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Overview

JHAVEPOP shares with Pascal Pointers, an educational application developed by Jeffrey Popyack at Drexel University, the goal of helping students become comfortable with pointers. Like Pascal Pointers: JHAVEPOP automatically creates snapshots of the memory after each pointer operation in the source code is executed. JHAVEPOP is packaged with programming exercises that engage students in programming and debugging tasks. The figure to the right summarizes the main functionality of JHAVEPOP. Given as input a source program written in (a subset of) either Java or C++, it generates a visualization of the program as a sequence of snapshots. The next two sections describe the default layout algorithm and the input language accepted by JHAVEPOP.

Layout manager

Underlying grid

Figure 1: Grid layout with default horizontal spacing equal to half the box width

Layout algorithm

Input language

• the name of a pointer variable in a declaration is preceded by a "*" in C++ but a reference variable in Java is not,
• the derefencing operator is "->" in C++ but "." in Java, and
• C++ uses the NULL macro while Java uses the null reference.

• The first line (that is, all characters before the first newline character) constitutes the title of the visualization that is reproduced unchanged at the top of the pseudo-code pane.
• The second line is a required "comment" (see below) that specifies the size of the memory grid, the input language (Java or C++), as well as a few other options to be described shortly. This line is processed by JHAVEPOP for setting up the visulization but, like all comments appearing in the source code, it does not appear in the pseudo-code pane.
• The third and following lines of the input file, up to the end-of-file marker, make up the actual source program.

Comments

• which syntax (Java or C++) the following source code adheres to,
• whether or not the underlying grid shown in green in Figure 1 above will be drawn on each snapshot, which can be useful at design time,
• the horizontal spacing between reference boxes, when the default value (equal to half the box width) is not adequate, and
• the font size for the pseudo-code pane.

  // gridsize 2 3 java drawgrid 2.0 smallfont

Figure 2: Grid layout with horizontal spacing equal to twice the box width

Figure 3: Grid layout with horizontal spacing equal to 0.0

Source code

Node n1;

Node *n1;

Node n2 = n1;

Node *n2 = n1;

Node n3 = null;

Node *n3 = NULL;

Node n4 = new Node('A',null);

Node *n4 = new Node('A',NULL);

n1 = n4.next;

n1 = n4->next;

n1.next = null;

n1->next = NULL;

n3 = new Node('B',null);

*n3 = new Node('B',NULL);

n1.info = n4.next.info;

n1->info = n4->next->info;

n1.next.info = 'C';

n1->next->info = 'C';

delete n1;

if (n1==null) {...} else {...};

if (n1==NULL) {...} else {...};

while ( (n1!=null) && (n1.info=='A') ) {...};

while ( (n1!=NULL) && (n1->info=='A') ) {...};

for( n1=n2; n1!=null; n1=n1.next) {...};

for( n1=n2; n1!=NULL; n1=n1->next) {...};

break;

break;

Node head = Utils.createList( 'A','B','C',"tail" );

Node *head = createList( 'A','B','C',"tail" );

n1.redrawListHorizontally();

n1->redrawListHorizontally();

• createList is a predefined routine (a static method in Java and a free function in C++) that takes in a variable number of arguments, namely 1 or more character literals and one string argument, and returns a pointer/reference to the head of a newly allocated linked list containing one literal per Node instance. Unless it is equal to the empty string, the last argument specifies the name of a newly allocated pointer/reference to the last Node in the list (see Example 15 above).
• redrawListHorizontally is a parameterless instance method/member function that redraws the linked list in the same row and starting at the same column as that of the head of the list, that is the pointer/reference on which the method/function is called. This is the only situation in which JHAVEPOP may move a node from one grid cell to another after its initial allocation. In particular, this function/method may not be called if the new position of any one of the linked list Node objects will clobber another Node object that does not belong to the linked list. Of course, its action also fails when the linked list is too long to fit within the grid's horizontal size (number of columns to the right of the list's head). Finally, note that JHAVEPOP nevers moves the location of any pointer/reference, even when the node it points to itself moves.

Layout directives

  Node n = new Node( 'A', null ); // 1 2 MIDDLE left

  Node n1; // 1 2
  Node n2; // 2 2 ANY left

EBNF grammar

Non-terminals

Terminals (tokens)