Go Easy with Kotlin, create as many as Class in .kt but if you are having problem with, then also create some class in .java
There is no issue if your app has both the classes, as per google or as .kt and .java after compilation converted to byte code for JVM, because App Modulation of Android can read only bytecode.
Google, noted in a later keynote that this is only an additional language, not a replacement for its existing Java and C++ support.
The aim of Kotlin is to be an enhancement to Java.
Also, already Kotlin is running before that announcement with the help of additional plugin and there is a option to convert Java Code to Kotlin. If you are having issues with the kotlin content available for Android, give it a try.
I advice you to use both to be a versatile Developer, even I want people also give C/C++ Language a try for performing some tasks, this helps Android App in Securities and Speed orientation.
Good Luck.
Learn Java if:
✅ You want to work on enterprise applications.
✅ You are interested in cloud computing, web services, and microservices.
✅ You prefer a well-established language with strong community support.
Learn Kotlin if:
✅ You want to specialize in Android development.
✅ You prefer a modern, concise syntax.
✅ You want to explore functional programming & modern backend frameworks.
Can You Learn Both?
Yes! Java and Kotlin are interoperable, meaning you can use them together in the same project. Many companies use Java for backend and Kotlin for Android development.
If you have time, learning both will make you a more versatile developer.
Final Thoughts
Both Java and Kotlin have their strengths. In 2025:
🚀 Best approach? Start with Java if you’re new, then explore Kotlin to expand your skill set. 🚀