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.


===============================================================================
Steps [0] through [4]
===============================================================================

class Problem7
+-------------------------------------------------------------------+
|      +---+      +---+      +---+      +---+      +---+      +---+ |
| args | *-+-+ F1 | * |   F2 | * |   B1 | * |   B2 | * |   B3 |   | |
|      +---+ |    +-+-+      +-+-+      +-+-+      +-+-+      +---+ |
+------------+------+----------+----------+----------+--------------+
             |      |          |          |          | 
      <------+      | [1]      | [2]  [3] +--+   [4] +------+
class Frippy        |          |             |              |
+-------------------+----------+-------------+--------------+--------------+
|                   V          |             |              |              |
| +----------+   +----------+  >+----------+ | +----------+ | +----------+ |
| |   +----+ |   |   +----+ |   |   +----+ | | |   +----+ | | |   +----+ | |
| | x |    | |   | x |0   | |   | x |0   | | | | x |    | | | | x |    | | |
| |   +----+ |   |   +----+ |   |   +----+ | | |   +----+ | | |   +----+ | |
| |          |   |          |   |          | | |          | | |          | |
| |   +----+ |   |   +----+ |   |   +----+ | | |   +----+ | | |   +----+ | |
| | F |    | |   | F |null| |   | F |null| | | | F |    | | | | F |    | | |
| |   +----+ |   |   +----+ |   |   +----+ | | |   +----+ | | |   +----+ | |
| +----------+   +----------+   +----------+ | +----------+ | +----------+ |
|                                            |              |              |
+--------------------------------------------+--------------+--------------+
                                             |              |
class Boingo                                 |              |
+--------------------------------------------+--------------+--------------+
|                                            |              |              |
| +----------+   +----------+   +----------+ +>+----------+ +>+----------+ |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ | |
| | a |    | |   | a |    | |   | a |    | |   | a |null| |   | a |null| | |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ | |
| |          |   |          |   |          |   |          |   |          | |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ | |
| | b |    | |   | b |    | |   | b |    | |   | b |null| |   | b |null| | |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ |   |   +----+ | |
| +----------+   +----------+   +----------+   +----------+   +----------+ |
|                                                                          |
+--------------------------------------------------------------------------+

class String
+----------------------+
|                      |
| "grok"               |
|                      |
+----------------------+

===============================================================================
Steps [5] through [7] (atop [0] through [4]
===============================================================================
   
class Problem7
+-------------------------------------------------------------------+
|      +---+      +---+      +---+      +---+      +---+      +---+ |
| args | *-+-+ F1 | * |   F2 | * |   B1 | * |   B2 | * |   B3 |   | |
|      +---+ |    +-+-+      +-+-+      +-+-+      +-+-+      +---+ |
+------------+------+----------+----------+----------+--------------+
             |      |          |          |          | 
      <------+      | [1]      | [2]  [3] +--+   [4] +------+
class Frippy        |          |             |              |
+-------------------+----------+-------------+--------------+--------------+
|                   V          |             |              |              |
| +----------+   +----------+  >+----------+ | +----------+ | +----------+ |
| |   +----+ |   |   +----+ |   |   +----+ | | |   +----+ | | |   +----+ | |
| | x |    | |   | x |0   | |   | x |0   | | | | x |0   | | | | x |    | | |
| |   +----+ |   |   +----+ |   |   +----+ | | |   +----+ | | |   +----+ | |
| |          |   |          |   |     [7]  | | |          | | |          | |
| |   +----+ |   |   +----+ |   |   +-\-/+ | | |   +----+ | | |   +----+ | |
| | F |    | |   | F |null| |   | F |nuXl*-+-+>| F |null| | | | F |    | | |
| |   +----+ |   |   +----+ |   |   +-/-\+ | | |   +----+ | | |   +----+ | |
| +----------+   +----------+   +----------+ | +----------+ | +----------+ |
|                                            |        ^     |              |
+--------------------------------------------+--------+-----+--------------+
                                             |        |     |
                                             |    [6] |     |
class Boingo                                 |        |     |
+--------------------------------------------+--------|-----+--------------+
|                                            |        |     |              |
| +----------+   +----------+   +----------+ +>+------|---+ +>+----------+ |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +--|-+ |   |   +-\-/+ | |
| | a |    | |   | a |    | |   | a |    | |   | a |nu|l| |   | a |nuXl*-+-+-+
| |   +----+ |   |   +----+ |   |   +----+ |   |   +--|-+ |   |   +-/-\+ | | |
| |          |   |          |   |          |   |      |   |   |          | | |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +-\*/+ |   |   +----+ | | |
| | b |    | |   | b |    | |   | b |    | |   | b |nuXl| |   | b |null| | | |
| |   +----+ |   |   +----+ |   |   +----+ |   |   +-/-\+ |   |   +----+ | | |
| +----------+   +----------+   +----------+   +----------+   +----------+ | |
|                                                                          | |
+--------------------------------------------------------------------------+ |
                                                                             |
class String                                                                 |
+----------------------+                                                     |
|                      |                [5]                                  |
| "grok"<--------------+-----------------------------------------------------+
|                      |
+----------------------+

===============================================================================
Steps [8] through [12] (atop [0] through [7])
===============================================================================
   
class Problem7
+--------------------------------------------------------------[10]-+
|      +---+      +---+      +----+     +---+      +---+      +---+ |
| args | *-+-+ F1 | * |   F2 |null|  B1 | * |   B2 | * |   B3 | ? | |
|      +---+ |    +-+-+      +-+--+     +-+-+      +-+-+      +---+ |
+------------+------+----------+----------+----------+--------------+
             |      |          |          |          | 
        <----+      | [1] [11] X [2]  [3] +--+   [4] +------+
class Frippy        |          |             |              |
+-------------------+----------+-------------+--------------+--------------+
|       [12]        V          |             |              |       [12]   |
| +--\-------/   +----------+  >+----------+ | +----------+ | +--\-------/ |
| |   \----+/|   |   \-/--+ |   |   +----+ | | |   +----+ | | |   \----+/| |
| | x |\   / |   | x |0[9]|1|   | x |0   | | | | x |0   | | | | x |\   / | |
| |   +-\-/+ |   |   /-\--+ |   |   +----+ | | |   +----+ | | |   +-\-/+ | |
| |      X   |   |          |   |     [7]  | | |          | | |      X   | |
| |   +-/-\+ |   |   +----+ |   |   +-\-/+ | | |   +-\-/+ | | |   +-/-\+ | |
| | F |/   \ |   | F |null| |   | F |nuXl*-+-+>| F |nuXl| | | | F |/   \ | |
| |   /----+\|   |   +----+ |   |   +-/-\+ | | |   +*/-\+ | | |   /----+\| |
| +--/-------\   +----------+   +----------+ | +----+-----+ | +--/-------\ |
|                       [8] ^----------------+------+ ^     |              |
+--------------------------------------------+--------+-----+--------------+
                                             |        |     |
                                             |    [6] |     |
class Boingo                                 |        |     |
+--------------------------------------------+--------|-----+--------------+
|       [12]         [12]           [12]     |        |     |              |
| +--\-------/   +--\-------/   +--\-------/ +>+------|---+ +>+----------+ |
| |   \----+/|   |   \----+/|   |   \----+/|   |   +--|-+ |   |   +-\-/+ | |
| | a |\   / |   | a |\   / |   | a |\   / |   | a |nu|l| |   | a |nuXl*-+-+-+
| |   +-\-/+ |   |   +-\-/+ |   |   +-\-/+ |   |   +--|-+ |   |   +-/-\+ | | |
| |      X   |   |      X   |   |      X   |   |      |   |   |          | | |
| |   +-/-\+ |   |   +-/-\+ |   |   +-/-\+ |   |   +-\*/+ |   |   +----+ | | |
| | b |/   \ |   | b |/   \ |   | b |/   \ |   | b |nuXl| |   | b |null| | | |
| |   /----+\|   |   /----+\|   |   /----+\|   |   +-/-\+ |   |   +----+ | | |
| +--/-------\   +--/-------\   +--/-------\   +----------+   +----------+ | |
|                                                                          | |
+--------------------------------------------------------------------------+ |
                                                                             |
class String                                                                 |
+----------------------+                                                     |
|                      |                [5]                                  |
| "grok"<--------------+-----------------------------------------------------+
|                      |
+----------------------+