The big idea: Object-oriented programming keeps a thing's data and the operations on it in one place.
A book's title and its borrow() method live together, rather than the data sitting in one list and the code that handles it in another.
A Book class, the objects made from it, static variables and encapsulation. Step through all four.
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Class — the template
Says what every one of this kind of thing will have and can do.
A class holds no values itself: no class ever sat on a shelf. It is the plan, not the object.
Object — one actual thing
Made from the class, with its own values.
Three Book objects share the same attributes and methods, but each holds its own title — changing one changes nothing about the others.
Encapsulation — private data, public methods
Attributes are private; the only way in is through public methods.
So every change goes through code that can check it first, rather than any part of the program writing whatever it likes.
Inheritance — a more specific kind
A new class takes everything an existing one has, and adds or changes what makes it different.
Ebook inherits from Book — same title and author, plus a file size. Nothing is copied.
Polymorphism — same call, right behaviour
Different classes can offer the same method and each do it their own way.
Call describe() on any Book and each kind answers appropriately, without the calling code needing to know which kind it has.
The two that get confused: Inheritance is about reusing what a more general class already has.
Polymorphism is about calling the same method on different kinds and each responding properly. Inheritance is often how polymorphism becomes possible, but they are not the same idea.
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What OOP gives you
- Models the real world — a Book object behaves like a book
- Reuse through inheritance, with no copied code to drift apart
- Encapsulation means invalid states can be refused at the door
- Large programs split into parts people can work on separately
- Easier to extend: a new kind of Book needs no change to existing code
What it costs
- More to design — classes and relationships must be worked out first
- Deep inheritance chains become hard to follow
- More code for a small task: a script does not need classes
- A small overhead in memory and speed
- Poor class design is harder to fix than poor procedural code
When it earns its place: OOP pays when there are real things with both data and behaviour, when a program is large or long-lived, or when several people work on it.
For a fifty-line script that reads a file and prints a total, it is overhead with no return.
Libraries of classes: Languages ship with ready-made classes: ArrayList and Scanner in Java, list and datetime in Python.
Using them saves writing and testing code that already works, and other programmers recognise them at once.
How this is tested — you must EVALUATE — weigh what OOP gives against what it costs, for this scenario. It comes up two ways:
Paper 2 — working with code
- Evaluate the use of OOP, 4-6 marks
- Define and illustrate the five concepts
- Say whether OOP suits a described program
Paper 2 — the algorithmic-thinking question
- Decide from a description with no code shown
- Justify the choice from the program's size and lifetime
The classic trap: Listing advantages only. Evaluate requires the costs too — the design effort, the extra code, and the fact that a small script gains nothing from any of it.
A school is building a library system handling books, ebooks, members and loans, to be maintained for many years. Evaluate the use of OOP.
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