Java - Interfaces & Abstract Classes

Interfaces & Abstract Classes

Interfaces

interface Shape {
  double area();
  default String unit() { return "sq"; } // default method (Java 8+)
  static Shape square(double a) { return () -> a * a; } // static method
}
Shape s = Shape.square(3);
System.out.println(s.area());

Functional Interfaces

@FunctionalInterface interface Combiner { int apply(int a, int b); }
Combiner c = (a,b) -> a + b;
int r = c.apply(2,3);

Abstract Classes

Can hold state and partial implementations; subclasses fill in the abstract pieces.

abstract class Polygon implements Shape {
  protected final int sides;
  Polygon(int s) { this.sides = s; }
}
class Square extends Polygon {
  private final double a;
  Square(double a){ super(4); this.a = a; }
  public double area(){ return a*a; }
}

When to Use Which

• Use interfaces to define capabilities and allow multiple inheritance of types.
• Use abstract classes when you need shared state/behavior with controlled extension points.

Try it

1. Implement a Shape interface with a lambda for a square and test area().
2. Create an abstract class with a protected field and a concrete subclass that uses it.