CS100, Spring 2000, Solutions to Review Question 7 for Prelim 2

7. We have done a little something extra in these sample solutions.

   First, we have numbered the statements and the corresponding
   values, e.g. arrows.  You are *allowed* but not *required* to do
   so.  TIP: Do it if you find it useful, e.g. it helps you keep track
   of what is happening; otherwise, do not do it.
   
   Second, we show intermediate stages of the box diagram to give you
   a feel for the steps as they happen over time.

   class Frippy {   |   public class Problem7 {
       int x;       |       public static void main(String[] args) {  [0]
       Frippy F;    |           Frippy F1 = new Frippy();             [1]
   }                |           Frippy F2 = new Frippy();             [2]
                    |           Boingo B1 = new Boingo();             [3]
   class Boingo {   |           Boingo B2 = new Boingo();             [4]
       String a;    |                                                
       Frippy b;    |           B2.a = "grok";                        [5]
   }                |           B1.b = new Frippy();                  [6]
                    |           F2.F = B1.b;                          [7]
                    |                                                
                    |           B1.b.F = F1;                          [8]
                    |           ++B1.b.F.x;                           [9]
                    |                                                
                    |           Boingo B3;                           [10]
                    |           F2 = null;                           [11]
                    |           // *** diagram up to this point ***  [12]
                    |       }
                    |   }
   
[0] parameter $args$ is like a local variable; it is a reference, but
    in CS100 we don't care what it refers to.

                    creates a new Frippy object, including giving the instance
                  / variables their default values
[1] $new Frippy()$ 
                  \ returns a reference to the new object.

    $F1 = new Frippy()$ the value (reference) is stored in F1

[2], [3], [4] are like [1]

====

[5] the old value $null$ is crossed out.
    the new value (reference to "grok") is placed into $a$.

[6] $new$ creates a new object.
    the reference to this object is stored in $b$.
    the reference is a "new" value, so the old value $null$ is crossed out.

[7] the old value $null$ is crossed out.
    the new value (reference to a Frippy object) is drawn.

=====

[8] old value $null$ is ossed out; new value drawn in

[9] old value 0 is crossed out; new value 1 is drawn nearby

[10] local variables are *not* given default values;
     thus, the value is undetermined and indicated by $?$.

[11] the old value (reference) is crossed out by drawing a tidy X
     over the arrow; the new value $null$ is drawn in.

[12] cross out all unused object boxes.