Implement a Shape interface using OOP

Problem

You are given the following interface (language-agnostic):

interface Shape {
  double area();
  double perimeter();
}

Design and implement concrete classes for common shapes (at minimum):

• Circle(radius)
• Rectangle(width, height)

Optionally add one more shape (e.g., Triangle(a,b,c) or Square(side)).

Requirements

• Use OOP best practices (encapsulation, clear naming, polymorphism).
• Validate inputs (e.g., non-negative dimensions; for triangle, triangle inequality).
• Make objects safe to use (e.g., immutable fields if possible).
• Provide a small example showing how a caller can compute total area/perimeter of a list of Shape objects.

What to submit

• Class definitions + brief demonstration code (no need for UI).