A simple example showing how to call native Android Java code from Unity. http://www.what-could-possibly-go-wrong.com/creating-a-native-android-plugin-for-unity3d/
The UnityProject directory contains the Unity project (set up with Unity 5.2), with everything you need to build it for Android.
The AndroidPlugin directory contains the source to the native Android plugin used in the Unity project (AndroidPlugin.jar), which has been tested against Android 5.0 (API level 21).
To build the Unity project, just open it in Unity, connect an Android device and go to Build
and Run in the menu. There is already a compiled version of the Android plugin (AndroidPlugin.jar)
in the Unity project Assets folder so there is no need to compile it first just to run the
program.
To build the Android plugin, first make sure you have installed the
Android SDK and Ant.
First, either make sure your ANDROID_HOME environment variable is set to your installation
of the Android SDK, or edit the sdk.dir=... line in the local.properties file inside
the AndroidPlugin folder.
Once that is all configured, navigate to AndroidPlugin and run
ant jar
Then copy the compiled AndroidPlugin.jar from AndroidPlugin/bin to UnityProject/Assets.