the outside
  world
• Solution
• Memento

6
Memento

• Privileged classes
• Other objects have only a more restricted access
  to the object
• Preserve their encapsulation
• Roles for objects

7
Contd

• The Originator is the object whose state we want
  to save.
• The Memento is another object that saves the
  state of the Originator.
• The Caretaker manages the timing of saving the
  state, saves the Memento, and, if needed, uses
  the Memento to restore the state of the
  Originator.

8
Description

• Memento is supposed to have two interfaces, a
  private one accessible only to the Originator and
  a public one accessible to the Caretaker.
• Memento gives the Originator a wide interface
  and the Caretaker a narrow interface.
• C Friend functions(Originator to be a friend
  of memento)
• Java Inner Classes (memento inner class of
  originator)

9

• public class Originator
• private int number
• private File file null
• // create a Memento
• public Memento getMemento()
• return new Memento(this)
• // restore the state of the originator
• public void setMemento(Memento m)
• number m.number
• file m.file
• //state that can be used for read/write from/to
  disk
• private class Memento implements
  java.io.Serializable private int number
• private File file null
• // private constructor - only the originator
  can create
• private Memento( Originator o)
• number o.number
• file o.file

10
Questions

• ? ? ?
• ?
• ? ?
• ?
• ? ?
• ?