#4 most askedVery commonEasyCore Java & OOP

Abstract class vs interface

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When would you use an abstract class instead of an interface in modern Java?

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☕ Java example · compiled and run to verify it works

interface Shape { double area(); default String describe() { return "area " + area(); } }
abstract class Base implements Shape {
    protected final String name;          // shared state needs a class
    Base(String name) { this.name = name; }
}
class Circle extends Base {
    final double r;
    Circle(double r) { super("circle"); this.r = r; }
    public double area() { return Math.round(Math.PI * r * r * 100) / 100.0; }
}
public class Main {
    public static void main(String[] args) {
        Shape s = new Circle(2);
        System.out.println(s.describe());
    }
}

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