Java Resources

Tutorials

The Official Java Tutorials for various tutorials authored by Sun / Oracle
Thinking in Java by Bruce Eckel, for a free online textbook on Java programming
Java2s: Java Tutorial for a free, online tutorial covering many of the basics

Generics for using types as parameters that are checked at compile-time
Lambda Expressions for how to use lambdas in Java 8
Lambda Quick Start for more on lambdas
Java Puzzlers for lots of subtle edge cases and gotchas in the Java programming language
Security Managers and the Java JDK for details on security managers
How to Tune Java Garbage Collection for details on Java's garbage collection

Reference

Java Language Specification for the specification of the Java programming language
Java Virtual Machine Specification for the specification of byte code and the virtual machine

Libraries

Guava: Google Core Libraries for Java for everything ranging from advanced collections to parsing URLs

java.util.logging.Logger for the builtin logging capabilities (since Java 1.4)
Apache Log4j for a similar but slightly more powerful logging system

JUnit for writing Java unit tests
Mockito for easily generating mocks, modifying their output, and verifying their invocations
Hamcrest for general purpose matchers used to easily express test assertions
Roboelectric for writing Android tests that run on an Android device or emulator

Protocol Buffers for automatic, cross-language, cross-platform serialization with good backwards compatibility
java.io.Serializable for builtin, automatic serialization of Java objects (hard to maintain compatibility)
java.io.Externalizable for builtin, manual serialization of Java objects (more manual labor involved)

Google Cloud Data Flows for large scale, parallel processing pipelines and workflows
Apache Spark for an alternative to cloud data flows for creating large scale parallel processing pipelines
java.lang.Thread for low-level thread creation
java.lang.ProcessBuilder for low-level process creation
java.util.concurrent for utilities and data structures for writing multithreaded code

JFlex for auto-generating a lexer from a lexical specification to break up input into tokens
Java CUP for generating a parser to build up and process an abstract syntax tree from lexed tokens

java.net.Socket for making low-level TCP connections
java.net.ServerSocket for receiving low-level TCP connections
java.net.DatagramSocket for low-level UDP connections
java.net.HttpURLConnection for sending HTTP requests
Google HTTP Client Library for more advanced HTTP request sending and HTTP response processing
Google App Engine for writing scalable web servers in Java
Spark for creating very simple web servers
gRPC for a high-performance remote procedure call (RPC) system based on protocol buffers

Thymeleaf a templating engine for emitting HTML (e.g. when implementing a server with a web-based UI)
GWT for building UIs in Java and generating HTML/CSS/JavaScript from it
Android XML and android.view.View for developing Android UIs
Swing for building desktop GUIs (though consider creating a webapp, instead)

Tools

Java Standard Edition (Java SE) for the official compiler provided by Oracle
OpenJDK for the open source implementation of Java SE
Android SDK if developing for Android (requires Java SE or OpenJDK)

Bazel (my recommendation) for Google's general-purpose, open source build system
Gradle for a general purpose, declarative build system with plugin support for Java
Apache Maven for a declarative XML-based system (recommend against; popular, but slow and complex)
Apache Ant for an imperative XMl-based system (recommend against; just a more verbose, confusing Make)
GNU Make for a low-level, imperative build system (ok for small projects, otherwise avoid)

Cloud 9 for an online IDE (what I'm using for this website, in fact)
Code Envy for yet another web-based IDE
Code Anywhere for yet another web-based IDE option
Nitrious.io for yet another web-based IDE option
Android Studio if developing for Android; has good support for Android UIs and based on IntelliJ IDEA
IntelliJ IDEA for a Java-specific, desktop-based IDE with strong refactoring / code understanding capabilities
Eclipse for a popular, general purpose, desktop-based IDE with good builtin support for Java
NetBeans for a free IDE with good drag-and-drop user interface creation for Java Swing GUIs

IDEOne an online compiler for quickly testing / validating small snipppets
FindBugs for automated defect detection
Checker Framework for static analsis and verification of nullness/non-nullness
Javadoc for automatically extracting/generating documentation
JDB for debugging Java programs
SWIG for auto-generating JNI code to allow C++ and Java code to talk to each other
Jenkins for continous integration and testing