Polymorphism, with a runnable example

One loop that handles types it has never heard of, and the difference between overloading and overriding.

12 min read · Lesson 4 of 6 · Pro

The problem, in code you have probably written

Java

for (Object o : things) {
    if (o instanceof Circle) {
        total += ((Circle) o).circleArea();
    } else if (o instanceof Rect) {
        total += ((Rect) o).rectArea();
    } else if (o instanceof Triangle) {
        total += ((Triangle) o).triangleArea();
    }
}

Every new shape means editing this loop. If the same chain appears in five files, that is five edits, and you will forget one. This is the shape of code that polymorphism deletes.

The polymorphic version

Java

abstract class Shape {
    abstract double area();

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