Here's a list of resources for Java programmers.
Java 10 Documentation - Oracle.
JSE Javadoc - Oracle.
Java Language Specification - James Gosling, Bill Joy, Guy Steele, Gilad Bracha, Alex Buckley, Daniel Smith.
stackoverflow.com - An outstanding resource for technical problems of all sorts.
Javadoc Style Guide - Oracle.
Java Look And Feel Guidelines - Oracle.
Java articles - Oracle.
JUnit Testing Framework - Kent Beck, Erich Gamma, and others.
Apache Ant Build Tool - (not as widely used nowadays).
Apache Tomcat - Popular and robust servlet container, for implementing web apps.
caniuse.com - view browser support for specific features.
Eclipse IDE - an IDE from IBM.
IntelliJ IDEA - an IDE from JetBrains.
Notepad++ - A text editor.
Putty - by Simon Tatham. Secure shell and secure ftp client.
Web Site Security Guidelines - Open Web Application Security Project. How to keep your web apps secure, and avoid the most common mistakes.
Portability Cookbook - Oracle
Java Community Process - jcp.org.
developerWorks - IBM.
Books:
Effective Java - Joshua Bloch, [Addison Wesley], 0-321-35668-3. Intermediate level. Probably the best reference for important core Java coding techniques.
Java Programming Language - Arnold, Gosling, Holmes, [Addison Wesley], 0-321-34980-6.
Head First Java - Kathy Sierra, Bert Bates, [Paw Prints], 0596009208. An excellent text for beginners.
Code Complete - Steve McConnell, [Microsoft Press], 0-7356-1967-0. Although written many years ago, many of the core ideas of this classic text can be applied to almost any programming context.
The Pragmatic Programmer - Andrew Hunt, David Thomas, [Addison Wesley], 0-201-61622-X. Part of an excellent series of practical texts.
Design Patterns - Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides, [Addison Wesley], 0-201-63361-2. A classic text from the 1990s.
Java Concurrency in Practice - Brian Goetz, and others, [Addison Wesley], 0-321-34960-1. A bible for multi-threaded programming.
Designing Web Usability - Jakob Nielsen, [New Riders], 1-56205-810-X. By a guru of web design.
Refactoring - Martin Fowler, [Addison Wesley], 0-201-48567-2.
Effective Enterprise Java - Ted Neward, [Addison Wesley], 0-321-13000-6.
Patterns of Enterprise Application Architecture - Martin Fowler and others, [Addison Wesley], 0-321-12742-0.