Learn Java — fundamentals, the modern way

A structured path through the Java language — from primitives to generics. Modern Java (17/21) idioms, runnable examples, and the common mistakes everyone hits.

📚 12 core topics 🔍 91 deep-dive pages ⚡ Runnable examples 🆕 Java 17 & 21

Foundation

Java Primitives — int, double, boolean and the Eight Built-in Types

The eight Java primitive types explained: int, long, short, byte, double, float, boolean, char. Sizes, ranges, defaults, autoboxing pitfalls, and modern Java idioms.

8 sub-topics

Variables in Java — local, instance, static, final, var

The four kinds of Java variable: local, instance, static and parameter. Scope, defaults, final, var (Java 10+), and the modern idioms that make code…

4 sub-topics

Java Control Flow — if, switch, for, while, break, continue

Every Java control-flow construct: if/else, switch (classic and expression form), for, enhanced-for, while, do-while, break, continue, labelled loops. Modern Java pattern matching.

7 sub-topics

Object-Oriented Programming in Java — Classes, Inheritance, Polymorphism

Java OOP from the ground up: classes, objects, constructors, inheritance, polymorphism, abstraction, encapsulation. Modern Java records, sealed classes and interfaces with default methods.

9 sub-topics

Java Methods — Definition, Parameters, Return Types, Overloading

Java methods explained: signature, parameters (including varargs), return types, overloading, static vs instance, recursion, the difference with constructors, and lambdas.

8 sub-topics

Second tier

Java Collections — List, Set, Map, Queue and the Java Collections Framework

The Java Collections Framework explained: List (ArrayList, LinkedList), Set (HashSet, TreeSet), Map (HashMap, LinkedHashMap), Queue, Deque. Performance, when to use each, modern factory methods.

10 sub-topics

Java Access Modifiers — public, private, protected, default

The four Java access modifiers (public, private, protected, package-private) explained with a visibility table, real-world examples and the modern module system considerations.

4 sub-topics

Java Exceptions — try-catch, throw, throws, checked vs unchecked

Java exception handling: the hierarchy (Throwable, Error, RuntimeException), try-catch-finally, try-with-resources, checked vs unchecked, custom exceptions, and the most common runtime errors.

11 sub-topics

Java Keywords — All 50+ Reserved Words Explained

Full list of Java reserved words and keywords: abstract, boolean, break, case, class, final, instanceof, new, static, switch, synchronized, this, throw, var, yield, sealed,…

12 sub-topics

Bonus topics

Java Operators — Arithmetic, Logical, Bitwise, Comparison

Every Java operator by category with precedence and associativity: arithmetic, comparison, logical, bitwise, assignment, ternary, instanceof. Modern pattern matching and the string concatenation trap.

7 sub-topics

Java Annotations — @Override, @Deprecated and Custom Annotations

Java annotations explained: built-in (@Override, @Deprecated, @SuppressWarnings, @FunctionalInterface), how to write a custom annotation with retention and target, and common framework uses (Spring, JPA,…

6 sub-topics

Java Generics — Type Parameters, Wildcards, Bounded Types

Java generics from scratch: type parameters, wildcards (? extends, ? super), bounded types, type erasure, raw types. The PECS rule and why generics disappear…

5 sub-topics

How to use this section

Each pillar page covers a core area of the Java language end-to-end — what it is, how it works, modern idioms (record, var, sealed classes, pattern matching), and the common mistakes. Each pillar links down to spoke pages that go deep on a single concept (one keyword, one collection, one exception type).

If you're new to Java

Read in this order: Primitives → Variables → Control flow → Methods → OOP → Collections → Exceptions.

If you came for one specific topic

Use the search bar in the header, or jump straight to the relevant pillar above. Spoke pages are linked from the pillar's table of contents.

Modern Java first

Examples target Java 17 (LTS) and Java 21 (LTS). Where a feature is newer, the Java version requirement is called out. Where the legacy approach is still common in the wild (e.g. SimpleDateFormat, raw HashMap, anonymous inner classes), it is shown alongside the modern idiom so you can read both old and new code.

Try every example live

Most snippets are runnable in the Java Online Compiler without setup. When a snippet uses an external library (Jackson, Lombok, JJWT), it's flagged.