Abstraction

What abstraction means in Java

• Show the essential operations, hide the implementation. abstract classes and interfaces are the language tools.
• An abstract class cannot be instantiated. It may have constructors, fields, concrete methods, and abstract methods (no body).
• A concrete subclass must implement every abstract method, or itself be abstract.

abstract class Shape {
    final String id;

    Shape(String id) {
        this.id = id;
    }

    abstract double area();

    void tag() {
        System.out.println("shape " + id);
    }
}

class Circle extends Shape {
    private final double r;

    Circle(String id, double r) {
        super(id);
        this.r = r;
    }

    @Override
    double area() {
        return Math.PI * r * r;
    }
}

public class AbstractDemo {
    public static void main(String[] args) {
        Shape s = new Circle("c1", 2);
        s.tag();
        System.out.println(s.area());
    }
}

Abstract class vs interface

Rules of thumb

• Need a constructor or non-constant instance state? Start with an abstract class.
• Need to mix into unrelated classes (Comparable, Runnable)? Interface.
• Template method pattern: abstract class defines the skeleton, subclasses fill in steps.

Partial abstraction and sealed types

• A class with zero abstract methods can still be abstract to block new (e.g. utility-ish bases). Rare.
• Java 17 sealed abstract class Shape permits Circle, Square makes the hierarchy closed — useful with switch exhaustiveness in later Java.
• Do not mark everything abstract “to be OOP”. If there is only one implementation and no shared code, a concrete class is enough.

Template method on an abstract class

abstract class Task {
    final void run() {
        start();
        work();
        end();
    }

    void start() {
        System.out.println("start");
    }

    abstract void work();

    void end() {
        System.out.println("end");
    }
}

public class BackupTask extends Task {
    @Override
    void work() {
        System.out.println("copy files");
    }

    public static void main(String[] args) {
        new BackupTask().run();
    }
}