Lecture java1: Object-Oriented Programming is About Data

1. "Java, the fashionable" VS "Java, what is it really"
   
   
2. The title object-oriented programming is a misnomer. It's about

   object-oriented computation        ---        class-based programming
   

   The first design goal is to lay-out the various classes of data that a program processes and how these forms of data interrelate.

   Sounds familiar? This is all the work we go through before we get to do the "real" stuff: programming.

   So what form of data does Java offer? We know from the first few weeks that we want:

   • basic forms of data
   • compound data
   • mixed data
   • arbitrarily large data
   Let's see how these things work out in the Java world.
   
   
3. basic forms of data:

   numbers (5, 7.45; no fractions, no complex): 5 + 55
   int, double

   Booleans (true, false): if true ...
   boolean

   Integer objects new Integer(5): new Integer(5).plus(55)
   Integer

   Boolean objects new Boolean(true)
   Integer
   
   

4. compound data: A monkey has four legs and needs 55 cubic-meters of living space.

   In the Scheme world:

   (define-struct monkey (legs space))
   ;; A monkey is a structure: (make-monkey number number). 
   
   ;; Creating a monkey: 
   (make-monkey 4 55)
   

   In Java:

    
   class Monkey {
     int legs; 
     int space:
     Monkey(int _legs, int _space) {
       legs = _legs; 
       space = _space; 
     }
   }
   
   // Creating a Monkey: 
   new Monkey(4,55)
   

   
   
   
5. mixed data: An animal is either a monkey, an elephant, or a spider.

   In the Scheme world:

   (define-struct spider (legs space))
   ;; A spider is a structure: (make-spider 8 .2)
   
   (define-struct elephant (legs space))
   ;; An elephant is a structure: (make-elephant 8 200)
   
   ;; An animal is either 
   ;;  - a monkey 
   ;;  - an elephant 
   ;;  - a spider. 
   

   In the Java world:

   interface Animal {}
   
   class Spider implements Animal {
     int legs; 
     int space; 
     Spider(int _legs, int _space) {
       legs = _legs; 
       space = _space; 
     }
   }
   
   class Elephant implements Animal {
     int legs; 
     int space; 
     Elephant(int _legs, int _space) {
       legs = _legs; 
       space = _space; 
     }
   }
   
   class Monkey implements Animal {
     int legs; 
     int space; 
     Monkey(int _legs, int _space) {
       legs = _legs; 
       space = _space; 
     }
   }
   

   
   
   
6. arbitrarily large data: a list of integers, a family tree, etc.

   In the Scheme world:

   (define-struct child (name father mother))
   
   ;; An family tree is either 
   ;;  - empty
   ;;  - or a structure: (make-child symbol FT1 FT2)
   ;;    where FT1 and FT2 are family trees. 
   

   In the Java world:

   interface FamilyTree {}
   
   class Empty implements FamilyTree {}
   
   class Child implements FamilyTree {
     String name; 
     FamilyTree father; 
     FamilyTree mother; 
     Child(String name, FamilyTree _father, FamilyTree _mother) {
       name   = _name; 
       father = _father; 
       mother = _mother; 
     }
   }
   
   // Creating Family Trees: 
   
   new Empty()
   
   new FamilyTree("Ben", new Empty(), new Empty())
   

   
   
   
7. So, we have layed out our data, now we are ready to design the methods. Yes, not programs -- methods.