Mini-Lab: A Whirl of Color

Using Conditional Statements

This set of Mini-Lab Exercises is part of a series in which students build a small program with several fish moving around in an aquarium. The set includes the following exercises:

Each section contains an Introduction to a problem or task, descriptions or examples of one or more Concepts to apply in solving the problem or completing the task, and an Exercise.

In the exercises that precede this one, students created three fish, moved them forward one step, and displayed them graphically. Therefore, students should be familiar with constructing objects, using variables, and invoking methods. Some familiarity with logical expressions is also required for this set of exercises.

About Face!


In our current program, fish move to the right and then get stuck at the right wall. This does not make sense. We can use a Simple Conditional statement to determine when a fish should turn around and start swimming the other direction.

Stop and Think

  • Which statements in Example 1 will be executed when there are no lemon squares? In what order will they be executed? Which statements will be executed, and in what order, when there are lemon squares?


  • In your previous testing of the program you may or may not have noticed that fish swim only to the right and get stuck at the right wall. To verify the problem, make a copy of the statement that sets the dimensions of the aquarium. (Stop and Think: where is the statement that sets the dimensions of the aquarium? In what class, and what method?) "Comment out" the original, and change the dimensions in the copy to be 100 x 200. Copy the code that moves and redisplays the fish to let them move a second time. Now run the program several times and make sure you see the problem.
  • Research the AquaFish specification to find out how to determine whether a fish is at a wall and how to make it reverse direction. Modify the main method to have each fish check whether to change direction whenever it moves forward. (Stop and Think: You could check whether to reverse direction or not and then move forward, or you could move forward first and then check whether to reverse direction. Does the order matter? Consider three cases: a) for a fish that was constructed along the left wall, b) for a fish that was constructed in the middle of the aquarium, and c) for a fish that was constructed along the right wall. Now ask yourself again: Does the order matter?)
  • Test your program in the narrower aquarium you created above. When you are satisfied that your program is behaving correctly, restore the aquarium to its original size.

One Fish, Two Fish, Red Fish, Blue Fish


Our aquarium is a little boring, since the fish are all the same color. We could specify the color of each fish as we construct it, giving each fish a different color. Or, to make things more interesting, we could decide on the color of each fish based on a random number. We can use the standard Java Random class to do this.

Exercise: Random Red and Blue Fish

  • Edit your main method to construct a random number generator right before you construct your three fish. Give your new variable a name that conveys its purpose. (Note: The AquaSimApplication class has an import java.util.Random; statement at the top of the file; this allows you to use the standard Java Random class even though it isn't one of the classes you defined.)
  • After you construct your random number generator and above the code that constructs your three fish, create a new integer variable that can store the a random number. Don't actually generate the random numbers yet, though. Give your new variable a name that conveys its purpose. For example, your variable declaration might look like the following line.
      int randNum;
  • Just above the statement that constructs each fish, set your random number variable to a different random number (0 or 1). Then use the variable in an if statement to randomly construct each fish as either Color.RED or Color.BLUE.
  • Identify in advance what behavior you expect from your program when you test it. Do you know which fish will be which color? Do you know how many fish you should get of each color? Test your program to make sure that your results are what you expect.
  • Update the appropriate internal and external documentation to reflect your changes.