Getting started 4 lessons

1 Getting Started with Java: JDK, JShell and Your First Program Install the JDK, try code instantly in JShell, and write, run and understand your first Java program. Beginner4 min read 2 Variables, Data Types and Strings Store values in variables, pick the right data type, do arithmetic without surprises, and work with Strings and text blocks. Beginner4 min read 3 Control Flow: if, switch and Loops Make decisions with if and the modern switch expression, and repeat work with for, for-each and while loops. Beginner3 min read 4 Methods: Writing Reusable Code Package logic into methods with parameters and return values, overload them, accept any number of arguments and understand recursion. Beginner3 min read

Object-oriented Java 3 lessons

5 Classes and Objects Model real things as classes, create objects with constructors, protect their data with encapsulation and understand static members. Beginner3 min read 6 Inheritance, Interfaces and Polymorphism Share behaviour with inheritance, define contracts with interfaces, and write code that works with many types at once through polymorphism. Intermediate3 min read 7 Exceptions: Handling Errors Gracefully Catch and throw exceptions, clean up with finally and try-with-resources, and create meaningful custom exceptions. Intermediate3 min read

Collections and generics 2 lessons

8 Collections: List, Set and Map Store groups of objects in Lists, Sets and Maps, choose the right implementation, and avoid the classic collection mistakes. Intermediate4 min read 9 Generics: Type-Safe, Reusable Code Understand the <T> in Java code — write generic classes and methods, add bounds, and read wildcards like <? extends Number>. Intermediate4 min read

Modern Java (functional programming) 9 lessons

10 Functional Interfaces: The Foundation of Modern Java Learn the single-method interfaces behind every lambda — Predicate, Function, Supplier, Consumer and friends — and compose them like building blocks. Advanced5 min read 11 Lambda Expressions: Code You Can Pass Around Write lambdas in every form, replace anonymous classes, sort and filter with them, and pass behaviour into your own methods. Advanced5 min read 12 Method References: Lambdas, Even Shorter Use the :: operator to turn existing methods and constructors into functions — and learn the four kinds of method reference. Advanced4 min read 13 Closures in Java: Capturing Variables and "Effectively Final Understand how lambdas capture variables, why Java requires them to be effectively final, and how to build functions that remember their configuration and state. Advanced6 min read 14 Optional: Say Goodbye to NullPointerException Model "maybe there's no value" explicitly with Optional — create, transform and unwrap it safely, and learn when not to use it. Advanced5 min read 15 Streams API: The Basics Process collections declaratively with streams — filter, map, sort and summarise data in readable pipelines, and understand laziness. Advanced5 min read 16 Advanced Streams: Collectors, groupingBy and flatMap Turn streams into reports — group and count with groupingBy, split with partitioningBy, flatten nested data with flatMap, and reduce, join and summarise in one pass. Advanced6 min read 17 Records, Sealed Types and Pattern Matching Model data with records, close type hierarchies with sealed interfaces, and take them apart safely with pattern matching in switch (Java 21). Advanced4 min read 18 CompletableFuture: Asynchronous Java Run slow work in parallel and combine the results with CompletableFuture — chaining, combining, waiting for many, handling errors and timeouts. Advanced5 min read