One of the main problem that face Java programmer when they start usining Android Studio is that their first HelloWorld Application code, is not written in Java. Alternatively, you will find a Java-like code, but differs slightly, this is Kotlin programming language.
There is a learning curve to master Kotlin, though, You can keep using Java language for Android development. When you start a new application using Android Studio, the default used code is Java, unless you check the Include Kotlin Support option at create Android Projct screen.
Rephrasing this in other words, If you want to use Java Language to develop your application, you need to uncheck Include Kotlin Support option, as shown in the figure below: