Script Language Interpreter

Project description

This Java console app is a script language interpreter that interprets and executes a script written on a custom script language.

Purpose

The project is made to practice Java Collections, I/O, and other Java SE standard package features.

Key feature description

The script is loaded from a text file (.txt) and executed line by line. The name of the file to be loaded is passed either as a command line argument or entered by the user from the keyboard (if there is no argument provided). The key features are:

• perform calculations using whole numbers
• save numbers or calculation results in user-defined variables
• use variables in calculations (resolving variables into numbers stored in the variables)
• print text strings or values of user-defined variables on the screen
• use script documenting comments that are ignored by the script interpreter
• check the script and print errors with the problem description

Calculation operations

Calculations are performed in whole numbers. The following operations can be performed:

• addition
• subtraction
• multiplication
• division

The expression for evaluation can contain opening ( and closing ) brackets to determine the order of operations.

Script operators

The user script should contain the following operators (each one on a separate line):

• set operator calculates the given expression and assigns result to the named variable. The variable name must start with “$” and can include Latin letters, numbers and underscore mark only. The expression must be a number, other variable or must consist of numbers, other variable(s) and calculation operations named above
• print operator prints the specified list of strings and variables to the screen: strings and variables in the list are separated by commas; strings are enclosed in double quotes
• # operator determines a comment. Any line that starts with # will be ignored
• blank lines are ignored

Usage example

The demo script file can be found in the root directory of the project (DemoScript.txt). It contains the following script.

# Printing some string
print "Hi, Alexey!"

# Performing "set" operations
set $n1 = 21
set $n2 = 121
set $sum = ($n1 - ($n2 - 42)) * (4 - 42 / $n1)

# Printing the script
print
print "You entered this script:"
print "set $n1 = ", $n1
print "set $n2 = ", $n2
print "$sum = ", "($n1 - ($n2 - 42)) * (4 - 42 / $n1) = ", $sum

The user passes the file name as a command line argument when launching the jar-file, or enters it from the keyboard after the corresponding request from the program (if the argument was not entered). The program displays the following result on the screen:

Hi, Alexey!

You entered this script:
set $n1 = 21
set $n2 = 121
$sum = ($n1 - ($n2 - 42)) * (4 - 42 / $n1) = -116

[ScriptLanguage] Program finished running. Press Enter to finish...

After the user presses Enter, the program will exit.

Project structure

This is a simple Ant-based Java application. The diagram of modules and classes is shown in the figure below.

• main package contains ScriptLanguage.java class which is the main Application class. It addresses to the classes of the underlying packages (to read and interpret a script) and handles the incoming exceptions
  • main.io package contains classes that read external data from different sources
    • KeyboardInputReader.java class reads user keyboard input (e.g., when user inputs script file name)
    • ScriptReader.java class reads script file for further script interpretation / execution
    • main.io.exceptions package contains custom exception class (FileReadingException.java) that is used to throw exceptions connected with I/O operations
  • main.process package contains classes for the script interpretation which includes script parsing and validation, as well as performing related calculations and processing
    • ScriptInterpreter.java class is designed to interpret the script and return the final result. It addresses to other classes contained in the sub-packages of main.process package to perform the job.
    • main.process.operations package contains classes each one of which performs a separate script operation
      • OperationPeformer.java is an abstract class that represents performance of an abstract operation
      • PrintOperationPerformer.java class extends OperationPeformer.java class. It is designed to perform print script operation
      • SetOperationPerformer.java class extends OperationPeformer.java class. It is designed to perform set script operation
    • main.process.exceptions package contains custom exception classes (UnsupportedScriptOperationException.java, WrongScriptExpressionExeption.java) that are used to throw exceptions connected with the script interpretation and execution