Sunday, January 29, 2012

Classes, Inheritance, and the Three Little Pigs

In object oriented programing, inheritance refers to the ability to create derived classes (or subclasses) of a class. These derived classes can reuse attributes or code defined in the original (or base) class. The subclasses can inherit the attributes and methods from their base class. In addition, these new classes can also contain code that is specific to the new class itself.

Once upon a time, three little pigs decided to leave home and venture out into the world. They traveled together for a few days until they came upon a charming village nestled within a large forest. Immediately taken with the beauty of the village, the pigs decided to settle there.

Each pig found an open plot of land and began setting up his new home. The first order of business was to build a house. The pigs, knowledgable in both architecture and object oriented programming, each created houses based on the common House class. The House class specified the basic attributes, such as NumberOfWindows, and functionality, such as HeatHouse, that people had come to expect in a house. However, each pig chose a unique subclass to suit his own needs.

The youngest pig decided create a house from the StrawHouse subclass. Many people have argued that this was done out of laziness, as the Build operation for StrawHouse houses requires little work. However, the truth is that the pig had always marveled at the concept of thatched roofs, and the possibility of creating an entire thatched house was exciting.

The middle pig decided to create a house from the WoodHouse subclass. Again the reason went beyond the cost of the Build operation. The second pig preferred the rustic look of wooden houses. Moreover, wooden houses had a nice HangPictureWithNail function that appealed to the pig.

The oldest pig was obsessed with safety. He built his house from the SecureBrickHouse subclass, paying a large upfront cost. He slept better at night knowing that his house provided unique functionality of LatchDeadbolt.

Then, one night, a big bad wolf wandered into the village. He spotted the first pig's house and hungrily eyed its inhabitant. He walked up to the straw house's door and pounded. Of course, knocking on the door was a function that could be applied to any house subclass.

"Little pig, let me in. Or I will huff and puff and blow your house in." threatened the wolf.

"Not by the hair on my chinny chin chin." responded the pig. Despite his bold words, he was worried. He knew that his house had a very poor response to WithstandWind. He very much doubted that his house could handle the breadth of a asthmatic turtle, let alone the wolf.

Fortunately for the pig, the wolf was unaware that houses in the StrawHouse class lacked a LockDoor function. That oversight gave the pig some time. As he heard the wolf begin to huff, the little pig made a small opening in the back of the house and ran for it.

The wolf was in the middle of his first puff when he saw the pig running. He grinned wickedly and gave chase.

The little pig reached his brother's wooden house mere seconds before the wolf. He slammed the door behind him and flipped the lock. His brother looked up from a newspaper.

"Wolf… huff… puff…" was all the first pig could get out between his own heavy breadths. It had been a long run uphill.

Outside the wolf surveyed the house. The construction of this house was more solid, but he was confident that he could still blow it down. And now there were two tasty pigs inside.

"Little pigs, let me in. Or I will huff and puff and blow your house in." threatened the wolf.

"Not by the hair on our chinny chin chins." responded both pigs in unison. Then they looked at each other and darted toward the back of the house. They knew that the house did not stand a chance.

The wolf, seeing the pigs darting out of the house, again gave chase. This time the two little pigs headed to the house of their older brother. It was a long run, but they knew his SecureBrickHouse could withstand a lot of huffing and puffing.

They made it to their brother's house before the wolf. Their brother was in his yard, digging a deep moat. Although he knew that a moat was unnecessary in this neighborhood, it would help him sleep better at night.

"Brother… wolf… huffing…" the two new arrivals panted.

The brother looked up and saw the approaching wolf. The three little pigs dashed into the house, threw the deadbolt, and closed the windows.

"I told you that the cost of the SecureBrickHouse would pay off," started the oldest pig in a lecturing tone. In his view, nobody ever paid enough attention to safety.

"Sure," responded the middle pig. "It is great for cases like this. But your house has a terrible implementation of RetainHeat. Do you remember how cold you were last winter? You had to borrow two quilts."

"And the way sound echoes in here is annoying," added the youngest pig. "Have you ever considered putting up some drywall?"

Outside the wolf reissued his threat. He received no answer. The occupants of the house were too busy arguing the relative merits of the different types of houses.

"At least we can all now agree that building a House was a better idea than building a ApartmentBuilding, right?" offered the oldest pig with a quick glance toward the middle brother. "I could never deal with tenants complaints all day. We got the base class correct."

The other brothers nodded in agreement.

Outside they could hear the faint sounds of rushing wind and a hyperventilating wolf.

"Could we create a new SecureWoodHouse?" asked the middle brother. He liked the warm feeling of wood paneling.

The other two pigs paused in deep thought. "Would we derive it from the WoodHouse?" asked the oldest brother.

"Of course," responded the middle brother, "But we could change the implementation of some of the key functions to make it more suitable to this kind of attack. Maybe add some support beams along the walls. And a LatchDeadbolt function, of course."

"Interesting…" said the oldest brother as he thought over the proposal. "What other functions are you thinking about? How about ShutterWindows?"

Outside the wolf had huffed and puffed until he had passed out.


Interested in learning more about object oriented programming? Read about it Marcus's visit to a cheese factory (Objects, Classes, and Inheritance).

Interested in computational takes on other classic fairy tales? Read Binary Searching for Cinderella or Goldilocks and the Boolean Bears.

On a random note, this is the 75th actual story on Computational Fairy Tales. Have a favorite story or found one that works? Have a request? Let me know at For updates follow @CompFairyTales on twitter.

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