What OOP Is

1A program as a small world of things

A Java program in this course is not first a list of steps. It is a small world of things that know facts and can do work.

Meera buys a cinema ticket. The ticket is the thing the program should talk about — not a pile of loose numbers named a, b and c with no owner.

Figure. Meera is a person in the story; the ticket is the object the program will talk about. The arrow is the purchase, not a Java method yet.

How it works

1. A program as a small world of thingsObject-oriented code is organised around things that know facts and can do work, not around a loose pile of numbers.

2An object is one particular thing

An object is one particular thing in that world. Meera's ticket is not tickets in general: it is seat A12 for the 6pm show, price 200 rupees.

Arun can hold a different ticket at the same time. Each object has its own copy of the facts. Changing Meera's seat does not move Arun.

Figure. Two ticket objects sit side by side. Same kind of thing, different facts. There is no arrow between them: one does not own the other.

3A class is the shared idea

A class is the shared idea those objects come from. Ticket says every ticket has a show, a seat, and a price.

The class is the mould; each printed ticket is an object. You do not need Java syntax yet: one idea, many instances.

Figure. Ticket at the top is the class — the idea. The two objects below are instances of that idea. The arrows mean made from, not a method call.

How it works

1. A class is the shared ideaOne class, many objects. The class names the facts; each object fills them in.

4Hide the inside; offer a small public surface

Encapsulation means the object's insides stay its business. Other code should not reach in and rewrite the price.

It asks the ticket to do something — print itself, say whether it is a student fare — through a small public surface. Later Java will use private fields and public methods for this idea, not as decoration.

Figure. The outer box is what other code may ask for. The inner box is the price, which stays the ticket's business. Not to scale as a real class file — it is a boundary picture.

How it works

1. Hide the inside; offer a small public surfaceAsk through the public surface. Do not reach in and rewrite the price.

5Idea first, then the .java file

The next lessons teach Java's file shape, types, operators, and control. Those are how you write.

This lesson is what you are writing about. If you skip the object idea, the first line of a Java file looks like a spell instead of a door into a class.

Figure. Treat the ticket as an object before you open a .java file. This lesson is what you are writing about. Syntax is how you write the idea down.

How it works

1. Idea first, then the .java fileTreat the ticket as an object before you open a .java file. Syntax is how you write the idea down.

Coding lab. Two tickets, two objects runs in the app, with checks on your output.