Must have tool, technologies & skills for a Java Developer

Java Development Kit (Java 8 or 9)

Git Distributed Version Control System

IntelliJ IDEA for Development Environment or Eclipse

Android Studio for Android Development

Atom or Notepad++ for misc editing tasks

Ubuntu/Centos/Windows OS

Microservices Architecture Styles for Software Development

REST over HTTP communication

Algorithms and Datastructures

Concurrency and multithreading in Java

Familiarity with any Cloud (AWS, Azure, Heroku, etc)

NoSQL and SQL database knowledge (MySQL, MongoDB, DynamoDB, HBase, Apache Cassandra)

Familiarity with Text Search Libraries (Lucene, Solr, Elastic Search, etc)

Basic Knowledge of Jquery and javascript

Freemarker/Velocity Template Engine

Spring Framework/ Spring Boot

Basic understanding of Servlets

Jetty/Tomcat/Undertow or other servers

HTML 5

Firebug Firefox/ Chrome Developer Tools

Head First Design Patterns in Java

Concurrency In Practice by Brian Goetz

Effective Java 2nd Edition by Joshua Bloch

Algorithms 4th edition : http://algs4.cs.princeton.edu/home/

http://www.stackoverflow.com

http://www.geeksforgeeks.org/fundamentals-of-algorithms/

http://www.careercup.com

Few articles on Java 6 at IBM website - http://www.ibm.com/developerworks/views/java/libraryview.jsp?search_by=5+things+you+did

Concurrency In Practice by Brian Goetz - http://www.briangoetz.com/pubs.html

Java Articles : http://www.oracle.com/technetwork/articles/java/index.html

http://www.oracle.com/technetwork/articles/java/index.html

Java SE Tutorial : http://docs.oracle.com/javase/tutorial/index.html

Java 8 docs: http://docs.oracle.com/javase/8/docs/