CSC302 2006S, Class 17: Java as Object-Oriented Language

Admin:
* Congratulations to Peter!
* Reading: Beyond Java 4, 5, 8. (Skipping Ruby.)
* Reading for Monday: Forthcoming.
* Homework: Semantics of D.
* Friday: Movie about Java.  (Guy Steele: Growing a Language)
* Syllabus updated.

Overview:
* Left over from Monday: Polymorphism vs. Overloading
* History: The development of Java
* Your perspective: Why did Java do so well?
* Context: What languages are currently popular?  (and how do we 
  measure popularity?)
* Alternative views of object-orientation

/Polymorphism vs. Overloading/

* Polymorphism: 
  * A function can take one parameter of many types
    e.g., in Java you write things like
      public Number square(Number x)
    that works on Integer, BigInteger, Double, etc.
  * More generally, in object-oriented langauges values in subclasses can be used in place of the superclass 
    * Consider interfaces to be a kind of superclass
  * Why do we like polymorphism?
    * Once we've written a polymorphic function (or other thing),
      we can reapply it to different things
      * Easier to read
      * Less duplicate code (compare to int square(int) + double square(double) + ....)
    * In Java, gives the programmer some freedom in the context of strict typing

* Overloading: You can write more than one function with the same name, and have the compiler/interpreter choose the correct one
  int square(int) ...
  double square(double) ...
  ...
  x = square(x) calls the corret one of the above, depending on the type of x
  x + y could be a number, if x and y are numbers, or a string, if x or y is a string
  * A kind of polymorphism?

/A biased and abbreviated history of Java/

* Java: Object-oriented, imperative, C syntax
  * White papers said: robust, secure, portable, ...
* Developed at Sun in late 80's early 90's because Sun was thinking about the future
  * Enterprise application development
  * (often for Embedded computing)
* Two "library research" questions
  * What features does such a language need?
    * Robust: Toasters shouldn't crash
      * Make it difficult for programmers to write incorrect programs
    * Secure: Only access what's reasonable 
      (Sandboxing)
    * Portable
    * Small footprint (hah!)
  * What existing languages might suffice?
    * None!
    * Few have security features
    * None are portable
    * But C++ has some good features
  * Roll your own
* Early 1990's: The Web appears
  * Whoops!  Needs a programming language
  * Sun marketers: Web pages can be embeeded systems.  Use our language.  And give it a good name.

Two of the main architects
* James Gosling - Author of the horror that is Gosling Emacs (boo)
* Guy Steele - Developer of Scheme (yay)

Java became BIG.  Why?

* Leveraged C syntax that lots of programmers knew; shallow adaptation ramp.
* Applets were cool.
* Lots of programmers converted from C++ to Java because ...?
  * Portability was a big win.
  * Sandboxing seems important.
  * "I hate memory management."
  * Change in attitude
* Good marketing!

/Stone's new favorite bug/

* Domain: Comparing name entered on Web form to names previously used (for validation)
  * Usually, says two different names are the same
  * Usually, says two identical names are the same
  * Ocasionally, says two identical names are different
    * Found the last problem primarily with "Nancy"
* Written in Perl

Correct
if ($newstr eq $oldstr)
Incorrect, but "works" because of coercision
if ($newstr == $oldstr)