Without wasting any more time, here is the list of some of the great Java books, which are absolutely FREE, you don't need to pay anything to download or read this book. All you need is an internet connection to download this books on your computer, laptop, iPhone or android smartphone.
Most of the new eBooks which I have added recently are absolutely free, legal and you can download them in PDF, EPUB or MOBI format for online and offline reading.
1. Introducing Java 8
Author: by Raoul-Gabriel Urma
Download: http://www.oreilly.com/programming/free/introducing-java-8.csp
Description: This is the latest Java book which is FREE. Many Thanks to O'Rilley who has published an introductory book on Java 8, titled with Introducing Java 8, A quick start guide to lambda expressions and streams. The author Raoul-Gabriel Urma, who is also an author of one of the best seller book of last year, Java 8 in Action, explains how improved code readability and support for multicore processors were the prime movers behind Java 8 features.
He’ll quickly get you up to speed on new classes including CompleteableFuture and Optional, along with enhanced interfaces and the new Date and Time API. No doubt a great guide for any Java programmer willing to learn Java 8 by himself. If you will follow examples given int his book, you will learn lambdas and Streams in real quick time.
2. Object-Oriented vs. Functional Programming
Author: by Richard Warburton
Download: http://www.oreilly.com/programming/free/object-oriented-vs-functional-programming.csp
Description: This book discusses the difference between Object oriented and Functional programming. Since Java 8 is now support some of the functional programming concepts e.g. lambda expressions, map, flatmap, reduce etc, it becomes important to learn how to work together in both OOP and FP at the same time.Java technologist Richard Warburton, author of Java 8 Lambdas, discusses similarities between these programming paradigms and points out that both FP and OOP are actually moving closer toward each another.

3. Java: The Legend
Author: by Benjamin Evans
Download: http://www.oreilly.com/programming/free/java-the-legend.csp
Description: Java has come a long way in last 20 years. It is no more the fancy language of developers but it has now become the mainstream of any application development in the world. The use of Java in Android has taken Java into even more larger domain
This book will take you to journey of Java programming language starting from very first version to the current Java 8 version. Benjamins Evans, author of the modern Java book, the Well-grounded Java Developer will explain to you the design decision was taken at the start e.g. making it platform independent and always keeping it backward compatible and how they paid off.
4. Docker for Java Developers
Author: by Arun Gupta
Download: http://www.oreilly.com/programming/free/docker-for-java-developers.csp
Description: If you have worked in Java development and production services you know that how painful is to deploy a Java application. You have to deploy code, configuration, database changes, file systems changes etc. Though this is the tried and tested approach and working well from a long time, it can be improved.

In this free Java EE eBook, Arun Gupta, author of the Java EE 7 Essentials explains how you can use Docker to deploy Java application both on server and cloud. This can improve both deployment and startup time of your Java application in Windows, Linux, and Mac OS X.

5. Microservices for Java Developers
Author: by Christian Posta
Download: http://www.oreilly.com/programming/free/microservices-for-java-developers.csp
Description: Microservices is a new buzz word in Java world, touted to replace huge, monolithic SOA applications with hundreds and thousands of micro web services. In this free Java Microservice eBook, Christian Posta, a Principal Middleware Specialist/Architect at Red Hat explain to you whether Microservice architecture is right for your organization or not. He outlines both benefits and drawbacks of Microservices and explains to them how to implement them using popular frameworks like DropWizard and Spring Boot. This free Java book is full of useful examples.
6. Modern Java EE Design Patterns
Author: by Markus Eisele
Download: http://www.oreilly.com/programming/free/modern-java-ee-design-patterns.csp
Description: There are not many good Java EE design patterns books available in the market, forget about free books, so when I found this free Java EE eBook, I got really excited. Since Java EE landscape is quickly changing with the advent of Microservice and it is challenging Java EE's monolithic SOA like architecture. In this book, Markus Eisele explains Java EE with MicroServices and explores design patterns like aggregator, proxy, pipeline etc.

7. Data Structures and Algorithms with Object-Oriented Design Patterns in Java
Author: Bruno R. Preiss
Download: http://www.brpreiss.com/books/opus5/index.html
Description: Great book to learn data structure and algorithm in Java programming language. Filled with a lot of samples but non-trivial examples of implementing different data structures e.g. stack, queue, linked list in Java. Since data structure and algorithm are very important for any Java programmer and quite a common topic in Java interview, it is an absolute must to have a strong command in both. If you are preparing for Java job interviews then you can also take a look at some of my favorite algorithm and data structures questions, it may help in your preparation. If you like to read paperback edition, you can order it from Amazon as well.

8. Java Application Development on Linux
Author: Carl Albing and Michael Schwarz
Download: http://javalinuxbook.com/
Description: A perfect Java book, if you are developing or running Java application on Linux environment, which is the case in most of the investment banks. You can download entire books as PDF, along with all example programs. Carl Albing and Michael Schwarz has done an excellent job to put everything needed to run and support a Java program in Linux environment including how to start, stop, or kill Java process, checking logs with some handy useful UNIX commands. Paperback edition of this book is also available here in Amazon.
9.Core Servlets and JavaServer Pages
Author: by Marty Hall and Larry Brown
Download: http://pdf.coreservlets.com/
Description: Servlets and JSP are fundamental Java technologies for developing web applications in Java. Core Servlets and Java Server Pages, teaches you basics of these technologies. You can access all chapter of this book as FREE PDF online, they are also available for download. The good thing about this Java books is that source code and lecture notes are also available for FREE download. If you like to read paper book then you can also purchase, paperback edition of this book here

10. The Java Language Specification, Java SE 7 Edition (Java Series)
Author: James Gosling, Bill Joy, Guy L. Steele Jr., Gilad Bracha, Alex Buckley (Author)
Download: http://docs.oracle.com/javase/specs/jls/se7/jls7.pdf
Description: Official Java language specification for Java SE 7 edition is available online to view as HTML and download as PDF. This is the best FREE resource in Java, as it's from source and contains most up-to-date details about Java Programming language. If you like paperback edition, you can also purchase this books from amazon here.

11. The Java Virtual Machine Specification, Java SE 7 Edition
Author: Tim Lindholm, Frank Yellin, Gilad Bracha, Alex Buckley
Download: http://docs.oracle.com/javase/specs/jvms/se7/jvms7.pdf
Description: Knowledge of Java Virtual Machine is very important for experienced Java developers, to get the maximum out of JVM and avoid unnecessary optimization, which can be effectively done by JIT and JVM. For a beginner, It's too much ask, to know more about JVM, but since it's a free Java book, you can always download PDF and read it. Paperback edition of this Java book is also available for purchase on Amazon, here.
12. The Java Tutorial: A Short Course on the Basics (5th Edition)
Author: Sharon Biocca Zakhour, Soumya Kannan, and Raymond Gallardo
Download: http://www.oracle.com/technetwork/java/javase/downloads/java-se-7-tutorial-2012-02-28-1536013.html
Description: This is the official Java tutorials from Oracle, which explains different Java concepts in the form of short courses e.g. JDBC, JMX, JAXB. All tutorials are available online and you can also download them for free as eBook, available in both .mobi and .epub format, nice to read in iPhone and android phones. If you love paperback edition, you can also order it from Amazon. By the way, these tutorials are really good quality and the great way to explore different features of Java Programming language.

13. Thinking in Java 3rd edition
Author: Bruce Eckel
Download: http://www.mindview.net/Books/TIJ/
Description: Thinking in Java is a Jolt Award winner and one of the classic books to learn Java programming. The third edition of this books is freely available for download and you can download them as PDF format for the offline read. By the way, the fourth edition of this book is also available which covers most of new Java 5 concepts in detail, but it's NOT FREE. you can purchase that from Amazon. One more thing, first six chapters of Thinking in Java 4th edition is also available in PDF format for free download.
14. Introduction to Programming Using Java, Sixth Edition
Author: David J. Eck
Download: http://math.hws.edu/javanotes/
Description: This is another free Java book, which is available in both PDF and HTML format and teaches programming basics using Java programming language. I liked the chapter on Linked Data structure and Recursion, which teaches some of the key programming concepts with simple, non-trivial Java examples. You can also purchase paperback edition of this book on Amazon.

15. Processing XML with Java (A Guide to SAX, DOM, JDOM, JAXP, and TrAX
Author: Elliotte Rusty Harold
Download: http://www.cafeconleche.org/books/xmljava/
Description: XML is one of the most desirable skills along with Java. You often need to work with XML files in large projects, as it's one of the most widely used data transport formats. This Java book is a comprehensive and up-to-date collection of various XML technology and how to use them with Java programming language. You will learn about different XML parsers e.g. SAX and DOM, JDOM, XPATH, and XSLT etc. If your work involves, XML and Java, then this is the book you should read. This Java book is freely available for online read, and you can buy a paperback edition from any book store including Amazon.
16. Think Java (How to Think Like a Computer Scientist)
Author: by Allen B. Downey
Download: http://greenteapress.com/thinkapjava/
Description: Don't confuse this book with Thinking in Java, it's a different one. This is another great Java book for beginners which is available for FREE. You can download it as PDF or read it online on their site. It covers programming basics, object-oriented concepts, essential software development technique, debugging etc. It's actually tailored for students, who wants to give Computer Science advanced placement (AP) exam, but turn out be a great book for any beginner. If you just started learning Java, give it a try. Paperback edition of this book is available on Amazon here.

That's all guys, these are some really useful FREE Java books, especially the latest Introducing Java 8 book. As we all love free resources, download them as PDF or view online them as HTML. If you have slow Internet connection, then it's better to download PDF eBooks and read them offline. By the way, nothing can substitute a paperback book, eBooks are good but not for continuous reading. So don't forget to have a paperback edition of at least one Java book, when you start learning Java. If you are an absolute beginner then it's worth buying Head First Java 2nd Edition as well, one of the most recommended books for Java beginners.
Further Learning
Complete Java Masterclass
Java Fundamentals: The Java Language
Java In-Depth: Become a Complete Java Engineer!
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