Double Dispatch and Exceptions in Scala

This work is licensed under a Creative Commons Attribution 4.0 International License

Scrabble

Scrabble is a Scala project that implements a simple type system akin to those found in programming languages or calculators. It is not related to the board game of the same name.

Description

Scrabble uses a type hierarchy to represent different kinds of values, including Integer, Real, and XString which represent integers, floating point numbers, and strings respectively. These classes extend from the Value trait.

The add method is implemented in each class using a technique called "double dispatch". This allows each class to handle different types of addition operations. The Integer and Real classes handle numerical addition, while the XString class handles string concatenation.

The project includes a custom exception, InvalidOperationException, which is thrown when an unsupported operation is attempted, such as adding a string to a number from the right.

Example

val intVal = new Integer(5)
val realVal = new Real(2.5)
val strVal = new XString("Hello")

val result1 = intVal.add(realVal) // Returns a Real(7.5)
val result2 = strVal.add(intVal)  // Returns an XString("Hello5")
val result3 = intVal.add(strVal)  // Throws an InvalidOperationException

This showcases how the add method uses double dispatch to correctly perform addition operations between different types of values.

Building

To build the project, you need to have sbt installed. Then, you can compile the project with:

sbt compile

Testing

Unit tests are included in the project. To run the tests, use:

sbt test

Scala File Reader

Scala File Reader is a simple Scala application that reads and prints lines from a user-specified file.

Description

The application prompts the user to enter a filename, then it attempts to open, read, and print the lines from the provided file. It employs robust exception handling to gracefully deal with scenarios where the file may not exist, cannot be opened, or if an error occurs during the reading process.

An Option type is used to handle the BufferedSource, allowing it to start as None before the file is opened and then wrapping the BufferedSource in a Some once the file is successfully opened.

Example

Here is an example interaction with the application:

Enter the name of the file to read: example.txt
Hello, World!
This is a file.

In this example, "example.txt" is the name of the file the user wants to read. The application opens the file, reads the lines, and prints them to the console.

Building and Running

To build and run the application, you need to have sbt installed.

Compile the application with:

sbt compile

Run the application with:

sbt run

Handling Exceptions

The application includes robust exception handling for FileNotFoundException, IOException, and SecurityException. These exceptions handle scenarios like the file not being found, an error occurring while reading the file, and not having the necessary permissions to read the file, respectively.