Our current assignment is to provide the source code for a very basic calculator. Before
we even think about the graphical design that will capture user input, we need to build
the engine that converts a standard arithmetic infix expression into its numerical solution.
We prefer to calculate a solution by parsing an expression from left to right. However,
the infix expression is not calculated this way: we need to follow operator precedence and
parentheses. Luckily, the equivalent postfix version will allow us to parse an expression
from left to right and easily evaluate the solution.
We break this system into two parts:
Part 1:: Convert an infix expression into its equivalent postfix expression.
Part 2:: Evaluate the postfix expression to calculate the numerical result.
(1) Create a fresh directory to contain this assignment: CSC115/assn3 is a recommended
name. Download this pdf file and the .java files to this directory.
(2) Download and extract the docs.zip and javafiles.zip in the same directory. A
docs directory will be created to store the specifications for the public classes. The
following links will work if the documents are stored in the docs directory local to
this pdf file.
• StringStack specifications
• ArithExpression specifications
(3) Most of the classes are complete. Some are supplementary, and others are just for fun.
(4) Create two simple classes first: StackEmptyException and Node.
(5) Complete the StringStack class.
(6) Examine the individual methods of ArithExpression. Read the textbook, the specification document, and the notes inside the source code. Read the detailed instructions
in the next section of this document.
There are several files supplied in this assignment. A lot of the code has already been
written. Read through these segments and the documentation links to understand how they
fit together and how one class helps another. We provide details of every class in the next
sections. The following files are completely done and need no additions:
The Node and StackEmptyException classes:
The Node class needs to be created. The InvalidExpressionException class is provided as a model for the StackEmptyException that you must also create. Once you
have complete these two classes, then start the StringStack class.
The StringStack class:
The shell of the StringStack class is provided. Use the specification document to fill
in the necessary methods. Note that the underlying data structure must be a singly-linked
list that is initialized by the head data field. Since a Stack ADT is not concerned with
the number of elements, a count or size data field is not required. You may add any
private methods that you find helpful; for example, a printout of the contents is always
recommended for debugging and testing purposes. You must provide a test harness in the
main method that tests each of the methods.
The Tools class:
Every programmer should have a Tools class to store helpful methods that are not necessarily attached to a particular object. An indication that a method is a candidate for the
toolbox is that it needn’t be as restrictive as is required within the class it is being written
for. For example, the method isBalancedBy originated in the ArithExpression class,
since very good infix expression must have balanced parentheses. During the writing of
the header comments, it occurred to us that it didn’t matter that the string was an arithmetic
We thought that finding balanced parentheses would be useful for any string, or for other
types of parentheses. So we gave the method a little more flexibility and put int into the
toolbox. There it can be used not only for arithmetic expressions, but also for parsing html
meta-tags or source code braces. This code also uses recursion, a subject covered during
The TokenList class:
This completed class is the very simplest of lists. It holds only String objects, only appends to the end of a list, and does not allow for the removal of an object. We use the
TokenList for each of the data fields in the ArithExpression class, one field holds the
infix expression as a set of tokens (numbers or operators) and the other holds the postfix
expression as a set of tokens. Having a set of tokens allows us to parse the individual items
in an expression.
The ArithExpression class:
The ArithExpression class is the engine of the calculator and the most intensive part of this
assignment. Before completing the methods of this class, make sure that StringStack
has been thoroughly tested and you understand the purpose of each of the supporting
The constructor is complete: it checks for balanced parentheses and tokenizes the incoming string by calling the completed tokenizeInfix method. When the constructor is finished, the data field infixTokens will be full of tokens, which are differentiated by the
• an operator; one of the following:
• a round parenthesis; one of the following:
• a string that isn’t identified as one of the above items. Note that if the expression is valid,
it should be a legitimate number, but we are not checking for that level of accuracy in
The two simple methods getInfixExpression and getPostfixExpression are useful
for debugging; finish these first.
The method infixToPostfix needs to be completed. The difficult part is developing a
complete algorithm that can take any infix expression and convert it to a postfix expression.
Fortunately there is an algorithm in the textbook: you may choose this one, or develop one
of your own. It is fairly complex, so we recommend you modify the textbook’s, which
uses a stack to parse the infix and create the postfix. It is also possible to do this using
recursion, but if you use the textbook’s algorithm, then use the StringStack.
The evaluate method parses the postfix expression contained in the postfixTokens
data field after infixToPostfix is done. The algorithm for this is also discussed in the
textbook; it uses a stack. You are to use the StringStack for this. HINT: look at the
Double class in Java for a means to turn a String object into a double data typle.
Both the infixToPostfix and evaluate methods will need to call smaller private methods. The completed isOperator is an example of such a method. You are welcome to use
and / or modify it to suit your programming needs.
Requirements for both StringStack and ArithExpression:
Follow the instructions within the incomplete source code provided. All methods must
meet the specifications in the documentation files. Both StringStack and ArithExpression
must contain a set of test cases in the appropriate main method, demonstrating that thorough testing was done. Only when everything was tested, should you use the graphical
calculator to input a variety of tests.
Using the GUI Calculator
The Calculator is provided for your enjoyment, and as inspiration for anyone wishing
to investigate computer graphics using Java. Enjoy using this as a tester for a number of
expressions once you have verified some tests in main. However, if you find an error, use
this information as an indicator that you need to go back to the internal tester, to find the
source of the error.
To play with the Calculator, simply compile it and run it in the directory. If ArithExpression.java
compiles, then the calculator will run. Note that this a GUI; it does not do any work. It delegates all the work to the Memory class and to your ArithExpression class. Initially,
hitting an ENTER key or clicking the = button will print out the value -1.0. Eventually, it
should produce the correct answer. If the input is incorrect, and ArithExpression works as
it should, you will see INPUT ERROR in the textbox.
Submit the following completed file to the Assignment folder on conneX.
Please make sure you have submitted the required file(s) and conneX has sent you a confirmation email. Do not send [.class] (the byte code) files. Also, make sure you submit your
assignment, not just save a draft. Draft copies are not made available to the instructors, so
they are not collected with the other submissions. We can find your draft submission, but
only if we know that it’s there.
A note about academic integrity
It is OK to talk about your assignment with your classmates, and you are encouraged to
design solutions together, but each student must implement their own solution.
Marks are allocated for the following:
• Proper programming style is demonstrated, as per the coding conventions on CSC115
• Use of private helper methods where applicable: if similar code is used in more
than one method, then a separate private method should be created that is then
called by the methods using similar operations. See the array based version of the
list as an example.
• Internal testing in both StringStack and ArithExpression main methods of all
public methods that you complete.
You will receive no marks for any Java files that do not compile.