Java Core

• Top 50+ Core Java interview Q&As
• Top 50+ Core Java Interview Questions And Answers
• JavaGuide (Chinese)
• Java Tutorial
• Java Generics Tutorial

SOLID Principles

• ref.1 (mirror) | ref.2 | ref.3 (clearer on DIP)
• The Open/Closed Principle and Strategy Pattern

Design Patterns

• Design patterns
• MVC Pattern

Java Concurrency

Multithreading

• Java Thread Tutorial: Creating Threads and Multithreading in Java
• Java Multithreading and Concurrency Interview Questions

Synchronization

• volatile keyword in Java

Java Collections

• Overview of Java Collections Framework API (UML diagram)
• Java Collections
• Collections Comparison
• Java Collections - Arrays.asList() Examples

Java Serialization

• transient keyword in Java
• TRANSIENT AND FINAL INSTANCE VARIABLES IN JAVA

Java Stream

• Java 8 Stream Tutorial
• A Guide to Streams in Java 8: In-Depth Tutorial With Examples

Java Lambda

• Java lambda expressions
• Functional Interfaces In Java
• Java 8 - java.util.function.Function example

JVM

• JVMInternals

Others

• Abstract Class and Methods
• Interface vs Abstract Class in Java
• Determine if a number is 2 to the power of n
• Java template class code
• JIT compiler
• How to sort by two fields in Java?
• sort-list-of-objects-using-comparator-java
• Java HashMap Inline Initialization
• equals() and hashCode() methods in Java
• Working With hashcode() and equals()
• Anonymous Inner Class in Java
• Why have var in Java