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Table of contents

The Curriculum

Nine lessons across five chapters. Read them in order if you're new, or dip in where you need to. Every lesson is self-contained and includes runnable code.

01

Foundations

§ 1.01
Hello, World — your first Java program
Set up the JDK, understand the anatomy of a Java class, and run a program from the command line.
12 min§ 1.02
Variables, primitive types, and references
The eight primitive types, the difference between values and references, and how the compiler infers types.
18 min§ 1.03
Control flow: if, switch, and loops
Branching and iteration, including modern switch expressions and the enhanced for loop.
20 min
02

Object-Oriented Java

§ 2.01
Classes, fields, constructors, methods
Model the world with classes — encapsulation, constructors, the `this` reference, and method overloading.
25 min§ 2.02
Interfaces, abstract classes, inheritance
Polymorphism done right — when to extend, when to implement, and why composition usually wins.
28 min
03

Collections & Generics

§ 3.01
Lists, Sets, Maps — the Collections framework
ArrayList, HashMap, HashSet, and the Iterable contract. Choose the right structure for the job.
30 min§ 3.02
Generics, bounded types, and wildcards
Type-safe containers, PECS (producer-extends, consumer-super), and erasure's sharp edges.
32 min
04

Modern Java

§ 4.01
Streams, lambdas, and functional pipelines
Transform collections declaratively. map, filter, reduce, collect — and when to fall back to a for-loop.
35 min
05

Data Structures in Java

§ 6.01
Arrays and ArrayList — the workhorses
Fixed-size arrays vs dynamic ArrayList. Memory layout, amortized O(1) append, and when each one shines.
22 min§ 6.02
LinkedList and the Deque interface
Doubly-linked nodes, O(1) insert at either end, and why LinkedList is almost never the right choice.
20 min§ 6.03
Stacks, queues, and ArrayDeque
LIFO, FIFO, and why java.util.Stack is a historical mistake you should avoid.
24 min§ 6.04
HashMap, equals, and hashCode
How hashing works, why you must override equals and hashCode together, and the load factor that keeps lookups O(1).
28 min§ 6.05
TreeMap, TreeSet, and ordered structures
Red-black trees under the hood. O(log n) sorted access, range queries, and floor/ceiling lookups.
26 min§ 6.06
PriorityQueue — heaps in disguise
Binary heaps, O(log n) insert and extract-min, and the canonical k-largest pattern.
22 min§ 6.07
Graphs — adjacency lists and BFS
Modeling graphs in plain Java with Map<Node, List<Node>>, then walking them breadth-first.
30 min
06

Concurrency

§ 7.01
Threads, executors, and virtual threads
From `Thread` to `ExecutorService` to Project Loom — concurrency the modern way.
40 min