Welcome

Welcome to Object-Oriented Programming Blueprint with Java, a course about thinking in objects. It is designed for students of computer science who are meeting object-oriented programming for the first time and who want to understand why it exists, not just how to type its syntax.

Java is the vehicle, not the destination. We use it because it forces you to be explicit: every value is part of a class, every responsibility lives somewhere on purpose, and every relationship between objects has a name. By the end of the course you should be able to look at a real-world problem — a library, a hospital ward, a vending machine, a ride-share system — and decompose it into a small society of collaborating objects.

What this course is about

You will spend most of your time on:

• Core OOP thinking — what an object is, what it means for objects to send messages, and how a system emerges from their interactions
• Object modeling — turning vague descriptions ("a school has students") into precise classes, interfaces, and relationships
• Encapsulation and abstraction — hiding what callers should not see; exposing what they need
• Relationships — composition, aggregation, dependency, inheritance, and the trade-offs between them
• Responsibility-driven design — deciding which object should know what and do what
• Software architecture fundamentals — packages, layering, and how OOP keeps large codebases maintainable

This course intentionally does not spend much time on web frameworks, build tools, dependency injection containers, ORMs, or production operations. Those skills are valuable, but they distract from the more important — and more durable — skill of being able to design a clean object model in the first place.

How to use this site

Every page mixes prose with three kinds of interactive widget:

1. Executable Java code blocks. Each block is compiled by javac and run on the JVM inside your browser via CheerpJ. The first run in a session is slow because the JVM has to boot; subsequent runs are fast.
2. Multi-file challenge cards. A real object model lives in many files: classes, interfaces, abstract classes, enums. You will see workspaces with several .java files side by side and fill in the missing parts.
3. Multiple-choice questions. Short single-answer checks at the end of most pages, with a per-choice explanation so a wrong answer still teaches you something.

A tiny taste

This is the simplest possible object: a Lamp that can be turned on and off. Notice that the lamp keeps its own state (isOn) and exposes behavior (turnOn, turnOff, describe) rather than letting outside code touch its fields. That is the seed of every idea in this course.

Course outline

Origins of OOP

A short story: the software crisis of the 1960s and 70s, the birth of OOP in Simula and Smalltalk, and how the Gang of Four popularized reusable design thinking. This is the why.

Java and objects

Your first Java class, then a look at what really happens in memory when you write new Dog() — the JVM's heap, references on the stack, and what null means.

Core OOP

Encapsulation, constructors, methods as messages, static vs. instance, enums, and utility classes. The everyday vocabulary of object design.

Relationships between objects

Composition, aggregation, dependency, inheritance, abstract classes, interfaces, and polymorphism. How small objects compose into bigger systems.

Design thinking

Responsibility-driven design and how to organize a codebase into packages and layers without losing your mind.

Capstone

A guided modeling exercise: design a small library management system end-to-end, exercising every idea in the course.