Simple coding task - a file differentiator

There are various people and not everyone is honest. Some would pretend a file is a JPG, while in fact, it’s a dangerous, malicious binary. Write a program, that prevents that from happening!

1. You accept all extensions. No matter the extension, attempt at reading will be done.

2. You verify "magic numbers".

3. Program must say if the extension is true (a txt file is a text file, an JPG is a JPG).

4. If extension isn’t handled, program honestly states so (throwing an appropriate exception, one that fits this situation).

5. We expect JPG, GIF and TXT to be handled. Anything more is a plus.

6. If extension lies, program states: Extension is A, while actually it’s a B.

Anything that is NOT stated in the above list, is not necessary. Console-only program is absolutely fine, no GUI is necessary.

Tip	see Steps, at the bottom of the page for how to proceed.

Magic numbers?!

If you never heard of magic numbers, don’t worry. They are good resources to study:

1. Unix file command, which tells you what a file is.

2. https://en.wikipedia.org/wiki/List_of_file_signatures

3. https://en.wikipedia.org/wiki/File_format=Magic_number

4. https://en.wikipedia.org/wiki/Magic_number_(programming)=Magic_numbers_in_files

Prerequisites

1. JDK11+.

2. Maven 3.6 +.

3. A sheet of paper and a pen.

Steps

Work on branches!

Tip	use mantra!

1. read the specification requirements, both functional and non-functional

2. conceptualize: write down nouns and verbs from the specification introduction

3. draw a flow chart for a simple version of the program

JShell

Identify key problems, play with API, do few experiments for solving crucial parts. .. save the results, add, commit, push

Impera

2nd branch. Rework the JShell file into a Java file, with proper class and main method. Use just java to launch it, code imperatively for now.

Tip	so far you have been increasing your familiarity with the problem. It’s time to do more. For instance: let’s introduce OOP.

Branch OOP

1. if you haven’t so far - use mantra and have a full-blown project with pom.xml, Maven, etc.

2. draw CRC diagrams or class diagrams from UML.

3. try finding out how your objects will work. Do you need more than 1 package?

4. name your main package appropriately

5. remember about encapsulation and polymorphism and composition!

Branch TDD

1. add TestNG to your POM - on test scope of course

2. create a TODO.lst file and have their the list of your test scenarios

3. scenario by scenario do the TDD mantra: red-green-refactor

4. 80% coverage please

Branch FP

1. introduce functional API: streams and lambdas

2. perhaps write a version of the app where you do everything in one line (stream!)