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The following eBooks/Guides can be downloaded as PDF files:
Advanced Java
Android UI Design
Apache Hadoop Tutorial
Hibernate Tutorial
Java 8 Features
Java Abstraction
Java Annotations Tutorial
Java Design Patterns
Java Interview Questions
Java Reflection Tutorial
Java Servlet Tutorial
JAXB Tutorial
JDBC Tutorial
JMeter Tutorial
JPA Mini Book
JPA Tutorial
JSF 2.0 Programming Cookbook
JUnit Tutorial
JVM Troubleshooting Guide
Multithreading and Concurrency Questions
Spring Interview Questions