https://codecombat.com
http://codingbat.com/java
https://www.codehunt.com
https://www.codewars.com
http://www.learnjavaonline.org
https://www.sololearn.com/Course/Java/
https://www.codeschool.com/learn/android
https://www.codementor.io/collections/learn-android-development-bwba0mlle