(Note: Thanks to all the kind people who wrote asking me to finish this
story. I've wrestled all around with this thing and finally feel like I
am starting to win. I have at least one more section to go after this -
I promise I'll get it done faster than this section now that I've
figured out where to take it. Drop me a note if you liked this or have
constructive criticism.) 

Debbie came back two hours later with groceries plus some curtain stuff
from one of those ‘big box' stores. She called to me and put me to work
unloading the car. It was like everything was perfectly normal - except
for the gut wrenching feelings I had every time I looked at my wife. 

"Okay," she said as she wrapped up the last of those little plastic
grocery bags. "We've both had some time to think about what I said
earlier. I want to hear your thoughts, but first I have a couple of
things to say. Is that alright with you?" 

"Yes. I guess so." 

"First, and most importantly, I love you. I want to spend my life with
you. I pray that we find a way through all of this." She paused and
looked at me. I just nodded. "Second, keeping this from you was killing
me. I am so scared of what my revelations might do to us, but at the
same time I am so relieved to get this out in the open." 

"I can see that," I said. 

"So, where do we stand?" she asked. 

"I don't know. I love you. I've loved you since we met. I don't want
things to be messed up either. I guess the bottom line is, you are no
different today than two weeks ago. I still love you. I'm surprised,
but heck, it is in the past. Whatever you did in the past created this
lovely creature I'm married to. I think we'll be fine." 

She smiled sadly at me and said, "But the creature you really married
isn't what you bargained for. You bought a pig in a poke and it turned
out to be an alley cat with low morals." 

"Your past doesn't matter. It is our present and future that does." I
pulled her into my arms and held her close. "That's all in the past. It
doesn't matter." 

She stiffened and pulled away. I saw tears forming in her eyes as she
said, "But it isn't just the past that I'm talking about. It is the
future. Don't you see?" 

"Uhhh..." Mouth to brain! Mayday Mayday Mayday! Need immediate
intelligent input! "But...I...you..."

Andreas Wilting, a PhD student with the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany, visited the Raffles Museum from 16 to 18 Dec 2009 to examine specimens for his study on the phylogeography of cats and civets in Southeast Asia, in particular their evolution in the Sunda region. Andi is working on a research and conservation project called ConCaSa (Conservation of Carnivores in Sabah) in the Deramakot Forest Reserve (DFR), Sabah, which is home to some endangered mammal species.

Andy with our civet cat specimen.

[Photograph by Sum Foong Yee]

from Tom. He is Karen’s older brother from northern Michigan. Karen and I have been married for 19 years and I’ve never met him before, that I can remember. Karen hasn’t seen him in 12 years, and then it was only one afternoon for a family reunion. He flew out for a long weekend to visit everyone. We had a great time.

Buddy and Paulie were even getting aquainted…lol

Today has been rather uninteresting. I woke up, lolled around in bed for a while, then got up and went on Facebook. I still have a cold. I got it on Thursday night. IT FAILS. I feel like there’s a stinging lake of magma at the back of my throat, my hearing is muffled, my ears ache, my nose leaks and I feel lousy. I watched some episodes we recorded from the Simspons marathon on Thursday – including my favourite episode ever, Gone Maggie Gone. It’s a spoof of the Da Vinci Code, which I have never read, but it involves several puzzles as well as its usual insanely funny humour. And believe me, I looooove puzzles.

Then my sister went off to gym, and by a little clicking and Wikipedia’ing, I found myself reading Cyanide and Happiness. At first glance, I don’t like it, but I also spent at least fifteen minutes clicking ‘Random’ on xkcd. xkcd is THE BEST webcomic I have ever read. I haven’t read many, but you gotta love xkcd.

Now, I should say something here. There is a boy who lives up our street, and he is a few months older than me. He comes over every now and then, asking if my sister and I are available to play, and usually we’re not. But I was home with Dad (Mum’s at a rehearsal and sister’s at gym), in my oversized pink nightgown, my head all stuffy and my mood far from pleasant, and yet when the boy comes to the door Dad decides to let him in. I waited until he’d been distracted by our PS3, then got dressed in a rush. It took about ten minutes to get rid of him, since I was not in the mood to entertain unexpected visitors. I have a cold, you know.

Now I’m sitting alone typing this, wearing an Elmo t-shirt that does not match the floral citrus-coloured three-tiered skirt that Gran made me, listening to the constant loop of the background music for Arthur’s Pet Chase (not actually that interesting, for a kiddie computer game) and my throat hurts. Dad’s getting my sister from gym, and I’m just waiting.

If you like someone, and you don’t know if they like you but you’re fairly sure they might, is it a bad thing to be scared of rejection and not make a single move? Just wondering.

Love,

Julia

Surprise the first visitor to the library on International School Library day or on a special day with a gift.

beanie’s preschool class has a mascot named Merly who goes home with a student each week.  and finally, FINALLY it is beanie’s turn.  she’s been waiting since day one.

merly is supposed to explore and have adventures and is kinda helped along my mom and dad to get into some strange places… but beanie will not let merly out of her sight!!  she’s been taking his picture:

at the end of their time together, the parents help to write a story about merly’s visit… and tonight beanie was convinced she’d have to bring merly back to school tomorrow and we HAD to write a story.  i *think* merly is staying with us for the holiday, so we will have 2 weeks of stories once merly returns to preschool!

    i can tell merly’s having fun, but he’s worn out!

During Christmas time these are the opening hours of the Central Library:

Monday 21st of December until Wednesday 23rd of December: 9 a.m. until 8 p.m.

Thursday 24th of December: 9 a.m. until 3 p.m.

Closed from Friday 25th of December until Sunday 3rd of  January.

From Monday 4th of January onwards (until 28/01/2010) extra long opening hours are planned, due to exams.

This means from Monday until Thursday 9 a.m. until 10 p.m. Friday from 9 a.m. until 5 p.m., Saturday from 10 a.m. until 1 p.m.

Best Season’s Greetings.

Being new to this whole blogging thing I wanted to get your professional blogging opinion.

Do you normally leave comments when you visit a blog? Do you only leave comments when you like / dislike a post? 

Why am I asking? Well, I started this blog for selfish reasons really. As a new mom, I wanted something to keep my mind busy while I stayed at home and raised my daughter. It sort of became a little addiction. At night, when I should be sleeping, I’m up surfing other mommy blogs - reading about funny ’spilt milk’ stories and adding a few of my own.  Did you know that there is a whole virtual mommy network out there?

I guess it is a little dark under this rock I’ve been living under. 

I had no idea! And so many of these blogs are totally pro too. Not just the basic wordpress templates but really elaborate ones that make money. Shit. Some mothers are really organized!

I’ve had kikkoo for about a month now and I’m interested to know who visits this site. Are you lost on your way to surfing for something totally unrelated? Did you intend to come here? Where you curious about “kikkoo” and thought you would click on a link? Are these questions too intimate for the virtual world? Do they cross some sort of unwritten rule?

Regardless, I interested in knowing. Who are you?!

Maybe there is no one out there? Maybe I’m just writing this and no one will respond because I don’t have anyone who comes by this site and, well, that is okay too…I guess.

Nope, I lied. It kinds of feels like I’m the last person picked for the team.

Netlog-gebruikers kunnen vrienden worden met de groep Centrale Bibliotheek Leuven

Netlog?

Netlog is een sociaal portaal voor meer dan 55 miljoen jongeren in Europa.
Je eigen profiel met een gastenboek, blog, vrienden, foto’s en video’s. Zoek naar vrienden, leuke evenementen en muziek.

If you plan to visit Thailand for your vacation and you find that Thailand has Thunderstorm everyday, you MUST NOT CHANGE YOUR PLAN!!

Thunderstorm in Thailand is not like what you have seen in New York, Washington or England. In Thailand, everyday is mostly sunny. Thunderstorm will happen at night, beginning at 5-6PM; and it is not longer than 3 hours. Almost raining is about 2 hours. So you can have a lot of time at noon to travel around Thailand.

If you are in Thailand, you can know the thunderstorm before it happens for 1 hour. You have to have a compass with you, and you look at the South West of the sky. Regular round cloud does not cause the rain. The only cloud that that cause the thunderstorm is 100-story-skyscraper cloud. You will see it about 5PM, and it will rain at 6PM. If you understand all this, you do not have to carry an umbrella for whole day. Sw Eden is Thai, and Sw Eden do not carry any umbrella, too.

Hey hey Ken Zi:Kill is not Thai, so he have to carry the umbrella.

The best time to take a tour in Thailand is in the Winter. Most of foreigners who visit Thailand are from colder countries. Thai winter is not that hot as summer. Some people said that Thailand has 4 seasons, are Hot, Hotter, Hottest and Hot Water (rain). In fact, we are the super cool country!! Sw Eden means Thailand is so interesting. Most of the winter day, you will not see any thunderstorm. You can also have a good time with photography in this season, because the shade and shadow are so shape. Winter does not have much cloud. You will see clear-light-blue sky back of your view. Sw Eden loves winter in Thailand a lot. The only thing Sw Eden hates is that Sw Eden does not have a water heater at home, so it is so cold when Sw Eden takes a bath! But you can have a good time with hot bath in every hotel and apartment (All have water heaters).

**If you feel that some swindlers try to bring you into their tour, you can ask me about the trip and its price.

Moon in NYC, Photo by Sw Eden on Flickr

St Andrews was founded on the legend that the bones of the Christian apostle St Andrew were brought here, over 1000 years ago. Credulous pilgrims flocked from all over Europe to worship the saint’s relics, and he became the patron saint of Scotland. The saltire cross – the flag of St Andrew – was subsequently adopted as the national emblem.

From those early days, religion was a strong influence in the development of St Andrews. Celtic monks built the church of St Mary on the Rock, the ruins of which still stand near the harbour, and the imposing St Rule’s Tower is all that remains of the first church of the Augustinian priory in St Andrews.

The great Cathedral, constructed in 1160, was for seven centuries the largest building in Scotland. Its majesty helped to establish St Andrews as the centre of the country’s religious life, which in turn helped the town to become hugely influential in Scottish political circles.

Medieval St Andrews had two friaries: one founded by the Dominicans (Blackfriars), whose ruined chapel still remains, and later another for the Franciscans (Greyfriars) for whom the only mark left on the town is a street name.

Religion was also important in the story of St Andrews Castle. The original castle was built sometime around the year 1200 A.D. and served as both fortress and residential palace for the Bishops and Archbishops of St Andrews. It later housed John Knox and other Protestant reformers.

The 16th Century was the most turbulent period in the history of St Andrews. The (Catholic) Archbishop of St Andrews, Cardinal David Beaton, was the leader of the anti-Protestant movement within the Scottish church and had orchestrated the trials and executions of several “heretics”. When Beaton arranged for the trial and burning at the stake of leading Protestant George Wishart in 1546, the town became an even more volatile place. Before long, Beaton too was dead – a victim of an assassination.

The Calvinist John Knox came to live and preach in the town, but was captured from St Andrews Castle by French Catholic forces in 1549 and enslaved. When Knox returned in 1559, he preached a fiery sermon in Holy Trinity Church which reputedly sparked off a riot amongst the congregation. St Andrews Cathedral was sacked by Knox’s followers, and this effectively signalled the end of the town as a dominant religious force in Scotland.

St Andrews is scattered with reminders of these bloody times. Martyrs’ Monument, on The Scores, commemorates the Protestant reformers who died, and markings on the streets indicate where executions took place.

Even after the end of the tumultuous Reformation, St Andrews was unable to rest in peace. Men and women accused of witchcraft were thrown into the sea below The Scores. If they drowned in “Witch Lake” they were declared innocent; if they swam to shore, they were deemed guilty, seized and burned at the stake at nearby Bow Butts.

See also: the History page for details on visiting the St Andrews Castle, Cathedral and Museum.

Thinking of inviting your parents or relatives to visit USA?

Documents required for inviting someone on Visitor Visa:

To invite your parents, friends, or any other relative to USA, you (the sponsor) need to provide them with a few documents, which are required in order to get the visitor visa approval by the USA consulate.

Sponsor in USA needs to send:

1. Completed and notarized Affidavit of Support form I-134. You can download it from USCIS web site.
[ Form Filing Guidelines-help | Sample Form 134 ]
2. A letter from Bank/banks (on their business letter head), stating your account summary.
(Talk to your bank and ask for your Account Verification letter. which is usually on the bank’s office stationary and should have the Date of Account Opened and Current Balance)
3. Copy of your Bank Statement, showing a good balance in order to justify your financial position.
4. Certificate Letter from your employer (on company’s business letter head), stating that you are a permanent employee of the company, and also stating your salary.
5. 1 or 2 recent pay stub copies.
6. Copy of a few recent income tax returns or last few W2 forms.
7. If you are a Visa holder Copies of your – Visa (H1/L1), H1 Approval Form (I-797), and I-94. (If visa has expired but has a renewed petition, a photocopy of the renewed petition.)
If Green Card Holder: Copy of Green Card front and back.
If USA citizen: Copy of certificate of citizenship.
8. A neat photocopy of your complete Passport, including even the blank pages.
Note: If you are sponsoring your in-laws, photocopies of all pages including blank pages of your spouse’s passport and your marriage certificate is also required.
9. If sponsoring your parents, original birth certificate of the sponsor child (son or daughter) is required.
Note: If you are inviting your in-laws, then spouse’s birth certificate is required not yours.
10. An application/letter from you to the U.S. consulate in India (address it to the particular consulate e.g. Delhi), requesting for issuing a visitor visa to the concerned person, stating that you will be taking care of every requirement for these people, including tickets, boarding and other expenses in USA.
11. A formal invitation letter addressed to the person(s) you are inviting to U.S.. Also stating that you will be taking care of all their needs while in U.S., including tickets, boarding and other expenses in here.

More than 50 people passed out in a matter of minutes while watching a show Festival Film Indonesia (FFI) in 2009 in Kota Batu-Malang, Sunday (6 / 12).

Some visitors who pass out of them are forced to use a respirator. The heat of the sun and the narrowness of the venue FFI allegedly triggers the collapse of many citizens. Also no maximum security committee.

Kasie Trantib Satpol PP Kota Batu, Bambang said the design Suyono event location has never been coordinated with the previous side. Security so that these proceedings can not be maximal.

“I do not quite understand the concept of this event, so do not engage fully to security processes,” he confirmed, Monday (7 / 12).

Winning the nomination announcement event itself FFI conducted yesterday at the Animal park covering an area museum only about 20 meters by 100 meters.

Location was not too free dijubeli thousands of people who come from different parts of Kota Batu and Malang Raya. Presence Raffi Ahmad, Luna Maya and Olga Syahputra absorbing more people. At that time the air temperature around the heat.

“The event just started a few minutes, but the number of visitors who collapsed has reached 50 people. This is because the audience is behind surging forward to the fore.

So who was in front of the crushed, there is fortunate not to critical, “said Supriyanto, one of the officers of the Indonesian Red Cross (PMI) City of Stone that helped evacuate the victims fainted yesterday

PETER TANICO- Getting 2 Know Tanico

PETER TANICO – Getting 2 Know Tanico -
WORLDWIDE DISTRIBUTION THIS WEEK!!!

Enjoy this Getting 2 Know Tanico promo video with a sample of all 16 tracks on the album (listed below.) Enjoy!

Exhibition on comics from 9th of December onwards.

More info here.

I always worked “domain driven”. I like objects, that’s the truth. It’s like the game Lemmings where you told to that little ones with the green hair what to do. And I like the idea of low coupling, too. So, for me, persistence and domain model was always in different places. In other words, I don’t like my model knowing about persistence.  But I always concerned about the fact that sometimes we have some rules that will use our database (through a repository, dao, whatever) and they are related to an inheritance of our “entities”. It stop worrying me when I meet the visitor/double dispatch patterns, because they let the inheritance to chose what to do,  even when they don’t know what they are doing (?).

Some days ago, a friend of mine told me about his experiences with ActiveRecord an how wonderful it is. I won’t talk about it, I know the pattern but I didn’t use it. I don’t like the idea of the model knowing how to persist and having access to the entire repository BUT for polymorphic rules, it is just simple and I like that (a lot). So, we talked for hours about it, I read some blog posts and decided to try it in the near future (I hope I will post about it in the near future of that near future).

Two days after this long talk, I had to design a solution for a problem that sounds like this: Suppose you have to implement a rules system like outlook’s. I mean, you have rules to apply to every new email. These rules could be applied according the subject, the sender, etc. and they could do every possible transformation: move it to a folder, adding a priority, even changing the subject (the actual problem wasn’t about emails, but I won’t talk about the project I’m working on. Confidencial stuff, you know). And when a rule is created, it could be applied to every past email. So, I have a rule that, at least in the first approach, will be implemented using the database. So, my initial model was something like:

My first thought was “ok, I need a kind of IRuleVisitor…” and then I realized the rules could be… hundred. I mean, that was what the requirement says. So, goodbye Visitor (I won’t have an interface with 100 methods, coupled to every kind of rule. no, no, no!!!). And the ActiveRecord ghost started to whisper. “If I would use ActiveRecord, every db implementation will be on the rule class itself and it will be just fine. How do hay split them here?”. Since I hadn’t any idea of how to solve it, I went to play the guitar for a while. But my head continued working in background.

Next morning I had a pretty nice solution and I want to share it with you (“you”… since it’s my first actual post, I think there won’t be a lot of “you’s” for a long time):

My Rule class have an abstract method void ApplyUsing( IRuleExecutor ruleExecutor). This IRuleExecutor interface has one single method: void Execute<R>( R rule ) where R : Rule.

The magic here is that every rule implement this method in the same way: ruleExecutor.Execute( this ). This remaind me to the visitor pattern, in which every subclass implements the Accept method like visitor.Visit( this ).

The interesting part is the implementation of this IRuleExecutor. It needed a new interface, IRuleImplementation<R> where R : Rule and, in order to find the right implementation for each rule,  it has a reference to the application context (I called it “object factory”), so:

public void Execute<R>( R rule ) where R: Rule {
   objectFactory.Get<IRuleImplementation<R>>().Execute( rule );
}

So, I have one class by rule for persistence and one class by rule implementation. Each class has 2 or 3 lines of code and, since all is generic, it doesn’t need to cast (ok, I know the object factory is doing it a lot, but… it’s another story). I called it “Visitor on steroids”.

Disclaimer: I know it has nothing to do with ActiveRecord, I’m just making conversation (or something like that).

These are notes I made while preparing for SCEA 5. I would like to thank my friend Sajitha for helping in scribing these notes.

1.     GOF DESIGN PATTERNS

1.1.             Abstract Factory

Provide an interface for creating families of related or dependent objects with out specifying their concrete classes

Advantages

• One level of abstraction higher than factory pattern – It is a factory method that returns one of several factories
• It encapsulates a group of individual factories having a common theme
• Isolates concrete classes
• Makes exchanging product families easy
• Promotes consistency among products

Disadvantages

• Supporting new kind of products is difficult

Example      EJBHome interface that creates new EJBObjects
javax.servlet.jsp.JSPFactory – Abstract class that defines factory methods available to JSP at runtime

1.2.             Adaptor

Convert the interface of a class to another interface that the client expects

Advantages

• Allows two or more incompatible objects to interact
• Reuse of older functionality
• Adapter class ‘adapts’ adaptee to target by committing a concrete adapter class
  • Adapter is a subclass of adaptee
  • Introduces only one object for adaption
  • It will not work when a class and all its subclasses have to be ‘adapted’
• Object adapter has an instance variable of type Adaptee
  • Single adapter works with all of adaptees sub-classes
  • Makes it harder to override adaptee behavior
• Adapter is usually used to avoid changing existing classes [not an up-front design choice when both systems do not exist] Unlike Bridge pattern – both ends of adapter already exist and it glues them together

Example      Vendor specific subclasses of an abstract DAO

1.3.             Bridge

Decouple an abstraction from the implementation so that the two can vary independently

• Abstraction                  =  Entity
• Implementation            =  Behavior
• Similar to Adapter – The Entity adapts the behavior to a different interface [Entity interface]
• Differs from Adapter – It is a design choice [both ends do not exist for Adapter
• Similar to Strategy and State in structure
• Differs from Strategy and State because these two design patterns have a single hierarchy of classes, so the behavior varies but the entity does not.

Advantages

• Two hierarchies of classes that vary independently
• The abstraction [entity] has-a implementation [behavior]
• Used rarely, often combines with AbstractFactory – [creating a family of products – Entity and Behavior]
• Hides implementation details from client
• Allows to assign and change behavior of entities at runtime

Disadvantages

• Behavior interface will be wide if the entities are highly orthogonal [Many varieties of operation]
• Delegation from Entities to Behaviors may degrade performance
• If a new entity is not satisfied with the current behavior interface, then changing interface needs extensive work

Example     DAO Implements Bridge pattern to separate services

1.4.             Builder

Separates the construction of a complex object from its representation so that the same construction process can create different representation

• Special case of strategy applied to create object
• Director has a fixed high-level procedure, but the actual construction process depends on what is being built.  This construction process is encapsulated in a Strategy object called the Builder.  E.g. Builder separates the push scheduler and data formatter for HTTP GET request

Advantages

• Lets you vary a products internal representation
• Isolates code for construction from representation
• Finer control over construction process
  • Director constructs the object step by step
  • Each step is implemented by a concrete builder
  • Once all steps are done Director retrieves the result

Example

1.5.             Chain of Responsibility

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request.  Chain the receiving objects and pass the request along the chain until an object handles it

Advantages

• Reduced coupling – Client need not know the receiver of request
• Added flexibility in assigning responsibilities to objects

Disadvantages

• Receipt of request is not guaranteed
• Chain may get lengthy and introduce performance problem

Example      Commonly used for parsers

1.6.             Command

Encapsulate a request as an object to parameterize clients with different requests, queue, or log requests and support undo operations

Advantages

• Easy to add new commands
• Can be assembled into composite commands
• Decouples the invoking object from the object that performs an operation
• Similar to Strategy – execute is the strategy method
• Differs to Strategy –
  • command is a user triggered action
  • Various commands can be unrelated – e.g. Strategy represents a single business rule such as tax calculations.  All Strategies do the same thing in a different way e.g. State Tax calculation verses Federal Tax calculation
  •  

Disadvantages

Example

1.7.             Composite

Compose objects into a tree structure to represent whole-part hierarchies

Advantages

• Lets clients treat individual objects and compositions of objects uniformly
• Defines hierarchies of primitive objects and composite objects
• Makes it easier to ass new kinds of components

Disadvantages

• Leaf has meaningless methods like add()
• Design could be made overly general, making it harder to restrict the components of a composite e.g. Make a composite have only certain components
• Type system will not enforce those contraints

Example      Aggregate Entity Beans,   XML DOM Model,     Swing/AWT containers

1.8.             Decorator

Attach additional responsibilities to an object dynamically

• Proxy is a special case of decorator pattern.  Proxy is used with regard to security and networking and typically a class with a proxy cannot be accessed directly.

Advantages

• Modifies the behavior of individual objects without having to create a derived class and do this at run time
• More flexibility then static inheritance [ ‘has-a’ not ‘is-a’]
• Avoids feature laden classes high up in hierarchy
• Each feature is implemented in separate decorator class

Disadvantages

• A decorator and its component are not identical.  So cannot rely on object identity when using this pattern
• It results in a lot of little objects composing the system

Example    The java.io streams library implementation uses decorators to add responsibility to the stream

1.9.             Façade

Unified interface to a set of interfaces in a system.

Advantages

• Reduces number of objects a client has to deal with
• Promotes weak coupling and less compilation dependencies
• Clients can still use subsystem classes [i.e. they do not have to go to the façade]

Example      Session entity façade pattern in J2EE

1.10.        Factory Method

Defines an interface to create an object but it lets the subclasses decide which class to instantiate

• A public creation method typically of the class that the method belongs to

Advantages

• Gives subclasses a hook for returning subclass

Disadvantages

• Have to subclass creator just to create a particular concrete product

Example      EJBHome interface,   Connection factories for EIS

1.11.        Flyweight

Use sharing to support large number of fine grain objects efficiently

Advantages

• Key concept is distinct between intrinsic and extrinsic state – Intrinsic state is stored in the flyweight, Extrinsic state depends and varies with flyweight’s context and so cannot be shared
  • Client objects are responsible for passing extrinsic state to flyweight
• Minimizes memory occupation by sharing as much data as possible with other similar objects
• Reduction in number of objects to handle

Disadvantages

Example      Pooled EJBs,    Pooled database connections

1.12.        Interpreter

Given a language, defines a representation for its grammar along with an Interpreter that uses the representation to interpret sentences in the language

• Special case of Composite is applied to Parsing

Advantage

• Implementing grammar is easy
• Easy to change and Extend grammar

Disadvantages

• Complex grammars are hard to maintain. At least one class per rule.

Example     

1.13.        Iterator

Provide a way to access elements of aggregate object with out exposing its underlying representation.

Advantages

• Supports traversals of objects in a collection
• Variation in traversal achieved just by changing iterator

Example

1.14.        Mediator

An object that encapsulates how a set of objects interact

Advantages

• Centralized control
• Loose coupling by preventing objects from referring each other

Example

1.15.        Memento

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

• Related to Command pattern regarding undo pattern

Advantages

• Preserves encapsulation boundaries
• Simplifies the originator [Object does not have to cache its own state as it changes]

Disadvantages

• Expensive if large data resides in Memento’s state
• It may be difficult in some languages to ensure only originator can access memento’s state
• Hidden cost of caring [and deleting] members

Example      dfs

1.16.        Observer

Defines a one-to-many dependency between objects, so that when one object changes state, all the dependents are notified and updated automatically

Advantages

• Abstract coupling between subject and observer
• Support for broadcast communication

Disadvantages

• Unexpected updates – Since observers are unaware of each other’s presence, they can be blind to the cost of changing the subject.  A seemingly innocuous operation on the subject may cause a cascade of updates to observers and their dependent object
• Without additional protocol to help observers discover what changed, they may be forced to work hard to deduce what changed

Example      Swing/AWT Listeners and Observers

1.17.        Prototype

Specify the kind of object to instantiate using a prototypical instantiate using a prototypical instance and create new objects by copying this prototype

Advantages

• Add/remove concrete products just by registering a prototypical instance with a client
• Define new kinds of objects by instantiating existing classes and registering the instances as prototypes of client objects
• The client exhibits different behavior by delegating responsibility to the prototype
• Reduce sub-classing – Factory pattern often produces hierarchy of creator classes that parallels the product class hierarchy The prototype pattern allows cloning a prototype instead of using a factory method to create a new object

Example

1.18.        Proxy

Provide a surrogate or place holder for another object to control access to it.

Advantages

• A remote proxy provides a local representation for an object in different address space
• A virtual proxy creates expensive objects on demand
• A protection proxy controls access to the original object
• A smart reference is a replacement for a bare pointer that performs additional actions when an object is accessed [e.g. counting number of references, loading persistent objects to memory on access, etc ]

Example    EJB’s remote interface is a proxy for a bean

                   Proxy is used in RMI

1.19.        Singleton

Control the number of instances of a class to a constant usually one and provide single global point of access.

Advantages

• Avoid polluting namespace with global variables
• More flexible than class variables or methods

Example      Math class in Java

1.20.        State

State pattern allows an object to alter its behavior when it’s internal state changes.  The object will appear to change its class.

Advantages

• Localizes state specific behavior and partitions behavior for different states
• Makes state transition explicit – All variables of state change together in a single call.
• State objects can be shared like flyweights if states do not have instance variable [Type is a state]
• State objects are often singletons [Gof page 313]
• Similar to Strategy [& Command] – Single class/interface with many subclasses.  The behavior is that of Strategy / Command design pattern
• Differs from Strategy [& Command] – Intend is different.  Behavior is varied by changing state of object and not by performing[or invoking] a single action.
• Similar to Bridge – State is the implementation of behavior for abstract object
• Differs from Bridge – Bridge separates abstraction from implementation and is not based on the state of the object. Bridge pattern can implement any behavior on any instance of the abstraction

Example

1.21.        Strategy

Defines a family of algorithms, encapsulates each one and makes them interchangeable [Alternative to sub classing – Context ‘has-a’ instead of a ‘is-a’ behavior]

Advantages

• Easier to extend a model to incorporate new behavior

Disadvantages

• Clients must be aware of an understand different strategies
• Increased number of objects
• Single interface for all strategies so simpler implementations may not need all the parameters given – Communication overhead.

Example

1.22.        Template Method

Defines a skeleton of an algorithm in an operation deferring some steps to subclasses

• An alternative to Strategy when there are invariant parts of the algorithm

Advantages

• Fundamental technique for code reuse
• Inverted control structure [parent class calls subclass’s method]

Example      The director in builder pattern may be implementing this pattern – only difference being Builders are not subclasses of Director

1.23.         Visitor

It represents an operation to be performed on the elements of an object structure. 

• Visitor lets us define a new operation without changing the classes of the elements on which it operates
• Each new operation is a subclass of the abstract Visitor

Advantages

• Visitor is after-the-fact substitute for multiple inheritance
• When an element is visited, it calls the visitor operation that corresponds to its class. The element supplies itself as an argument to this operation to let the visitor access its state
• Gathers related operations and separates unrelated ones. [Related operations in visitor and unrelated in their won visitor subclasses
• Visitor can visit objects that do not have same parent class

Disadvantages

• Adding new concrete elements is hard [needs new visitXXX method visitor] Consider whether it is most likely to change the algorithm [visitor] applied over an object structure or the classes that make up the structure
• Breaking encapsulation to allow visitor to access state

Example      jaxb XJC implementation uses Visitor pattern