It is not easy to find someone who knows everything about Java. There is so much in Java that even the professionals who do understand all the details. Just make sure that you hire a professional who is qualified and proficient in app development.
Do not hire a single developer for the whole project. Android App development is a lengthy process that needs a thorough check. Look for people who are team players, that is, who can work well with others on a given set of instructions. All the team members must understand their roles and act accordingly to complete the project on time. Check the experience each developer has in their field and ask them to show the apps they have created in the past.
Make sure that the developers make and give you a timeline for the completion of the project and the milestones during the development phase.