If you are an android developer, you should basically know Java, XML, SQL and other web standards. Knowing Java is obviously important because android is powered by Java, so it is necessary for you to be familiar with Java. Though you don’t need to be an expert, you should be familiar with Java and also some android programming languages. XML is mostly used for defining application interface, so you should be able to use XML in order to develop Android applications. SQL is useful for databases and User Interface (UI) development. You should be familiar with the concepts of database and UI especially when the application you develop requires a database.