Java Multi-threading:
http://www.tutorialspoint.com/java/java_multithreading.htm
http://www.javabeginner.com/learn-java/java-threads-tutorial
http://tutorials.jenkov.com/java-concurrency/index.html
http://beginnersbook.com/2013/03/multithreading-in-java/
Java Networking:
http://www.tutorialspoint.com/java/java_networking.htm
http://tutorials.jenkov.com/java-networking/index.html
http://docs.oracle.com/javase/tutorial/networking/TOC.html
Java IO:
http://www.tutorialspoint.com/java/java_files_io.htm