Java Roadmap: Basics to Advanced

The goal of this series is to reach Spring Boot and microservices. Both are built on Java, so Java comes first. If you have never written Java before, start here and read in order.

What you will learn

1. OOP fundamentals: classes, objects, and the four pillars, from Java's point of view.
2. How Java works: JDK, JRE and JVM, what makes Java platform independent, and how to install it.
3. Going one level deeper: types of variables, type casting, access specifiers, methods, constructors, abstract and inner classes, and the other kinds of classes.
4. Memory management: how Java uses the Stack and the Heap, and how the garbage collector frees memory.
5. Operators and control flow statements.
6. Exception handling and generic programming.
7. Java Collections: the collections used every day, and how they work inside.
8. Java 8 features: lambdas, functional interfaces, predicates, and the Stream API. These come up often in interviews.
9. Multithreading and concurrency: the different ways to run code in parallel safely.
10. File handling and newer Java features.

After Java

Unit testing comes next: checking that a method returns the expected output for known inputs (for example, sum(1, 2) must return 3), using JUnit and Mockito. After that come Spring Boot and microservices.