Introduction

This tutorial covers:

• Evaluating arithmetic expressions using the Interpreter Pattern (see Dr. Cartwright's lecture notes)
• DrJava, a home-grown GUI user-friendly Java Development Environment.
• Solutions to homework 1

I. Interpreter Pattern

The following are examples of integer arithmetic expressions:

• -234
• 234 + 567 * 92

The question is how can we write a program to evaluate such expression?  The first step is to define appropriate classes to represent these expressions.  This requires knowing the rules that define integer arithmetic expressions.  Here are the grammar rules:

• ArithExpression ::= ConstExpression | SumExpression | ProdExpression | NegExpression
• ConstExpression ::= int
• SumExpression ::= ArithExpresssion  '+' ArithExpresssion
• ProdExpression ::= ArithExpression '*' ArithExpression
• NegExpression ::= '-' ArithExpression

These rules constitute what is called the defining "grammar" for the "language" of integer arithmetic expressions.  ArithExpression, ConstExpression, SumExpression, ProdExpression, NegExpression are called the non-terminal symbols of the grammar.  ArithExpression is a special non-terminal called the start symbol.  int, +, *, - constitute the terminal symbols.  Evaluating an integer arithmetic expression defined by the above grammar is an example of what is called "interpreting a language."  There is a well-known OO design solution to the problem of language interpretation.  It is called the interpreter pattern!  In this pattern, we represent the above grammar by defining a class for each of the non terminal symbols.  The start symbol is defined as an abstract class with all of the other classes as its sublcasses. We then add a method generically called interpret for each class to do the job of interpreting!  Applying the interpreter pattern to our grammar gives the following class diagram in UML. Notice that the eval method is the generic interpret method.

Exercises:

1. Implement (i.e. write Java code) for each of the classes in the UML diagram as specified. Use DrJava. Your labbies will show you how to use DrJava. (solns)
   
2. Write test code to test your design solution (the solution contains very incomplete test code).

II. DrJava

• Edit and Save code.
• Compile code.
• Debug code.
• Execute compiled code.
• Step through code.
• Load existing code.

III. Homework #1 Solutions

If time permits, your labbies will explain the soultions to the homework #1.