Assignment 5

This assignment is based on Exercise 7.21 (Turtle Graphics) from the textbook (but is not really the same). Please follow the instructions as given below.

This is a more challenging but mainly a longer exercise than the previous exercises. The steps involved are not particualry complex, but there is quite a number of separate steps involved in getting to a working solution.

Take an early look at what this exercise requires, and be prepared to ask questions while there is still time to get answers. Note that this program is a command-line style program, not a GUI program.

Please submit at least the following file for the assignment (together with any additional Java source files you find necessary):

  •, which implements the TurtleGraphics class with just the two public methods, enterCommands, and executeCommands, as described below. The supplied test program (TurtleGraphicsTest) expects to find the TurtleGraphics class in the same package as the test program.)

Note: that this assignment requires only that this one TurtleGraphics class (plus any other classes you might create to support it) to be submitted. There is a provided test program and a data package ( that supplies all the needed inputs to test and verify the public implementation of this class.

You may define and use other methods within the TurtleGraphics class. These methods should be private. Such private methods do not affect the public interface of the class.

The same test program will be used to evaluate the submitted work. The test program is described below.

A “skeleton” (i.e., initial but incomplete) version of the class TurtleGraphics, and the complete test program, TurtleGraphicsTest, are supplied in either of the two zipfiles described in the About the Assignment topic, where the content of each is discussed in more detail. There are, also in each file, the test data used by the TurtleGraphicsTest program, a Debug class that you may want to use, as well as all the accompanying Javadoc.

The Scenario

The concept of “turtle graphics” is based on an invention made in 1967, in a language called “Logo” ( The idea is that the “turtle” is a mechanical robot that walks around the floor of a room. It carries a “pen.” If the pen is down, it marks a track on the floor when it moves. If the pen is up, it just moves, leaving no mark. The robot is controlled by a “turtle program,” which is a sequence of commands, each controlling an action of the turtle. In this exercise, the commands are represented by integer values. For this program, the floor is assumed to be 20 by 20 units (although the program should be written so that this can be easily changed, by, for example, defining a constant, say, FLOOR_SIZE).

There are two phases to the processing of the commands of a turtle program, just as there is with any programming language.

  • The first phase is reading the program, checking it for correct form and syntax, and storing it in some defined but convenient “compiled” form for later execution.
  • The second phase is the execution, where the “compiled” form is read and the actions indicated are actually taken.

The class to be written, TurtleGraphics, implements both of these phases, by means of its two defined methods.

The two methods of the TurtleGraphics class

The definition of the commands is reproduced here:

Table of Commands
Command Meaning
1 Pen up
2 Pen down
3 Turn right by 90°
4 Turn left by 90°
5,d Move in the current direction “d” spaces
6 Display (print) the current state of the floor
9 Indicates the end of the program
(nothing after this command is part of the
program, and is to be ignored)

There are two public methods of the TurtleGraphics class,

  • enterCommands, which is the “compiler,” and reads the commands of the program and decides whether they are validly formed or not, and the
  • executeCommands method, that actually carries out the steps described under Meaning in the above table.

enterCommands method 

The enterCommands method acts as the “compiler” for a turtle program. Each call to this method enters a new program, overwriting any previous program. The signature and return value of the method is:

public List<String> enterCommands(Scanner source)

The Scanner parameter passed to this method is the source of the “turtle program.” The integer values representing the commands may be obtained from this Scanner instance as has been done for any Scanner seen in previous assignments.

When passed to the enterCommands method, the method should analyze the values delivered by the Scanner.

The tasks to be carried out by the enterCommands method are:

  • to see that the input consists only of the recognizable and valid commands (see the table above);
  • that a “move” command is followed by another integer value (which represent a number of steps of movement); after a “move” command, there must be another integer or the program has an error;
  • to check the program is terminated properly. The program must end with the proper end-of-program command. (Any data beyond this is “junk” and must be ignored)
  • the method should create informative messages; in particular, there should be error messages for any problem recognized in the program (see the examples in the About the Assignment topic). The messages (which are Strings) must be added to a List, and this List is the value to be returned by the method. (The test program will take care of actually printing the messages).
  • The method may also, if you wish, echo the values input as a “listing” of the program, by adding a representation of those values to the same List (as is done in the samples shown in the About the Assignment topic.
  • the enterCommands method should transform the program into a more convenient representation, i.e., more convenient than having to re-analyze the commands read from the Scanner, and at the same time store this more convenient representation in an internal structure ready for execution (see the executeCommands method below).

The TurtleGraphics class is not instantiated again for each new program (it does not have a constructor with any defined requirements). This requires that the enterCommands method must be “serially reusable,” meaning that it should reset all the state of the instance (the variables) that need to be in an initial state in order to process a program.

executeCommands method

The program that was entered may be “executed.” The program is executed by calling the executeCommands method, which is also done by the test program. (Do not call the executeCommands method from the enterCommands method.) The program created in one instance of the TurtleGraphics class may be executed as often as desired. The signature and return value of the method is:

public boolean executeCommands()

This method does not need any parameters, as there is no further input required and it will execute the program most recently processed and stored by the enterCommands method. Executing the program actually does the work to move the turtle over the “floor,” and as it moves, if its pen is down, it makes a mark, otherwise it just continues moving and leaves whatever is already on the floor undisturbed and unchanged. The executeCommands returns a value of false if there is no valid program to execute or if some error arises in the execution of the program.

The floor and how the turtle moves

Assume that the turtle drawing is in a box that has a floor 20 units by 20 units in size (as mentioned above). You yourself may specify what you want the behavior to be when the turtle gets to the edge of the box (since this is not mentioned in the problem description—your choice—but you must write some commentary to say what you have decided the turtle will do when reaching a boundary, so it may be determined whether you have done it as you intend).

When a “display” (that is, a “6”) command is found in the program, the current state of the floor is rendered to the standard output. (A single “*” is suggested in the textbook as a character to represent a mark on the floor, but two characters such as “* ”—an asterisk followed by a space—gives a much better appearance.)

With the pen “down,” the turtle marks the floor only when it is asked to move (not when the pen is put down). When the turtle starts a move it marks the current position where it is, and then moves one position in the required direction (but does not mark the floor again). This is repeated for every position it moves. That means, if the turtle is required, say, to move 3 units, it will mark the current position at the start of the move, and also will mark the next two positions, but will be left over the position after that without marking it. (So, with a move of 3 units, only three positions are marked, not four.)

With the pen “;up”, the turtle simply moves, making no mark, and not disturbing any mark over which it may move.

Format of the Turtle Program

Spaces, end-of-line, blank lines have no significance in a turtle program (just like in a Java program) other than to separate the numbers that make up the commands and distances. A single comma immediately after a number is like a single space. A comma may be replaced by a single space, and the program will be unchanged, and will produce exactly the same drawing. The single comma is allowed in a turtle program just for visual effect, intended to make it easier to read the program when a command has a related move value. It does not have to be there, so need not be checked. All of the five following one-line programs (that draw the box shown in the textbook) are equivalent.

2 5,12 3 5,12 3 5,12 3 5,12 1 6 9

2   5 12 3 5 12    3 5 12 3 5,12  1  6 9

2  5  12  3  5  12  3  5  12  3  5  12  1  6  9

2, 5,12 3, 5,12 3, 5,12 3, 5,12 1, 6, 9, 24,

2, 5  12 3,  5, 12,  3 5, 12,  3 5,12 1 6     9

Initial conditions

When the turtle starts executing a program, it is to be at the point 0,0 (top left-hand corner) of the floor. When the turtle moves, it moves in the direction it is currently facing. The initial direction is in the positive x direction, that is, to the east. Also, the pen must be initially up, so that it makes no mark on the floor.

Test Program

The supplied test program tests drawing samples additional to just the textbook example. These test programs are shown in the About the Assignment topic.

For more information about test program supplied, also see the topic About the Assignment. If you wish, you may write additional test programs and add them to the list of programs. You will note that the test program uses some inputs that are intended to be incorrect, in order to assess that the TurtleGraphics class deals correctly with such problems.