Add Java example for counting products using Streams and Map

[hirushi1999]

Hi @IamBisrutPyne, I would like to contribute a Java example for Hacktoberfest 2025.

The code demonstrates how to use Java Streams and Maps to count the number of products in each category.

I think this would be a useful addition to the repository as it covers practical usage of Streams, grouping, and maps in Java. Please let me know if I can proceed.

[IamBisrutPyne]

Hey @hirushi1999 👋

This is an excellent idea!
Please proceed with your PR.

Crucial for Hacktoberfest

• One contribution per submission: Please only submit one file per PR.
• Correct file naming and repository structure: Self-explanatory.

Good luck, and Happy Hacking 🌟

[hirushi1999]

Thanks a lot, @IamBisrutPyne! 🙌