ITEC 120 2012fall dbraffitt ibarland

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lab06a
Objects intro
compound data (fields)

I will give examples of objects using class Song. You'll do similar work for a class Student.

• 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; }

• In the registrar's computer system, a student has a name, a sid, a gpa, a major, and a number of credit hours completed.

  Your task: Make a class to represent a student.

• Fields; picture ("memory diagram").
• Your task: Draw a picture (memory diagram) for a variable which holds a reference to a Student.
• 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 ); } }

• Your task: Write the signature for the natural Student constructor.
• Your task: Complete the body for the constructor, by just passing the arguments to the mysterious super.
• Your task: Call the constructor, from the code pad. To the left of the result “<object reference>” is a small red box. Double-click on that, to inspect it.
• Your task: Make a second student object, and inspect that one too. (You see that we can have multiple student objects.)
• Your task: Write a test function which
  1. declares a variable to hold a Student;
  2. calls the Student constructor;
  3. initializes the variable with the result of calling the constructor.
  Now, make a second Student, also storing it into a second variable.
• Your task: Draw a memory diagram, adding what happens at each step of your test function.

Your task

Scenario: In the registrar's computer system, a student has a name, a sid, a gpa, a major, and a number of credit hours completed.

• Create a class Student to represent this information — start with fields only, and make sure your code compiles.

• What is the signature for the Student constructor?
  Write the constructor; for its body use the mysterious call to super as above.
  (On Friday we'll see how to replace that call to super with our own, actual code to initialize the fields.)

• Call the constructor, to create an example of a Student. (Either call in the code pad; if that doesn't work right-click on the class.)
  Inspect the result in BlueJ, by clicking on the small red return value (in code pad), or the Inspect button if you called the constructor interactively.

• As usual, make a function testStudent(); have it declare two variables s1,s2 which can hold students and initialize each of them.

• Draw a picture (memory diagram) of what things look like after the computer declares and initializes your two variables.
  (As you do this, be aware of how the picture gets built step-by-step -- there are six things that happen to make that picture, and each of those correspond to an intermediate picture. For example: What is the picture immediately after declaring the first variable and calling the constructor, but before actually completing the initialization of the first variable.)
  Do this on paper, and turn it in as you leave lab (with both you and your partner's name on it).
• (If you have time:) Consider the function onDeansRoll(Student) (true for students whose gpa is above 3.6).
  • Write a stub function for onDeansRoll.
  • Write three test cases function for onDeansRoll (two can use the variables you already have declared; have the third test case call the constructor without saving the result in a variable).
  • We have not yet talked about how to implement the function, so don't do so yet.
    (If you finish the stub/tests for this and the next function raise your hand, and a (peer) instructor will tell you how to access an object's field from inside the function, using the “.” operator.)
• Consider the function sameMajorAs, which takes in two Students and tells whether they have the same major.
  • Write a stub function for sameMajorAs.
  • Write two test cases function for sameMajorAs (one can use the variables you already have declared; have the other test case call the constructor without saving the result in a variable).
  • We have not yet talked about how to implement the function, so don't do so yet.
    (If you finish the stub/tests for this and the previous function raise your hand, and a (peer) instructor will tell you how to access an object's field from inside the function, using the “.” operator.)

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• Bonus: Write tests, a stub, and finally code for enrollNewStudent which takes in a name and sid, and returns a new Student with 0 hours enrolled and a 4.0gpa.

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©2012, Ian Barland, Radford University Last modified 2012.Oct.01 (Mon)

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