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The JHAV É Project. JHAV É : Java Hosted Algorithm Visualization Environment Goal: Development of a comprehensive suite of visualization-based materials for teaching algorithms and data structures, work was partially supported by a National Science Foundation Grant, CCLI-EMD # 0126494

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The jhav project
The JHAVÉProject

  • JHAVÉ: Java Hosted Algorithm Visualization Environment

  • Goal: Development of a comprehensive suite of visualization-based materials for teaching algorithms and data structures, work was partially supported by a National Science Foundation Grant, CCLI-EMD #0126494

  • Principal Investigators:

    • Scott Grissom (Grand Valley State University)

    • Myles McNally (Alma College)

    • Thomas Naps (University of Wisconsin - Oshkosh)

  • Website:

Jhav supports effective av how
JHAVÉ “supports” Effective AV -- How?

  • “Stop-and-think” questions

  • Documentation window

  • Pseudo-code window

  • Input generators

  • Audio accompaniment

A tour of the jhav website
A Tour of the JHAVÉ Website

Jhav organization
JHAVÉ Organization

  • Client-server mode of operation

    • Generation of visualization script occurs by running algorithm on server

    • Viewing of script handled by “dumb” rendering client

  • Webstart facilitates deployment

    • Users just need to access the appropriate website to run the client on their machine

  • JHAVÉ supports a variety of scripting languages through plug-ins

The jhav client server model
The JHAVÉ Client-Server Model

Algorithm Choice


Appropriate Input Generator Served



Input Generator Presented


Appropriate Script Generation Program Run

Vis Script

Script is Rendered


Data Flows


Overall gaigs vis script structure
Overall GAIGS VIS Script Structure

  • A GAIGS visualization script is defined in a show file

  • The general script structure is:

    • one or more snapshots

    • followed by an optional question collection

  • The show file could be created by hand, or (more usually) as the output of a script generating program

Example of overall script structure
Example of Overall Script Structure


<snap> … </snap>

<snap> … </snap>

<snap> … </snap>

<questions> … </questions>


A Show File with Three Snapshots and a Question Collection

A simple but complete example
A Simple, but Complete, Example

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPEshowPUBLIC"-//JHAVE//DTD GAIGS SHO//EN" "gaigs_sho.dtd">






<list_item color="#0000FF">




<question_ref ref="0"/>



<question type="MCQUESTION" id="0">

<question_text>Color of the next queue item?</question_text>


<answer_option is_correct="yes">green</answer_option>





A Simple Visualization Script with a Multiple Choice Question

Generating scripts
Generating Scripts

  • GAIGS XML scripts can be generated by programs written in any programming language

  • However, the JHAVÉenvironment is designed to directly support programs written in Java

    • Input to programs must be specified on the command line

    • The first command line parameter is the file name the script is to be written to

  • Support classes are available which can be used to directly generate the required GAIGS XML

Support classes for script generation
Support Classes for Script Generation

  • ShowFile: Handles the actual writing to the script file

  • Structure Classes: Basically one for each of the GAIGS built-in structures

    • Linear Structures: GAIGSstack, GAIGSqueue, GAIGSlist

    • Arrays: GAIGSarray (includes bar graphs)

    • Trees and Graphs: GAIGStree, GAIGSgraph

    • Text: GAIGStext

  • Question Classes: Support various aspects of generating questions in scripts

    • XMLfibQuestion, XMLmcQuestion, XMLmsQuestion, XMLtfQuestion

The showfile class
The ShowFile Class

  • The ShowFile class is responsible for all writing to the script file

  • Constructors:

    • ShowFile(String fileName)

      • file is opened, and preliminary XML written to it

  • KeyMethods:

    • writeSnap(String title, Double titleSize, GAIGSdatastr… ds)

      • writes to the file the XML for a snap with the title and each of the listed structures

    • close()

      • writes any questions and closes the file

The gaigsstack class i
The GAIGSstack Class I

  • GAIGSstack functions in the usual way as a stack (with push and pop operations

  • But also stores color and other information in a way that can remain hidden (if desired) from the client class

  • Constructors:

    • GAIGSstack()

      • create a stack using defaults for location and color

    • GAIGSstack(String n, String c, double x1, y1, x2, y2, size)

      • create a stack with name n, color c, location <(x1,y1),(y2,y2)>, and fontSize size

The gaigsstack class ii
The GAIGSstack Class II

  • Key Methods:

    • pop()

    • push(Object o)

    • push(Object o, String c)

  • Key Inherited Methods (from GAIGSlist)

    • isEmpty()

    • peek()

import exe.*;


publicclass Example1 {

staticfinalString title = "Stack Example";

staticfinaldouble titleSize = 0.08;

publicstaticvoid main (String [] args) throws IOException {

GAIGSstack stack = new GAIGSstack();

ShowFile show = new ShowFile(args[0]);

int itemsToAdd = Integer.parseInt(args[1]);

for (int i = 0; i < itemsToAdd; i++) {


show.writeSnap(title, titleSize, stack);





Example Code: Using the ShowFile and GAIGSstack Classes

ShowFile constructor

ShowFile writeSnap

ShowFile close

<?xml version="1.0" encoding="UTF-8"?>





<title>Stack Example</title>


<bounds x1="0.0" y1="0.0" x2="1.0" y2="1.0" fontsize="0.05"/>

<list_item color="#FFFFFF">








Example Output: itemsToAdd == 1

Gaigsarray i
GAIGSarray I

  • GAIGS provides support for one and two dimensional arrays

  • Row labels can be specified, and if the array is 2-d column labels as well

  • If the array is a 1-d array of int, it can be shown either in the usual format or as a bar graph

  • Here we just briefly consider the 1-d case, please consult the GAIGS API for complete details on this class

Gaigsarray ii

  • Constructor:

    • GAIGSarray (String s, boolean bar, color c, double x1, y1, x2, y2, size)

      • create a label with name s, color c, location <(x1,y1),(y2,y2)>, and fontSize size. Display as a bargraph if bar == true

Gaigsarray iii

  • Key Methods:

    • set(Object o, int loc) and set(Object o, int loc, String c)

      • set location loc to have value o, optionally with color c

    • get(int loc)

      • return the value at location loc

    • setColor(int loc, String c)

      • set the color of the item at location loc to c

    • setName(String s)

      • set the name of this structure to s

Activity bubblesort visualization
Activity: Bubblesort Visualization

  • Our exercise will be to create a complete visualization for the (infamous) Bubblesort algorithm

  • Supplied code will create the snapshot shown below:

Supplied code
Supplied Code


import java.util.Random;

import exe.*;

publicclass Sort {

staticfinal String TITLE = null; // no title

staticint arraySize; // # of items to sort

static GAIGSarray items; // the array of items

Exercise Code: Preamble

publicstaticvoid main(String args[]) throws IOException {

// process program parameters and create the show file object

ShowFile show = new ShowFile(args[0]);

arraySize = Integer.parseInt(args[1]);

// define the two structures in the show snapshots

items = new GAIGSarray(arraySize, true, "BubbleSort",

"#999999", 0.1, 0.1, 0.9, 0.9, 0.07);

// initialize the array to be sorted & show it


show.writeSnap(TITLE, items);

for (int pass = 1; pass < arraySize; pass++)

for (int i = 0; i < arraySize-pass; i++)

if ((Integer)(items.get(i)) > (Integer)(items.get(i+1)))

swap(i, i+1);

// visualization is done



Exercise Code: Main method

// Load the array with values from 1 to the array size, then

// shuffle these values so that they appear in random order.

privatestaticvoid loadArray () {

Random rand = new Random();

for (int i = 0; i < arraySize; i++)


for (int i = 0; i < arraySize-1; i++)

swap(i, i + (Math.abs(rand.nextInt())

% (arraySize - i)) );


// Swap two items in the array.

privatestaticvoid swap (int loc1, int loc2) {

Object temp = items.get(loc1);

items.set(items.get(loc2), loc1);

items.set(temp, loc2);


Exercise Code: Support Routines

If you decide to do the activity
If you decide to do the activity … then

  • Decide when snapshots should be taken (when do the interesting events occur?)

  • Use coloring to show the ongoing actions of the algorithm

  • Use the name of the array to produce useful messages about the status of the algorithm

Adding interactive questions
Adding Interactive Questions then

  • GAIGS Scripts can be used to ask four types of questions:

    • True / False

    • Fill in the Blank

    • Multiple Choice

    • Multiple Selection

A Multiple Choice Question

Question basics
Question Basics then

  • All the questions in a GAIGS script are collected at the end of the XML File

  • Each contains a unique ID number/identifier

  • A snapshot can contain a question reference

    • The reference is by ID number/identifier

    • A question reference causes the question to appear when the snapshot is shown

Question generation support i
Question Generation Support I then

  • As for GAIGS structures, there are support classes for the generation of question XML

    • XMLtfQuestion: true / false

    • XMLmcQuestion: multiple choice

    • XMLmsQuestion: multiple selection

    • XMLfibQuestion: fill in the blank

  • Each support class allows the definition of the question text, choices, and correct answer(s)

Question generation support ii
Question Generation Support II then

  • To include a question in a snap, pass a question in ShowFile method writeSnap

    • This method also require documentation and pseudocode urls (which may be null)

  • writeSnap(String title, double titleSize, String doc_url, String pseudo_url, question q, GAIGSdatastr… ds)

    • writes to the file the XML for a snap with the title, titleSize, documentation and pseudocode urls, a question and each of the listed data structures

XMLtfQuestion then

  • Constructor:

    • XMLtfQuestion(ShowFile f, String id)

      • The id string must be unique within a script

      • ShowFile reference is a legacy code issue

  • Key Methods:

    • setQuestionText(String text)

      • sets the text which will be displayed as the question

    • setAnswer(boolean value)

      • set the correct answer to value

Xmltfquestion example
XMLtfQuestion Example then

int id = 0;

boolean swapIsDone = false;

XMLtfQuestion tf = new XMLtfQuestion(show, id + "");


tf.setQuestionText("Will a swap be done next?");

show.writeSnap(TITLE, null, null, tf, …);


XMLfibQuestion then

  • Constructor:

    • XMLfibQuestion(ShowFile f, String id)

      • The id string must be unique within a script

      • ShowFile reference is a legacy code issue

  • Key Methods:

    • setQuestionText(String text)

      • sets the text which will be displayed as the question

    • setAnswer(String text)

      • set text to be one of answers to be accepted as correct

Xmlfibquestion example
XMLfibQuestion Example then

int id = 0;

int swapsThisPass = 0;

XMLfibQuestion fib = new XMLfibQuestion(show, id + "");


fib.setQuestionText("How many swaps will be made this pass?");

show.writeSnap(TITLE, null, null, fib, …);

fib.setAnswer(swapsThisPass + "");

Probabilistic question asking
Probabilistic Question Asking then

  • The support classes allow the user to define the number of questions to be asked during a session, despite how many are “added”

  • To do this, use the alternative constructor for ShowFile:

    • ShowFile(String fileName, int count)

  • Exactly count questions will be asked (as long as at least that many questions have been added)

Next activity bubblesort questions
Next Activity: Bubblesort + Questions then

Return to your Bubblesort visualization and

  • Add a question which asks how many swaps will be made during the next pass (asked just before a pass)

  • Add a question which asks if a swap will be made (asked just before a comparison is made)

  • Use probabilistic questioning to limit the number of questions asked during a session