How can I validate that my mobile app idea is worth investing time?

As a software developer, can you share your experience of creating a SaaS product as your side project?

Just to clarify, I want to hear from those developers who successfully developed their own side product while doing a full-time job.

Is Java worth learning given the fact that languages like Python, Go etc. are gaining more popularity in industry. Also it is being replaced by Kotlin for Android app development?

I recently graduated as a computer science engineer and realized that Java is quite a "fatty" language. Also the languages stated above are becoming more and more popular. So given these facts should I invest in learning Java for my future career?