ITEC 120 2010fall ibarland

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lab07a
Mutation!
modifying an object's fields

Sometimes we want an object to change over time. For example, if a Song enters the public domain, we might want to update its copyright status:

Song s;
s = new Song( "Happy Birthday", "Patty and Mildred Hill", 31, true );

// Returns true:
Object120.equals(s,  new Song( "Happy Birthday", "Patty and Mildred Hill", 31, true ) )

s.isCopyrighted = false;

// Returns false:
Object120.equals(s,  new Song( "Happy Birthday", "Patty and Mildred Hill", 31, true ) )

We often use “setter functions” to change a field:

/** Set the isCopyright status of a Song.
 * @param copyable The new copyright status for the Song to have.
 */
static void setCopyright( Song s, boolean copyable ) {
  s.isCopyrighted = copyable;
  }


// Test that it works (from the Code Pad):


setCopyright(s, true);  // A *void* function -- must include semicolon in code pad

System.out.println( "Actual: " + s );
System.out.println1( "Expect: " + new Song( "Happy Birthday", "Patty and Mildred Hill", 31, false ) );

Updating Students

You can use your own Student code from last week's two labs, or you can use this provided solution.

• We will write a method setMajor which takes in Student and their new major, and updates the Student.
  • First, add a couple of test cases. Remember, setMajor doesn't return any value; to test it you must first call setMajor, and then check that your old modified Student actually equals the Student structure that you expect (use a new, different Student to compare to).
  • Then, write the function: first the signature, then a stub function to make sure it compiles (what is the smallest possible stub?), and finally the body of setMajor.

• We want to be able to update a Student's gpa when they complete a class. We realize that a grade point average is actually just a Student's total grade points divided by the number of credit-hours.

  It will take several steps in order to write recordGrade, which takes in three things: a Student, a grade they just received in class, and the number of credit hours that class is worth. (You don't need to worry about +/- grades, and you can use 'E' to represent a failing grade if you like, although it would be nice to allow 'F'.) The method recordGrade won't return an answer; it will just update the Student's fields to record the pertinent changes.

  • We realize that we need to update our representation of a student, not keeping track of their overall gpa, but instead just keeping track of their total grade points (as well as the number of credit-hours they've completed).
    Make this change to the class, including updating all test cases. Make sure everything still compiles before proceeding!
    (How will you modify enrollStudent?)
  • Write the function getGPA which returns a Student's gpa (using the total-grade-points field we just introduced above).
  • Write test cases for recordGrade.
    Like setMajor above, your test cases involve calling the void method recordGrade on one line, and then (on the next lines) comparing that modified Student against a different, new Student.
  • Once test cases are complete, write the signature (with an empty stub function so that it compiles);
  • Finally, write the body of recordGrade and check that it passes your tests.

Hint: To convert from a letter-grade to a gradepoint: I'll leave the full details to you, but note that Java does let you convert between a character and its ascii code.

In Code Pad, type (int)'A' or (char)((int)'A'+25).

(Recall that “(int)” means "casting", which is a very idiosyncratic way to call a conversion function. You are free to write your own (ordinary) function charToInt which hides the cast.)

Hint: How do you compute a grade point average?

First you compute the total grade points weighted by the number of hours, and then divide by the total number of credit hours (the total weight).

For example, if you get a C in a one-hour class (like Univ100), you get 2*1 grade points. If you get a B in a four-hour class (like ITEC120), you get another 3*4 grade points. If those were your only two grades, your gpa would be (2*1+3*4)/(1+4) = 2.8.

setCopyright(s, false);

if (Object120.equals(s,  new Song( "Happy Birthday", "Patty and Mildred Hill", 31, false ) )) {
  System.out.println( "(test passed)" );
}
else {
  System.out.println( "Actual: " + s );
  ( "Expect: " + new Song( "Happy Birthday", "Patty and Mildred Hill", 31, false ) );
}

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©2010, Ian Barland, Radford University Last modified 2010.Oct.26 (Tue)

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