OOP Implementation in Java: http://java-questions.com/oops-interview-questions.html
Association-Aggregation-Composition: https://www.infoworld.com/article/3029325/application-development/exploring-association-aggregation-and-composition-in-oop.html
SOLID Principles in Java :https://howtodoinjava.com/best-practices/5-class-design-principles-solid-in-java/
Java Collection: https://docs.oracle.com/javase/tutorial/collections/index.html
Java 8 Tutorial : http://winterbe.com/posts/2014/03/16/java-8-tutorial/
Java 8 new Stream API Demo based on tutorial: http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/
Android Native Code for Networking using HttpConnection APIs (Not with Libraries like Volley, Retrofit and if possible avoid OkHttpClient ): Rest Calls using HttpClient, and also ftp methods.
Others - Java Concepts Quiz: http://java-questions.com/keyConcepts-interview-questions.html