ITEC 120 2012fall dbraffitt ibarland

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lect08a
object equality

Non-static methods:
 - how to *call* them
 
        Song s1 = new Song( "Violet Hill", "Coldplay", 180, true );

        // Instead of   fitsOnDisk(s1,2.5), write:
        s1.fitsOnDisk(2.5);
 
 - how to *write* them.
 
  /* Instead of:
  static boolean fitsOnDisk(Song thisSong, double freeSpace) {
    return thisSong.length * MB_PER_SEC <= freeSpace;
    }
  We will write:
  */
  static boolean fitsOnDisk( /* Song thisSong, */ double freeSpace) {
    return thisSong.length * MB_PER_SEC <= freeSpace;
    }
 




We go through our class Song:
 - remove 'extends Object120' (and making fixes);
 For each method:
   - change how we *call* it, to be OO
   - change how we *define* it, to be OO
   - make sure our tests still work, before proceeding to the next function




Rule-of-thumb:
If the answer to a question depends on who you ask, then the method should be nonstatic.

Examples: 

  

    Hey Song, what is your length?
  
  

    Hey Song, do you fit on a disk with 23MB free?
  
  

    Hey Student, what is your gpa?
  
  

    Hey Student, do you have the same major as that Student?
  




Rule-of-thumb:
If the answer to a question does not depend on who you ask, 
then the method should be static.

Examples: 

  

    What is 47°F, in Celcius?
  
  

    How much disk-space is required for 4:07 of mp3 music?
  
  

    Test all my Robot functions!
  

Rule-of-thumb:
  If it makes sense to call a method, even before a single object has been created,
  then it should (must!) be static.




(2012-fall: we got to here, and spent tomorrow's lab introducing about equality.)
 
Beginning code: Robot.java,
ending code: Robot.java.


Equality:
  intensional ("equals(Object)"; if applied recursively it's a "deep" equality-check)
  extensional (shallow) ("==").

**Draw pictures** (memory diagrams):
  String s1, s2, s3;
  s1 = "election";
  s2 = s1;
  s3 = new String("election");

  s1.equals(s2)
  s1.equals(s3)
  s1 == s2
  s1 == s3

What's going on:
When comparing two object references,
 == returns true if (and only if) they
  reference (point to) the identically-same object.



(reached here)


What `equals` means:
If you don't do anything to say otherwise,
your own classes have a equals
   (inherited from java.lang.Object)
which just does the same thing == does (?!):
  // Inside class java.lang.Object, deep in the bowels of Java:
  public boolean equals(Object that) { return this == that; }

If you want equals to do something smarter,
you have to write it yourself 
(“overriding the version inherited from Object”).
That's what class String does, which is why
equals works for Strings.





Similar for Robots (and evilCopy); Draw the picture:
  Robot r1, r2, r3, r4;
  r1 = new Robot("cell phone",true,true);
  r2 = new Robot("cell phone",true,true);
  r3 = r2;
  r4 = r2.evilCopy();


 - three things extending Object120 gave us:
    - constructor (we fixed)
    - toString (override like you did for Treasure)
    - deep equality-test

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©2012, Ian Barland, Radford University Last modified 2012.Oct.17 (Wed)

Please mail any suggestions (incl. typos, broken links) to ibarlandradford.edu