Computer Science 120
Introduction to Programming

Spring 2011, Siena College

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Nibbles is a snake who moves around a field, looking for food. Unfortunately, Nibbles is not very clever and will try to eat anything it can reach. When it eats food, it grows. When it eats the electric fence around its field, it dies. When it gets all twisted up, it can even try to eat itself, which also causes it to die. The player of the game will control the movement of the snake. The objective, of course, is to eat as much food (and grow) as much as possible, without dying.

Nibbles is controlled using the arrow keys on the keyboard. Note that you must click on the window displaying the snake before the program will respond to the arrow keys! The green squares are the snake, and the red square is food. The rest of the field is unoccupied.

A working solution for this program will appear below. Click inside the applet to interact with it.

To help you get started, we have provided a starter BlueJ project with the framework of the program that includes much of the code to run the program, including several classes that you will not need to modify.

Your task is to write the Snake class.

Game Details

The snake constantly moves by extending its "head" in the direction indicated by the last arrow key the user presses. Normally, each time the head moves into a new cell, the last cell of the snake's tail is removed from the screen. If the snake manages to eat the food, it becomes a few squares longer, but does not grow all at once. Instead, for a few steps after it eats, the snake's head is allowed to move into new squares while none of the squares in the tail are removed, thus simulating the growth of the tail. Note that the snake can go straight or it can turn, but it cannot reverse its direction with a single click of an arrow key.

Design

The Nibbles program consists of 5 classes:

• Snake - an ActiveObject extension that animates the snake as it slithers around the screen,
• NibbleField - a class used to represent the 2D grid that the snake lives within,
• Nibbles - the WindowController that initializes the display and handles the user input,
• Position - a simple class that is used to represent positions within the NibbleField, and
• Direction - a class whose values are used to represent the four directions in which the snake can move: UP, DOWN, LEFT and RIGHT.

In the starter, you will find working code for all but the Snake class.

Below are details of each class.

The Nibbles Class

The Nibbles class extends WindowController. It begins by creating the field and the snake. It then receives the clicks of the arrow buttons and tells the snake to change direction accordingly.

The Position Class

The field on which the snake moves is divided up into a grid like a checkerboard. While the boundaries between the cells of this grid are not visible on the screen as the game is played, the cells determine where it is possible to put the food and the pieces of the snake's body. The picture below shows how the image of the snake and the food (shown above) might look if the grid in which the game is played was visible.

Each cell in this array can be precisely identified by two numbers indicating the row and column in which the cell is located. For example, in the picture the food is located in row 2 and column 6, and the head of the snake is found at row 3 and column 7. We can write these positions as (2,6) and (3,7). The Position class is used to combine such a pair of grid coordinates into a single object much as an object of the Location class encapsulates the x and y coordinates of a point on the canvas as a single object. Unlike canvas coordinates, the row and column numbers identifying a cell in our Nibbles grid must be integers. Also, the convention is to write the number that describes the vertical position of a cell before the number that describes its horizontal position. This is the opposite of the convention used with canvas coordinates.

The Position constructor takes a row and a column as parameters:

    public Position (int row, int col)

In addition to this constructor, the Position class provides the following methods:

    public int getRow();
    public int getCol();
    public boolean equals(Position otherPos);
    public Position translate(Direction dir);

The individual row and column values can be extracted from a Position using the getRow and getCol accessor methods. The boolean equals method checks if two Positions are equivalent. The translate method is explained shortly in the discussion of the Direction class.

The Direction Class

The class Direction is used to encode the direction in which the snake is currently moving. There are four possibilities: Direction.UP, Direction.DOWN, Direction.LEFT and Direction.RIGHT.

Internally, the representation of a Direction is similar to that used for Locations. A Direction is just a pair of values indicating how far the snake's head should move in the horizontal and vertical directions. Each of these two values can be either 0, 1 or -1.

The most important method used with Direction values is actually associated with the Position class rather than with the Direction class.

    public Position translate(Direction curDirection);

Given a Position named curPos and a Direction named curDir, an invocation of the form:

    curPos.translate(curDir)

will return a new position obtained by moving one step from curPos in the direction described by curDir.

In addition, the Direction class provides several methods that can be used to manipulate Direction values themselves:

    public int getXChange();
    public int getYChange();
    public boolean isOpposite( Direction newDir );

The getXChange and getYChange methods return the amount of horizontal or vertical motion (either 0, 1 or -1) associated with a Direction value. The isOpposite method returns true if the Direction passed as a parameter is the opposite of the direction on which the method is invoked.

The NibbleField Class:

The NibbleField represents the actual contents of the game grid. Most of the necessary information is encoded using a two-dimensional array containing one entry for every cell in the grid. The entries of the array hold FilledRects. If an entry in the array is null, it means there is nothing in the corresponding cell. If it is not null, it refers to the FilledRect drawn on the screen to represent the food or the part of the snake that occupies the corresponding cell in the game grid.

The only parameter expected by the constructor for a NibbleField is the canvas.

    public NibbleField(DrawingCanvas canvas);

The constructor uses the getWidth and getHeight methods of the canvas to decide how big the game grid should be. It also places a piece of food at a random location within the field.

The Snake will interact with the NibbleField using several methods:

    public void addItem(Position pos);
    public void removeItem(Position pos);
    public boolean outOfBounds(Position pos);
    public boolean containsSnake(Position pos);
    public boolean containsFood(Position pos);
    public void consumeFood();
    public Position getCenter();

The snake can ask the NibbleField to place a piece of its body (probably its head) in a given cell by invoking the addItem method. It can also remove a part of its body (usually its tail) from the screen by invoking removeItem.

Before moving, the snake can use the outOfBounds method to ask the field if a particular position is out of bounds. The snake can determine whether a given position in the field contains food or some part of the snake's body using the containsFood and containsSnake methods.

When the snake gets lucky enough to eat the food, it can tell the field to remove the food and place a new piece of food somewhere else on the field by invoking the consumeFood method. This method should be called before moving part of the snake into the cell that had contained the food.

Finally, rather than starting its life in some random position, the snake likes to start in the center of the grid. To do this it has to ask the field where the center is using getCenter.

The Snake Class

The most interesting class is the Snake. The Snake is an active object. It continuously moves the snake around the field checking at each step whether the snake has found food and should grow or has made an illegal move (out of bounds or into its own body) and should die.

If the user types an arrow key, the Nibbles controller gets the input and calls the setDirection method of the snake

    public void setDirection (Direction newDirection);

to inform the snake that the direction in which it moves should be changed. The snake should ignore the controller if the new direction is the opposite of the snake's current direction.

The snake moves and tries to eat inside its run method. Between each move, it pauses for a while so that it slithers at a realistic speed. The snake normally moves by stretching its head forward one cell and then removing the last piece of its body from the screen.

We urge you to implement this motion using two separate methods: one to stretch the snake by adding a cell next to its current head and the other to shrink the snake by removing its tail. This will make it easy to make the snake grow after it finds some food. After the snake finds food, you can make it grow by letting it take several steps in which it stretches but does not invoke your "shrink" method. Similarly, when the snake should die you should make it gradually disappear from the screen by repeatedly invoking your "shrink" method without calling "stretch".

To implement "stretch" and "shrink", you will keep track of the Positions of the parts of the snake's body in an ArrayList.

Each element of the ArrayList in the Snake class identifies the Position of one portion of the snake's body. For example, consider the snake shown in the pictures of the game field shown above and repeated below:

Since this snake occupies 10 cells of the field, 10 elements in a Position array would be used to describe it. The picture below suggests what the ArrayList would look like. As a shorthand in this figure, we have written pairs like (3,7) and (5,10) to represent Position values, but you should realize that you cannot actually do this in your code. Instead of (3,7) you would have to say new Position(3,7).

The cell at one end of the snake is in row 3, column 7 and this Position is stored in the 0th element of the ArrayList. The piece next to this piece is at row 3, column 8 and its Position is shown in the 1st element of the list. The positions of the remaining pieces of the snake's body are placed in the remaining cells of the ArrayList in consecutive order.

You cannot tell from either the picture of the game grid or the Position ArrayList which end of the snake is the head and which end is the tail. This is because the pieces of the snake can be stored in either order as long as you remember which order you decided to use while writing the code for your Snake class.

To move the snake, your code should first determine the cell that will become the new head based on the Direction in which the snake is moving. It should check if the cell it is going to move into is out of the bounds or if it is occupied by part of its own body. If either of these conditions apply, the snake dies. Next, your code should check to see if the food is in the cell that will become the new head. If so, the snake should eat the food and grow by one cell on each of the next 4 moves it makes. The snake eats the food using the consumeFood method of the NibbleField, which also generates a new piece of food somewhere else in the field.

As we have suggested above, your snake should grow by being allowed to make several moves in which the snake stretches but does not shrink. Thus, your run method will contain one main loop. Each time around the loop you will pick a new Position and "stretch" the snake to move into this new Position. Most times around the loop you will also shrink the snake by removing its last cell. For several iterations after eating food, however, your snake will skip the "shrink" step in the loop.

Finally, when the snake dies you should make it wither away rather than simply stopping the program. You can do this by writing a separate, simple loop that repeatedly shrinks the snake and pauses.

Lab Procedure

Your job is to write a complete version of the Snake class. The Snake constructor is passed the NibblesField that it wanders through as its only parameter. The constructor should create and initialize the ArrayList to hold the location of its body. It should use the field's getCenter method to decide where to place the snake initially. The snake should begin moving UP from this position. During the first few steps it takes, the snake should be allowed to grow to its initial length (3). You are responsible for defining a run method, a stretch method, a shrink method, as well as any additional methods you need to make the snake behave as described. You should not need to modify any other classes.

Start by constructing a snake that is only 1 cell long. Write a loop in the run method that will make this snake move in a straight line by calling stretch and shrink. As the snake tries to move out of the bounds of the game, have the snake detect the boundary and die. The snake should die by falling out of the main loop of the run method.

Once that is working, use the values passed in the setDirection method in response to arrow clicks to cause the snake to change direction. At this point, your program should work correctly until the snake runs into the food (at which point it will encounter an error in our NibbleField class).

Now, get the snake to eat the food. Change the loop that moves the snake so that after the snake eats the food, the snake will take several steps in which it stretches but skips the call to shrink. You will need a counter to keep track of how many steps the snake has taken since it ate to do this.

Now that your snake can grow to be longer than one cell, it has to worry about running into itself. Add code to make the snake die if it runs into itself. Add a short loop after the main loop that slowly removes all the pieces of the snake from the screen when it dies.

Submitting Your Work

Before the start of your next lab session, submit your Java program for grading. There are three things you need to do to complete the submission: (i) place a copy of your program into your csis120/hw folder under hw8, (ii) print a copy of your program and hand it to your lab instructor, and (iii) demonstrate the execution of your program for your instructor.

Grading Sheet

This lab is worth 25 points, which are distributed as follows: