Abstract Classes

Analogy: blueprint

An Abstract Class is like a Partial Blueprint.

It defines some things (like "All houses have walls"), but leaves other things undefined (like "What style is the roof?").

You cannot build a house from a partial blueprint. You must first complete it (Subclass it).

The Blueprint Builder

We have a House blueprint.

It already has Walls and a Roof.
But the Style is abstract.

You must choose a style to complete the class and build the house.

Blueprint Builder (Abstract Classes)

Abstract Class

Walls (Implemented)
Roof (Implemented)
Style (Abstract!)

"I know I have walls and a roof, but I don't know what I look like yet!"

Implement Subclass:

Incomplete Blueprint. Cannot instantiate.

The Code

Use the abstract keyword.

Java Example

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abstract class House {
    // Implemented Method (Concrete)
    void buildWalls() {
        System.out.println("Building walls...");
    }

    // Abstract Method (No Body!)
    abstract void designStyle();
}

// Concrete Subclass
class Castle extends House {
    @Override
    void designStyle() {
        System.out.println("Adding towers and a moat!");
    }
}