Assignment #3 Solution

Objectives




Review command line input and string to integer conversion



Practice using Exceptions



Exposure to postfix notation



Practice using an ADT to solve a problem



Introduction




In this assignment you will implement a program that evaluates expressions written using postfix notation. For example, the result of evaluating 3 7 9 + + is 19.




The program will accept a postfix expression on the command line and print the result of evaluating the expression using the algorithm below:




try

while there is more input




if next token is an operand

push value on the stack




if next token is an operator

pop two values from the stack




apply the operator to the two values just popped push the result of applying the operator on the stack

if one element left on stack




pop value and display it

else




invalid expression

catch EmptyStack or NumberFormatException




invalid expression




This assignment has two parts:




implement the Stack ADT using an internal linked data structure. (You may not use any of the Java classes that implement the java.util.List interface.)
implement a program that uses a Stack to evaluate postfix expressions


An important concept:




A completed array-based Stack is provided. Note that:

It also implements the Stack interface, making all the array-based implementation hidden from a user, who can only see public method headers. Therefore, a user need not care which version of the stack is used. The user, in this case, is the calculator class.



In the calculator, the Stack object can be instantiated as either an ArrayStack or as a LLStack. There is only one line in the source code that would be different. For the ArrayStack, it is



Stack stack = new ArrayStack();

For the LLStack, it is

Stack stack = new LLStack();




Part I – Implement the stack interface using a linked list




Create a class called LLStack in a file named LLStack.java. The class LLStack must implement the Stack interface specified in Stack.java using a student-created linked list structure to contain its elements.




Create an appropriate Node class for your linked list implementation in a file named Node.java Modify StackTester.java so that it tests your implementation of the LLStack class.




Part II – Implement the calculator




Implement a program in a file called Calc.java that accepts postfix expressions on the command line and outputs the result of evaluating the expression to the console. Your program must also handle invalid expressions gracefully.




You should break down your Calc program into at least two other methods besides main. Solutions that have all the code in the main method will lose marks for poor style.




Your calculator only needs to support integer operands for convenience, but note that integer division with integers will yield a different answer




Your calculator should support the following binary operators:




addition
subtraction
division
multiplication



Note that we are using the lower case letter x to represent multiplication, not the *. The reason for this is that the '*' character on the command line will be misinterpreted as a wildcard by the operating system.




The table below shows some of the test cases we will use and the exact output your program must produce for those inputs.




Command line
Output










java Calc 5 4 +






9
java Calc 5 4 -






1
java Calc 5 4 x






20
java Calc 10 2 /






5
java Calc 1 2
3
4
5
+ + + +
15
java Calc 4 +








Invalid expression.
java Calc 4 5
+ 6
3
/ x
18
java Calc 1 2
+ +
+


Invalid expression.
java Calc what is
this
Invalid expression.






Submission




Submit your LLStack.java, Node.java and Calc.java using Connex.




Please be sure you submit your assignment; don't just save a draft.




A reminder that 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.




We will be using plagiarism detection software on your assignment submissions.




Grading




You will be graded on the following:




Good style as per the coding_conventions.pdf file on conneX and use of methods to do the work.



Following the exact specifications in this assignment.
















































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