Design a shape hierarchy so new shapes can be added without changing existing code.

Depend on an abstraction and let each shape compute its own values.

interface Shape {
    double area();
    double perimeter();
}
class Circle implements Shape {
    private final double r;
    Circle(double r) { this.r = r; }
    public double area() { return Math.PI * r * r; }
    public double perimeter() { return 2 * Math.PI * r; }
}
class Rectangle implements Shape {
    private final double w, h;
    Rectangle(double w, double h) { this.w = w; this.h = h; }
    public double area() { return w * h; }
    public double perimeter() { return 2 * (w + h); }
}
double total(Shape[] shapes) {
    double sum = 0;
    for (Shape s : shapes) sum += s.area();
    return sum;
}

total never changes when a Triangle is added, which is the open-closed principle. Validate arguments in constructors so invalid shapes cannot exist. Avoid a type field with a big switch, because that reintroduces exactly the coupling the abstraction removed.