Here I am sharing some good resources to learn the concepts of java from bascis to advance levels:

Java Tutorials
https://docs.oracle.com/javase/tutorial/

Intro of Java Programming
https://en.wikipedia.org/wiki/Java_(...ming_language)

Getting Started with Java
https://intellipaat.com/tutorial/java-tutorial/

Learn Java
http://www.javatpoint.com/java-tutorial