ITEC 120 2010fall ibarland

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lect06a
Objects intro
compound data (fields)

Update: A version of Object120 has been released as of Oct.04 (v1.03 (adamant)); please use the updated version from now on. Note that you can call Object120.getVersion().

I will give examples of objects using class Song. Then you'll immediately follow up with the person next to you, for a class Robot.

• Review:
  Declare a var to hold an int (and draw picture); initialize it (udpate picture).
  Declare variable to hold a Foo (and draw picture). Initialize it, presuming that there is a function named “new Foo” takes in a String and returns a Foo. (update picture)
• Data analysis.
  a song has an artist, a title, a length, and whether it's copyrighted.

  class Song { String title; String artist; double length; // in seconds. boolean isCopyrighted; }

  
  a robot has an id#, a transformed shape, whether or not it's currently transformed, and whether or not it's evil.

  class Robot { String id; String shape; boolean isTransformed; boolean isEvil; }

• Fields; picture ("memory diagram").
• Constructor, and examples of the data.

  Constructors are just functions: You pass them arguments, and they return a value — in particular, they return a newly-constructed object (a Song or a Robot).

  Java forces you to write constructors a bit differently than most functions, though:

  • When writing the signature:
    • You don't mention any return-type at all.
      (Conceptually, that's because it's obvious from the fact that it's a constructor: the Song constructor returns something of type Song.)
    • The constructor name must be exactly the same as the class name, in Java.
    • You don't say “static” — that's implicit by the fact that it's a constructor.
  • When calling a constructor, you use the additional keyword new.
  
  For Songs:

  class Song extends Object120 { String title; String artist; double length; // in seconds. boolean isCopyrighted; // define the constructor: Song( String t, String a, double l, boolean c ) { //some voodoo (for now): super(t,a,l,c); } static void testSongs() { Song s; s = new Song( "Help!", "The Beatles", 122, true ); } }

  
  For Robots:

  class Robot extends Object120 { String id; String shape; boolean isTransformed; boolean isEvil; // constructor Robot( String i, String s, boolean t, boolean e ) { super(i,s,t,e); } static void testRobots() { Robot r; r = new Robot("WX-47", "phonebox", true, false) ; } }

• declare variables, initializing, full picture.
  We'll put these inside your test driver.

to teacher: Reached this far Oct.04.

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• Using fields.
  We'll only get the fields' values for now. (We'll talk about setting fields later (mutation)).
• A function taking in an object.
  Song: willFit -- given an amount of disk space, is an mp3 verison like to fit within that space?
  Robot: canLift -- a robot can lift 3x their own body weight, if *not* transformed.
  Song: longer than
  Robot: heavier than
• a function taking in an object: test cases, header/stub/comments; body.
• Syntax review.
• You try it: A student, by the design-recipe—the design recipe. We want to keep track of the student's ID, their gender, their major, and their gpa.
  Write a function onDeansRoll(Student) (true for students whose gpa is above 3.6).
  Write a function inSameMajor. that takes in two Students and checks whether they have the same major.

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©2010, Ian Barland, Radford University Last modified 2010.Oct.05 (Tue)

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