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# 2  Learning Objectives

1. Complete all of the learning objects of the previous projects.
2. To implement a GUI interface and respond to action events.
3. To use the linked list, stack, and queue classes.

# 3  Background

In the lecture notes and video lectures for Stacks and Queues : Sections 3 7 we discussed an application that evaluates an arithmetic expression written in infix notation such as:

(-1 – -2) * -(3 / 5)

Infix notation is the usual algebraic notation that we are all familiar with where a binary operator (a binary operator is an operator that has two operands) is written between the left-hand and right-hand operands. For example, in the expression above, the left-hand operand of the subtraction operator is -1 and the right-hand operand is -2. Some operators, such as the negation operator, are unary operators meaning there is only one operand (uni = one). For negation, the operand is written to the right of the negation operator. Therefore, this expression contains six operators: negation, subtraction, negation, multiplication, negation, and division.

In the algorithm that evaluates the expression, we also treat a left parenthesis as an operator, which it is not, but during the evaluation we have to push left parentheses onto the operator stack.

In an infix arithmetic expression, each operator has a precedence level, which for a left parenthesis, is different when it is on the operator stack as opposed to when it is not (this will become clear when you read the trace of the algorithm for the above expression; see page 3):

Operator          Normal Precedence Level                 Stack Precedence Level

(                                     5                                                         0

–                                    4                                                         4

*  /                                3                                                         3

+  –                               2                                                         2

)                                     1                                                         1

Right parentheses really don’t have precedence because they are not pushed on the operator stack, but we assign them a precedence level of 1 for consistency. The algorithm discussed in the notes did not handle the negation operator so I have modified it to handle negation. Here is the revised algorithm:

Method evaluate(In: pExpr as an infix expression) Returns Double

Create operatorStack — Stores Operators

Create operandStack  — Stores Operands

While end of pExpr has not been reached Do

Scan next token in pExpr — The type of token is Token

If token is an operand Then

Convert token to Operand object named number operandStack.push(number)

ElseIf token is an InstanceOf LeftParen Then Convert token to LeftParen object named paren operatorStack.push(paren)

ElseIf token is an InstanceOf RightParen Then

While not operatorStack.peek() is an InstanceOf LeftParen Do topEval() operatorStack.pop() — Pops the LeftParen

ElseIf token is Negation, Addition, Subtraction, Multiplication, or Division Then

Convert token to Operator object named operator While keepEvaluating() returns True Do topEval() operatorStack.push(op)

End While

While not operatorStack.isEmpty() Do topEval()

Return operandStack.pop()  — the result of evaluating the expression

End Method evaluate

Method keepEvaluating() Returns True or False If operatorStack.isEmpty() Then Return False

Else Return stackPrecedence(operatorStack.peek())  ≥ precedence(operator) End Method keepEvaluating

Method topEval() Returns Nothing rightoperandStack.pop() operatoroperatorStack.pop()

If operator is Negation Then operandStack.push(-right)

Else left operandStack.pop()

If operator is Addition Then operandStack.push(left + right)

ElseIf operator is Subtraction Then operandStack.push(leftright)

ElseIf operator is Multiplication Then  operandStack.push(left * right)

Else operandStack.push(left / right) End If

End Method topEval

Method precedence(In: Operator pOperator) Returns Int

If pOperator is LeftParen Then Return 5

ElseIf pOperator is Negation Then Return 4

ElseIf pOperator is Multiplication or Division Then Return 3

ElseIf pOperator is Addition or Subtraction Then Return 2 Else Return 1

End Method precedence

Method stackPrecedence(In: Operator pOperator) Returns Int

If pOperator is LeftParen Then Return 0

ElseIf pOperator is Negation Then Return 4

ElseIf pOperator is Multiplication or Division Then Return 3

ElseIf pOperator is Addition or Subtraction Then Return 2

Else Return 1

End Method stackPrecedence

It would be worthwhile to trace the algorithm using the above expression to make sure you understand how it works:

1. Create the operand and operator stacks. Both are empty at the beginning.
2. Scan the first token ( and push it onto the operator stack.
3. Scan the next token – (negation) and push it onto the operator stack.
4. Scan the next token 1 and push it onto the operand stack.
5. Scan the next token – (subtraction). Since the operator on top of the operator stack (negation) has higher precedence than subtraction, evaluate the top (note: negation is a unary operator so there is only one operand to be popped from the operand stack):
1. Pop the top number from the operand stack. Call this right = 1.
2. Pop the top operator from the operator stack. Call this operator = – (negation).
3. Evaluate operator and push the result (-1) onto the operand stack.
4. Now push the subtraction operator onto the operator stack.
6. Scan the next token – (negation). Since the operator on top of the stack (subtraction) has precedence less than negation, push the negation operator onto the operator stack.
7. Scan the next token 2 and push it onto the operand stack.
8. Scan the next token ). Pop and evaluate operators from the operator stack until the matching ( is reached. a. The top operator is a unary operator (negation):

Pop the top number from the operand stack. Call this right = 2.

Pop the top operator from the operator stack. Call this operator = – (negation). Evaluate operator and push the result (-2) onto the operand stack.

1. The top operator is a binary operator (subtraction):

Pop the top number from the operand stack. Call this right = -2.

Pop the top number from the operand stack. Call this left = -1.

Pop the top operator from the operator stack. Call this operator = – (subtraction).

Evaluate operator and push the result (1) onto the operand stack.

1. The top operator is ( so pop it.
1. Scan the next token * (multiplication). The operator stack is empty so push *.
2. Scan the next token – (negation). Since negation has higher precedence than the operator on top of the operator stack (multiplication) push the negation operator onto the operator stack.
3. Scan the next token ( and push it onto the operator stack.
4. Scan the next token 3 and push it onto the operand stack.
5. Scan the next token / (division). Since the operator on top of the stack (left parenthesis) has higher precedence than division push / onto the operator stack. Now do you see why the precedence of ( changes when it is on the operator stack?
6. Scan the next token 5 and push it onto the operand stack.
7. Scan the next token ). Pop and evaluate operators from the operator stack until the matching ( is reached. a. The top operator is binary operator (division):

Pop the top number from the operand stack. Call this right = 5.

Pop the top number from the operand stack. Call this left = 3.

Pop the top operator from the operator stack. Call this operator = /. Evaluate operator and push the result (0.6) onto the operand stack.

1. The top operator is ( so pop it.
2. The end of the infix expression string has been reached. Pop and evaluate operators from the operator stack until the operator stack is empty.
1. The top operator is a unary operator (negation):

Pop the top number from the operand stack. Call this right = 0.6.

Pop the top operator from the operator stack. Call this operator = – (negation).

Evaluate operator and push the result (-0.6) onto the operand stack.

1. The top operator is a binary operator (multiplication):

Pop the top number from the operand stack. Call this right = -0.6.

Pop the top number from the operand stack. Call this left = 1.

Pop the top operator from the operator stack. Call this operator = *. Evaluate operator and push the result (-0.6) onto the operand stack.

1. The operator stack is empty. Pop the result from the operand stack (-0.6) and return it.

# 4  Software Requirements

The project shall implement a GUI calculator which accepts as input a syntactically correct arithmetic expression written in infix notation and displays the result of evaluating the expression. The program shall meet these requirements.

1. The program shall implement a GUI which permits the user to interact with the calculator. Watch the Project 4 video lecture for a demonstration of how the application works.
2. When the Clear button is clicked, the input text field and the result label shall be configured to display nothing.
3. When a syntactically correct infix arithmetic expression is entered in the input text field and the Evaluate button is clicked, the program shall evaluate the expression and display the result in the label of the GUI.
4. When the input text field is empty, clicking the Evaluate button does nothing.
5. When the Exit button is clicked, the application shall terminate.
6. Note: you do not have to be concerned with syntactically incorrect infix expressions. We will not test your program with such expressions.

# 5  Software Design

Refer to the UML class diagram in Section 5.21. Your program shall implement this design.

## 5.1  Main Class

The Main class shall contain the main() method which shall instantiate an object of the Main class and call run() on that object. Main is completed for you.

Implements the addition operator, which is a binary operator. AddOperator is completed for you. Use AddOperator as a guide when completing DivOperator, MultOperator, and SubOperator.

## 5.3  BinaryOperator

The abstract superclass of all binary operators. BinaryOperator is completed for you. Note that BinaryOperator implements one abstract method evaluate() which all subclasses must implement. The subclasses are AddOperator, DivOperator, MultOperator, and SubOperator.

## 5.4  DivOperator

Implements the division operator, which is a binary operator. Complete the code in this file by using the AddOperator class as an example.

## 5.5  DList<E>

This is the DList<E> class from the Week 7 Source Code zip archive. It implements a generic doubly linked list where the data type of each list element is E. DList<E> is completed for you. For example, to create a DList which stores elements of the type Token you would write DList<Token> list = new DList<>(); much in the same way that we can create an ArrayList of Doubles by writing ArrayList<Double> list = new ArrayList<>();

5.6  Expression

Represents an infix expression to be evaluated. Use the provided pseudocode as a guide in completing this class.

## 5.7  LeftParen

Represents a left parenthesis in the expression. LeftParen is completed for you. Note that LeftParen is a subclass of the abstract class Parenthesis.

## 5.8  MultOperator

Implements the multiplication operator, which is a binary operator. Complete the code in this file by using the AddOperator class as an example.

## 5.9  NegOperator

Implements the negation operator, which is a unary operator. Complete the code in this file by using the AddOperator class as an example. Note, however, that negation is a unary operator so it only has one operand.

5.10  Operand

An operand is a numeric value represented as a Double. Implement the class using the UML class diagram as a guide.

## 5.11  Operator

Operator is the abstract superclass of all binary and unary operators, i.e., it is the superclass of BinaryOperator and UnaryOperator. Implement the class using the UML class diagram as a guide. Note that all of the non-constructor methods are abstract, i.e., none of them are implemented in Operator.

## 5.12  Parenthesis

Parenthesis is the superclass of LeftParen and RightParen. These are treated as a weird sort of Operator because we need to be able to push LeftParens on the operator stack when evaluating the expression. Parenthesis is completed for you.

## 5.13  Queue<E>

Implements a generic queue data structure using a DList<E> list to store the elements. This is the same class that was provided in the Week 7 Source zip archive. Queue is completed for you.

5.14  RightParen

Represents a right parenthesis in the expression. RightParen is completed for you.

## 5.15  Stack<E>

Implements a generic stack data structure using a DList<E> list to store the elements. This is the same class that was provided in the Week 7 Source zip archive. Stack is completed for you.

## 5.16  SubOperator

Implements the subtraction operator, which is a binary operator. Complete the code in this file by using the AddOperator class as an example.

## 5.17  Token

Token is the abstract superclass of the different types of tokens (i.e., symbols) that can appear in an infix expression. Token is completed for you.

## 5.18  Tokenizer

The Tokenizer class scans a String containing an infix expression and breaks it into tokens. For this project, a token will be either an Operand (a double value), a LeftParen or RightParen, or an arithmetic UnaryOperator (subclass NegOperator) or BinaryOperator (one of the AddOperator, SubOperator, MultOperator, or DivOperator subclasses). Tokenizer is completed for you. It is implemented as a finite state machine (FSM) which are commonly used in computing, especially when breaking a “sentence” of words or symbols into its component parts. If you have the time, you should study the code to learn how FSM’s work and are used.

5.19  UnaryOperator

UnaryOperator is the superclass of all unary operators. UnaryOperator is completed for you.

5.20  View

The View implements the GUI. Read the comments and implement the pseudocode.

## 5.21  UML Class Diagram

The UML class diagram is provided in the zip archive /uml folder as two UMLet files. Because the images in this document are small, there are PNG images of the two class diagrams in the /image folder. Your program shall implement this design.

## 5.21  UML Class Diagram (continued)

1. Format your code neatly. Use proper indentation and spacing. Study the examples in the book and the examples the instructor presents in the lectures and posts on the course website.
2. Put a comment header block at the top of each method formatted thusly:

/**

* A brief description of what the method does.  */

1. Put a comment header block at the top of each source code file formatted thusly (or use /** … */ comments if you wish):

//******************************************************************************************************** // CLASS: classname (classname.java)

//

// COURSE AND PROJECT INFO

// CSE205 Object Oriented Programming and Data Structures, semester and year // Project Number: project-number

//