Java -> android apps is one the things you can do with Java
But Java is really my area, someone else will have to answer that for you.
Of course, you could then learn SQL to strengthen the back-end of your website with a databas (DB), then you can make a powerful web-app.
if you want this, you can install XAMPP, this comes with a webserver, DB and PHP and you can build a webapp.
There is also a framework for PHP, called Laravel, which makes it easier to build web apps.