Java Guide is an app that teaches you everything you need to know about the Java language.

This app has a wide collection of trail-organized PDFs that gives you the opportunity to learn everything about the Java language from scratch. It's beneficial in a lot of ways, as this is used for mobile app development, big data processing, and embedding systems.

You don't have to have prior knowledge. Even if you have zero knowledge about this, learning with this tool will not be difficult.

The developer continuously updates this app and many more documents will be added.

This is available on Google Play Store.