Percolation Lab

Simulating percolation through a Porous Material

Alyce Brady
Kalamazoo College


Getting Started


The Percolation program simulates a substance percolating through a porous material, such as water or gas percolating through the ground.  The simulation shows percolation through a vertical two-dimensional slice through the material, represented by a bounded grid.  At the beginning of the simulation, cells within the grid represent solid material or empty porous openings.  A percolating substance is added to the porous material; as the simultion runs, the percolating substance spreads to other empty cells.

Through a graphical user interface, the user can control how the simulation program is run, stepping through it one step at a time or running it for many steps until the Stop button is pressed.  In each step, the percolating substance seeps a little further if it can, moving one cell at a time in one or more directions.

Through the exercises in this project, you will be creating a class to represent solid material, and creating another class to represent a simple vertical percolator (a substance that sinks, or percolates down, but does not seep sideways nor percolate up).  In a follow-up to this lab, you will create a class that can seep sideways or down, but still obeys gravity and does not percolate up.


You will be using several classes provided for you, shown in the diagram below, as well as creating new classes of your own. 

object diagram

Exercise 1 — Downloading and compiling a skeleton version of the program:

  1. Download the zip file ( that contains the starting code files described above for the Percolation Project, and unzip it.
  2. Also download grid.jar, the Java archive library for the Grid Package, containing classes such as Grid, GridObject, and ColorBlock, upon which the Percolation Program is built. Compile your project.
    You will have to make sure your project knows about the grid.jar library. In BlueJ, you can create a +libs folder and put the jar file there, or you can specify its location in the Libraries tab of the Preferences or Properties dialog box (under BlueJ->Preferences, Tools->Preferences or File->Properties, depending on the version of BlueJ you are using).


Becoming Familiar with the Program and User Interface

In the next exercise you will run the Percolation program and become familiar with the functionality of its graphical user interface.

Exercise 2 — Understanding the SolidCell Class:

  1. Look over the SolidCell class, whose objects represent solid matter in the porous material. Every class that represents objects in a grid must inherit from the GridObject class, but in this case it does so indirectly: the SolidCell class extends the ColorBlock class, which extends GridObject.
    • Notice that the new SolidCell class does not need any new instance variables, since it inherits all the state it needs from ColorBlock and GridObject.
    • SolidCell has two different constructors: one which takes no parameters, and one which takes one parameter, a color. Since a ColorBlock expects to be told the color of the block, the constructor with no parameters will always create itself as a black block.
    • Finally, the SolidCell redefines the act method from GridObject. The inherited method does absolutely nothing, but the redefined version prints out a debugging message if debugging is turned on.

Analysis Questions: (write these up to turn in at the end of lab)
  1. Which superclass constructor is being invoked by the two calls to super in the SolidCell constructors?
  2. Where does the location method being called in act come from? Who is the "Hey, you!" object receiving that method? (Hint: look at the class documentation for the ColorBlock and GridObject classes.)
    Class documentation for all classes in the Grid Package can be found here.


Exercise 3 — Becoming familiar with the program:

Compile and run your skeletal percolation program. You can't run a simulation with anything actually percolating yet, because you don't have any percolating substances, but you should be able to create a representation of a porous material.

  • Create a new grid using the appropriate button in the graphical user interface.
  • Using the Manually Populate Grid button, add a few solid cells to your grid.
    • How many types are available to you in the Type pull-down menu?
    • What happens when you click on an empty cell in the grid? What happens when you click on a grid location with a SolidCell in it?
    • What happens when you click the Done button?
    • What happens if you click on Manually Populate Grid when the grid already has some items in it?
  • Click on the Step Once button, the Step N Times button, and the Run button. What happens? What did you expect to happen? Why?
    Note: when you click on the Step Once button, the graphical user interface calls the step method in the program's controller object (SlowPercolationController), which calls the act method for all the cells in the grid. What does the SolidCell act method do?
  • What happens when you click on the Automatically Populate Grid button? What is the effect of choosing a density of 0%, 5%, 30%, 75%, or 100%?
  • What happens when you click on Manually Populate Grid after having automatically populated the grid? Why might this be useful?
File Menu:
  • Create a new grid using the File menu, not the New Grid button. Edit the grid, again using the File menu, and add some solid cells to your grid. Is the behavior any different from using the buttons?
  • Open the grid file called porousMaterialA.dat. (It should be in the same folder as your BlueJ project, along with several other data files and a folder called images.)
  • Edit the grid you opened to place a solid cell that cuts off one of the pathways from the top of the grid to the bottom.
  • Save your new, edited grid as porousMaterialA2.dat.
  • Test that you saved the file correctly by creating a new grid (to overwrite the current one) and then opening your saved file.
  • Open the other data files provided to you to see how they differ.

You do not need to formally write up and turn in the answers to the questions in this exercise, but if there are any you do not understand or to which you are unsure of the answers, you should be sure to follow up with the lab instructor or teaching assistant.


Using Images

You have been representing your solid cells using a solid block of color, which was possible because you extended the ColorBlock class. Grid package applications also provide a relatively easy way to associate a picture with a class, so that every object of the class in a grid is represented by the same picture.

Exercise 4 — Using images to represent solid cells:

  • Edit the PercolationApp class and find the commented-out code that associates the SolidCell class with marb1.gif (the statement spans across two lines). Uncomment these lines, then run the program creating solid cells in a grid.
  • If you want, you can change the code to use any of the files in the images folder in the Percolation project for your solid cells.
  • Alternative: You can also get a Grid Package program to use an image for objects of a class by placing an image with the same name as the class in the same directory as the BlueJ package file. For example, you could copy the marb1.gif file to the same directory as the class and change its name to SolidCell.gif.



Often when you write a program, it is useful to keep track of some information as you go. You can do this by printing that information. On the other hand, once you know you have your program working you will not want extraneous messages coming out when you run it. The Grid Package Debug class provides a handy way to put print statements in your code that will only print out only when you want them to, usually during debugging.

Exercise 5 — Debugging:

  • You already discovered in Exercise 3 that solid cells don't do anything when you click on the Step Once button, even though their act method is being called each time.
  • Edit the PercolationApp class and find the commented-out code that turns debugging on. Uncomment it and run your program. Do you see different behavior?
  • Before running the program again, click in the BlueJ terminal window and then choose Clear from the Options menu.


Vertical Percolation

Now it's time to create a class to represent a substance that will percolate through your porous material. We will start by simulating "vertical percolation," where the percolating substance percolates straight down but does not seep sideways nor up. This type of percolation might describe the behavior of a small solid, such as grains of sand, percolating through a material, although it would not provide a good simulation of a liquid or gas.

Exercise 6 — Creating a skeletal vertical percolation class:

  • Create a new class called VerticalPercolator. Eventually this will represent a substance that percolates down through your porous material, although to start with we will create a class that doesn't do anything yet. That means that to begin with it will be very similar to the SolidCell class. (You may want to copy and paste some code segments from SolidCell into VerticalPercolator as you go.) This class, though, should inherit directly from the GridObject class (not from the ColorBlock class).
    • Like SolidCell, your new VerticalPercolator class does not need any new instance variables, since it will inherit all the state it needs from GridObject.
    • VerticalPercolator only needs one constructor, since we are not associating colors with percolator objects. The single constructor should take no parameters, and invoke the superclass constructor, also with no parameters.
    • Finally, your VerticalPercolator should redefine the act method from GridObject. This will eventually be a significant method, but for now just have it put out a debugging message, similar to the act method in SolidCell but saying that the method is about to act:
           Debug.println("Percolator " + location() + " about to act...");
    • Double-check what classes you need to import. Read through the classes imported by SolidCell class and decide if any of those are needed in VerticalPercolator and what new classes might need to be imported.

  • Run your program, creating (or reading in) a grid with solid cells in it. Can you create vertical percolation objects in the grid? (Are there any other types in the pull-down menu to the left of the editing grid?)
  • In the PercolationApp class, uncomment the line that would add "VerticalPercolator" to the array of editableTypes. (Notice that one difference between arrays and ArrayLists is that arrays provide this short-hand method that you see for adding elements to the array.) Don't uncomment the line that would add "VerticalPercolator" to the array of percTypes. You will make use of the PercTypes array for the programming project.
  • Run your program, creating (or reading in) a grid with solid cells and adding vertical percolation objects to it. What do vertical percolation objects look like?
  • In the PercolationApp class, uncomment the two lines that will associate the VerticalPercolation class with an image. (You can switch to a different image if you like.)
  • Run your program again, creating (or reading in) a grid with solid cells and vertical percolation objects in it. Click on the Step Once button. Do you see different behavior (or at least different debugging output) for solid cells and vertical percolators? Is the behavior what you expected to see?
  • Between each run of your program, don't forget to click in the BlueJ terminal window and then choose Clear from the Options menu.
  • Save a grid with some solid cells and some vertical percolation objects in it to a file. Read in the file. What was saved to the file? Is the behavior what you expected to see?


Exercise 7 — Next steps toward percolation:

  • Next we're ready to think about what the act method in the VerticalPercolator class should do. Each cell just considers where it should percolate to; the global percolation effect is produced by each cell percolating one unit's worth in that time step. We can describe the logic for each cell of percolating substance in general terms:
    Determine what location(s) it should percolate to.
    Percolate there.
    We can write this right into the act method as comments and temporary debugging messages:
         public void act()
             Debug.println("Percolator " + location() + " about to act...");
             // Figure out where to percolate or spread to.
             Debug.println("Should figure out where to percolate to...");
             // Percolate there.
             Debug.println("Should now percolate to somewhere...");
  • Each of these two steps is a well-defined unit, so we will write a separate helper method for each. The first method, getPercolationLocation, does not need any parameters and will return a Location. Write this method, returning any arbitrary location for now (for example, location (0, 0)). (How do you create (construct) a location object for a given row, column combination? Look at the class documentation for the Location class to find out.) Then replace the middle debugging statement in act with a call to your new method, storing the returned location in a variable. Modify the final debugging statement in act to replace "somewhere" with the actual location the method will percolate to. What do you expect the behavior of this simple version of the program to be? Compile and run your program to test the modified act method and your new helper method. Edit your program as necessary so that your program does what you expect it to.
  • Write a second method, percolateTo. Does this method need any parameters? Should it return anything? For now, leave the body of the method empty except to move the final debugging statement in act to it. What do you expect the behavior of your program to be after making this modification? Compile and test your new method. Edit it as necessary so that your program does what you expect it to.


Exercise 8 — Fully implemented vertical percolation:

  • Update the getPercolationLocation method to return the correct location. First determine the row and column of the substance's current location, and then use those values to return a new location that represents the location below the current location. Run your program and make sure that your debugging messages show that you've calculated the right location.
  • Update the percolateTo method. The first thing it needs to do is to see whether the location passed to it as a parameter is a valid location (inside the grid, not outside) and is empty. Read the class documentation for the Grid class to find out what methods would be useful for this. Pay particular attention to the method details, including the description of the return values. You will find that one of the two obvious method choices can be used to check for both validity and emptyness. If the location you are trying to percolate to is invalid or not empty, just return without doing anything. If it is OK, then percolate there by adding a new VerticalPercolator object to the grid in the new location. The last thing you need to do is to tell the current controller about the new object you created. You can do this with code like the following (assuming that dupl is a variable representing the new object):
    Thus, the skeleton for this method, with comments, debugging messages, and the statement to notify the controller of the new object, might look like:
         Debug.println("Should now percolate to location " + newLoc);
         // If newLoc is not in the grid or is not empty, do nothing.
         // If newLoc is a valid, empty location, spread to that location.
         Debug.println("Should percolate to location " + newLoc +
                       " successfully.");
         // Tell the controller about the new percolating substance object.
  • Before you run your program, think clearly about what you expect the results to be. Then run the program and compare your actual results to your expected results. Edit your program as necessary so that your program does what you expect it to. Test your program with a variety of different grids, strategically placing your solid cells and starting vertical percolation substances to fully test your program.


Exercise 9 — Save and Submit Your Modifications

  • Update the class documentation for SolidCell and VerticalPercolator to accurately describe the purpose and behavior of the class from a user's perspective. Focus on what the program does, rather then how it does it. Include your name and the current date as well as the names of anyone from whom you received help. Update the documentation before each method, including @return and @parameter descriptions.
  • When you are done and your program works correctly, submit the source code for the SolidCell and VerticalPercolator.
  • Also submit your answers to the Analysis Questions in this lab.
  • This lab must be finished before the next class period.