Etch-a-sketch

First Laboratory: Expressions, Statements, and Interactions

Etch-a-sketch is designed to teach students about expressions and statements in an interactive environment where behavior is not 100% deterministic. Students write "recipes" for the movement behavior of a dot in a window. When the dot moves, it leaves a line from the start point to the end point.

• View Handout
• Browse Javadoc
• Usage notes
• Screen shots
• Executable jar
• Source jar
• To quickly see it click here (requires Java WebStart)
• Report bugs

Running the Program

There are several ways to run the code in class. You should choose the one that your students are ready for. They are presented in an order that represents an increasing usage of the command line utilities and options.

1. a) Click Here

      (requires Java WebStart). If you want to put a link on your course 
      web site, you may use a link like this (subject to our terms of use 
      like all materials on the site): 

      <a href="http://www.cs101.org/psets/etchasketch/demo/etchasketch.jnlp">Click Here</a>

2. a) java -jar etchasketch.jar
   b) File > Expand Classpath > (choose jar/dir of package of student code.)   
   c) File > Load Behavior > (enter fully qualified Class name for student code)

3. a) java -cp etchasketch.jar etchasketch.EtchControl
   b) File > Expand Classpath > (choose jar/dir of package of student code.)
   c) File > Load Behavior > (enter fully qualified Class name for student code)


4. a) jar -xvf nodenet.jar
   b) java etchasketch.EtchControl
   c) File > Expand Classpath > (choose jar/dir of package of student code.)
   d) File > Load Behavior > (enter fully qualified Class name for student code)

Usage Notes

• Recipes are esentially method bodies for a method with a double return value. This means every recipe must have a return statement of type double.
• Recipes must be compiled before they will do anything (recipe menu). You can tell if a recipe has been compiled since it was last edited by looking at the message below and to the right of the recipe entry area.
• Students may reasign which axis the recipe relates to and this will be shown by the color of the tab and the text below and to the right of the recipe entry area this is done using the Recipe menu.
• Recipies are executed iteratively, and so only the new position of the drawing dot needs to be returned. Several variables are available for use in recipies. The most useful of these is pos which holds the current position of dot along the currently assigned axis. For a full list see the help menu.
• The center of the screen is 0,0. 0,0 is NOT the upper left (standard GUI) or lower left corner (cartesian quadrant I).
• Simple recipies don't need anything beyond the application provided variables such as pos. More complex paths may require saved state between iterations. The students may declare fields in which to save state by clicking on the "Add Fields" button and entering standard variable declarations in the text box that appears.
• Fields do NOT disappear whe the field text area is closed by clicking on the button again. You must erase them and re-compile.
• Errors in student code are output to the error console which may be opend from the Drawing menu.
• The default mode of the program is to interpret the return value of recipes as a position, but this may be changed to Velocity or Acceleration by changing the advanced options (from the Drawing menu). Other advanced options are fairly self explanitory.
• Some students find it useful to compare two or more recipes side by side without clicking between tabs. This can be accomplished by selecting any recipe and selecting Standalone View from the Recipe menu. This view is exactly the same recipe as show on the tab in the main window. You will notice that text typed in one, appears in the other so there is no possible confusion about which copy is actually being used. They are just independant views of the same recipe object.
• Saving Recipies is buggy. Specifically, it is currently done via Object serialization, and any time the class serial ID changes all old save files cease to work. This will be fixed in the future, but This means you should not currently rely on saving behavior, particularly if you run the program using Java WebStart. WebStart will automatically grab a newer version if it exists on the server when you start it. The advantage is you get bugfixes immediately, and don't have to deal with download and command lines. The down side is that you have no control over when the files become useless. This is bug 117 When this bug is fixed this problem should disappear.
• This application contains a copy of a third-party tool called BeanShell for compiling code at run-time.
• Etch-a-Sketch now handles infinite loops in student code by stopping execution of a recipe if it doesn't return within several seconds. Virtually no correct student code should ever hold onto the thread of execution for over a second even on the slowest machines.

Screen Shots

Here are two typical recipes. The first is shown in the main control panel, and the second is shown in a standalone recipe window.

The drawing produced by compiling and executing these two recipes is shown below.