Object-oriented visualizations

• Encapsulation
  I wear a man's jacket on that day (with inner pockets!) and a brooch on the lapel. I say: I am an object. I offer you many interesting methods, like this nice brooch you see. If you use the method, I go into my pockets - my hidden variables and fields - and figure out what it is that you want and return that value. For example, you might ask me how old I am - that could be a method, getAge (Person p). I open my pocket and take out a card - you don't know if I have my birthday written on here and I look at my armband watch to determine how old I am, or if I keep my age on the card and wake myself up at midnight every night to add a day to the age on the card. After doing my calculations I will let you know the result, and this is all that you really wanted to know, not how I arrived at the result.

  [Note: check the contents of your pockets the day you do this and remove any embarrassing material. I grabbed one day what I thought was a packet of nose tissues, it turned out to be a sanitary napkin... guess I got quite red on that one!]

• Type casting
  Casting an object in Java to another type does not change the object, just the methods and fields that are available. A student noted that this is just like when he puts on a tuxedo in order to be admitted to the Yacht Club Dining Room. He's still the same Nicklas (who normally wears jeans), but now he is recognized by the doorman as a person who can be admitted to the club!
• Traversing the inheritance chain
  A student thought of this one: Trying to find the right value for a variable in an inheritance chain is like a chain of firefighters trying to put out a fire with just a bucket. The person next to the fire looks to see if water (= the variable or method we are looking for) is available. If not, the next guy (= superclass) is given a bucket and asked to fill it with water. This continues until one of the persons actually is near water. This one fills the bucket with water ( = this is what we were looking for!) and passes it back down the chain to the calling instance.
  The connection between fighting fires and programming is surely not lost on practicioners!
• Message passing
  We had an example in Java with little String Transformer objects (from Lynn Andrea Stein's "Introduction to Interactive Programming"). These are little objects that read in a string, transform it (like put it in Pig Latin or UpperCase or UbbyDubby) and return the transformed string. We had a little "playground" with connectors that would take a String from one object and feed it into another one.

  They did not understand that I could have more than one StringTransformaer, nor that I could have more than one of the same one. So I went around making "new" StringTransformers out of students by giving them a pile of empty index cards and instructions on how to transform. Then I tied them together with string by defining the left hand as the reading end and the right hand as the writing one and having them hold the ends of string. I piped a card with a value ("Maria") written onto it down the string to a PigLatinTransformer. He read his instructions, took a new card, wrote "aria-May" on it and piped this to a GreeterTransformer, who took a new card, wrote "Hello aria-May" on it and piped that to an UpperCaser, wo then sent "HELLO ARIA-MAY" down the pipe that was connected to the chalk GUI on the board. Crazy, but they understood!