CS 051 Homework Laboratory # 5
It's not Easy Being Green

Objective: To gain experience with loops and active objects.

The Scenario

For this lab, we would like you to write a program that plays the game Frogger. In this game, you control a frog that is trying to cross a busy 4-lane highway. Each lane has cars or trucks zooming by. The vehicles in a given lane all travel at the same speed, but vehicles in different lanes may travel at different speeds (and even in different directions if you would like). The user is in control of a frog. Clicking in front of the frog moves it forward one hop (one hop is the width of a lane of traffic), clicking behind moves it back, and similarly for clicking to the left and right of it. The goal is for the user to get the frog across the highway without it getting squished.

If the frog does get squished it should display an "OUCH!" message at the bottom of the screen. The user should be able to restart the frog from its original starting position by clicking the mouse once in the area below the lanes of the highway.

A demo version should appear below if your web browser supports Java.


A design for all of the classes used in this program (Frogger, Frog, Vehicle, and Lane) is due at the beginning of lab. While we expect a design for all aspects of the program, we urge you to write and debug the program as suggested in the Getting Started section later in this handout.

Preparing for this lab

Read this entire handout before doing anything else. In this handout we will begin by describing the classes you need to implement. After we describe the classes, we will outline one plan for proceeding with your implementation.

The Objects

As you learned in the past two weeks with the magnet and boxball labs, the key to good object design is the design of classes to represent the objects in your program. There are three different kinds of objects involved in the Frogger game. There is the frog, there are the lanes of traffic (four of them), and there are the vehicles that go on the road. (There are also some graphical objects on the screen to represent the lane markings on the highway, but we won't discuss these in detail as they are easily constructed. In fact, we have provided in the starter folder the code that draws the highway markings.)

The Frog

The frog is a very important object. The frog will be displayed on the screen by creating a VisibleImage using an image file we will provide. The Frog class will need an instance variable to keep track of this VisibleImage.

The constructor for a Frog should create the VisibleImage and place it just below the lowest lane of the highway, approximately halfway from each end. Your constructor will require several parameters: the Image of the frog, the location where the frog should start, how wide a lane is (so the frog knows how far to hop), and the canvas.

The frog clearly needs to be able to hop in each of the four directions in response to a user click. We suggest writing a single method hopToward that takes a Location parameter (e.g., point). Depending on which side of the frog the point is on, the frog should move in the appropriate direction. To keep the testing required to determine how the frog should jump simple, we suggest you divide the space around the frog as shown in the diagram below.

Unfortunately, the other thing that happens to a frog is that it gets splattered on the road. A vehicle will be responsible for determining whether it has killed a frog. To do so, it will need to ask the frog if any part of the Frog's body overlaps the VisibleImage displayed to represent the vehicle. To make this possible, your Frog class should include the definition of an overlaps method that takes a VisibleImage as a parameter and returns a boolean.

If a vehicle hits the frog, it kills the frog by calling a kill method on the frog. This causes an "OUCH!" message to appear at the bottom of the screen.

Finally, through the miracles of modern medicine, we'd like our frog to be able to come back to life. Add a method reincarnate which moves the frog back to its starting point and shows the image. Of course, you should not reincarnate a frog unless it is dead. So, include a boolean instance variable that keeps track of the condition of the frog (alive or dead), and check this variable in reincarnate. As described above, the user will reincarnate the frog by clicking the mouse below the highway. The Frog class should include an isAlive accessor method that returns a boolean to enable the onMousePress method to determine whether the frog should hop or possibly be reincarnated.

The Vehicles

The constructor for a vehicle will need parameters specifying where the vehicle should be located (two doubles or a single Location parameter), the Image used to display the vehicle on the screen, the velocity with which the vehicle should move, and the distance the vehicle should travel before disappearing. In order to check if the vehicle runs over the frog, you also need to pass the frog as a parameter to the Vehicle constructor.

Vehicle extends ActiveObject. This means you must

define the method public void run(){...}. The while loop inside run should:

  1. Save the current time. (Get the current time by calling System.currentTimeMillis()).
  2. pause for (at least) 30 milliseconds.
  3. Determine how long it actually paused for (e.g., subtract the time saved earlier from the current time) and move the appropriate distance. (Remember your equation from physics: rate * time = distance.) You need to do this to ensure smooth motion of your vehicles. With so many active objects, the computer cannot ensure that the length of pauses will be very precise.
  4. Find out if the vehicle squished the frog and kill the frog with the kill method if it did.

You may assume that all vehicle velocities will be positive. That is, your highway may be a one-way street. You are free (for extra credit), however, to add the ability to handle vehicles with negative velocities so that you can have some lanes where traffic goes from left to right and others where traffic moves from right to left.

The Lane

The purpose of an object of the Lane class is to constantly generate the vehicles that travel in a particular lane on your highway. As such, the lane does not actually correspond to any drawing on the screen. Instead, a Lane will be an ActiveObject that creates Vehicles.

The constructor for a Lane is quite simple. The lane was already drawn as part of the background in our window controller so we do not need to update the display. One thing we know is that all the traffic in a lane should drive at the same speed so that cars do not run into each other. The Frogger class should pick a random speed for each of the lanes and pass it in to the Lane constructor. We have found speeds in the range .033 to .133 pixels/millisecond to be good (though you may want to start with a slower set of speeds while you debug your program).

A lane's main responsibility is to periodically place a new car on the screen. It will do this inside the run method. In the while loop of the run method, the lane should generate a car, wait a while to allow a gap between cars, generate another car, and so on.

As you recall from above, the Vehicle constructor requires a lot of parameters: its starting location, its image, etc. So far all the lane knows is how fast the cars drive. Where will Lane get this other information? The car should be located at one end of the lane initially and should drive until it reaches the other end. If the lane knew where it was located, it could pass this information on to the car. Its location is relative to the entire highway. Our window controller can provide this information to the lane when it constructs it so that the lane can pass the information on to the vehicle.

What about the image? It turns out that the image can only be loaded from the window controller, so this information must also be passed down to the Vehicle by passing it to the Lane constructor. The Lane constructor remembers the image of the cars for its lane and passes this image to the Vehicle constructor. (Because of this limitation, it is simplest for all the cars in a lane to have the same image.)

Finally, the vehicle needs to know about the frog so it can tell if it hit the frog. Again, our window controller created the frog. It can pass the frog to the Lane in its constructor. The lane can remember the frog so that it can tell the vehicles about the frog in the Vehicle constructor call.

After generating one vehicle, the lane should pause for some time. The pause should be at least long enough so that there will be a one car-length gap between pairs of vehicles. The pause should never be so long that it leaves more than about four car lengths between vehicles.

If a vehicle is moving with a certain velocity, how long must it move before it's a full vehicle-length away from its original location? As an example, suppose the speed is 2 pixels/millisecond and the car is 140 pixels long. How long until the vehicle has gone one car length? How long until there is a gap of one car length between two vehicles? The answers to these two questions are different! Because the speed is actually a double and the pause time must be an int, you will have to "cast" the result from a double to an int by writing an expression like (int)(some double expression) to compute the pause time.

For simplicity, you may use a fixed value for the pause time when you first write your program, but eventually the pause time must be selected randomly. That is, the gaps between successive cars should not be the same, but should be distributed randomly with the constraint that there is at least one car length and no more than 4 car lengths between successive cars in the same lane.

The Image Files

There is an image file for the frog. We provide 8 images of vehicles. They show 4 different types of vehicles. For each type of vehicle there is an image of that vehicle facing right and another facing left. The image files are included in the starter folder. You can see their names listed in the left column of the Eclipse window. It is fine if you only use a single image file so that all your vehicles look the same and move in the same direction. Using multiple pictures is a feature that we hope you add (it does make the display look better), but it is not required.

The frog is 83 pixels wide and 48 pixels tall. The widest vehicle is 139 pixels wide. The tallest vehicle is 66 pixels tall. This information should help you figure out how to place the vehicles and frog within the lanes. Remember to use constants effectively so that you would not need to change many values if we introduce a much taller vehicle, for example.

Getting Started

The "starter" folder contains several files intended to hold Java code. The file Frogger.java should be used to write the extension of the WindowController that will serve as your "main program". The Frog.java file should be used to hold your code for the Frog class, Lane.java will hold the code for the Lane class, and Vehicle.java will hold the code for the Vehicle class. The Frogger.java file contains code that draws the highway background and markings for you. All the files contain skeletons of code which you will need to complete.

There are many ways of proceeding with this lab. Here is one suggested ordering:

  1. Read the begin method in Frogger.java to understand how we've drawn the highway background for you.
  2. Write the Frog class except for the kill and reincarnate methods.
  3. Modify the Frogger class to create the frog on the screen, and write onMousePress to control the movements of the frog. Make sure that the frog hops around appropriately.
  4. Write the Vehicle class.
  5. Test the Vehicle class by having the begin method in the Frogger class put one car on the road. (You don't want the code here in the end, but for now it lets you test the Vehicle class before you've written the Lane class.) Move the frog into the road to see if it gets killed by the car. Add code to reincarnate the frog and test it.
  6. Write the constructor of Lane. Move the code that creates a car from the Frogger class to the run method of the Lane class so that a lane generates a stream of cars on the lane. Make sure they don't bump into each other (they shouldn't as they are all going the same speed). Make sure that the frog gets killed if hit by any of the cars (though our cars will be hit-and-run - they don't stop!).

Advanced Features

You may notice that the sample version of Frogger we have provided (and depicted in the image at the beginning of this handout) has more features than we have required. Vehicles move in both directions and several different images are used to represent vehicles. You are encouraged, but definitely not required, to incorporate such extensions in your program for a small amount of extra credit. Only do so, however, after completing the construction of a program that meets the basic requirements.

Submitting Your Work

Before submitting your work, make sure that each of the .java files includes a comment containing your name. Also, before turning in your work, be sure to double check both its logical organization and your style of presentation. Make your code as clear as possible and include appropriate comments describing major sections of code and declarations. Use the Format command in the Source menu to make sure your indentation is consistent. Refer to the lab style sheet for more information about style.

Turn in your project the same as in past weeks. Be sure the project's folder includes your name in the title. Your lab is due Thursday at 11 PM.

Grading Point Allocations


Design preparation (3 pts total)
1 pt. instance variables & constants
1 pt. constructors and parameters
1 pt. methods and parameters
Style (5 pts total)
2 pts. Descriptive comments
1 pts. Good names
1 pts. Good use of constants
1 pt. Appropriate formatting
Program Quality (5 pts total)
1 pt. Good use of boolean expressions
1 pt. Not doing more work than necessary
1 pts. Check if killed and reincarnate at appropriate times
2 pts. Parameters used appropriately
Correctness (7 pts total)
1 pt. Car moves smoothly
1 pt. Car disappears at end of lane
1 pt. Car kills frog appropriately
1 pt. Cars spaced appropriately in lane
1 pt. Frog moves correctly on clicks
1 pt. Frog says "ouch" when killed
1 pt. Frog reincarnated properly

Computer Science

Department of Computer Science
Pomona College