Invoke methods dynamically: replacing reflection with functional programming

This question on stackoverflow has an answer that replaces calling methods using reflection with functional programming. I like it, but I don't understand why it works. The interface defines an execute method, but nothing explicitly implements that method!

Why does this work? Why does it even compile? What am I missing?

@FunctionalInterface
public interface Method {
    double execute(int number);
}

public class ShapeArea {
    private final static double PI = 3.14;

    private Method[] methods = {
        this::square,
        this::circle
    };

    private double square(int number) {
        return number * number;
    }

    private double circle(int number) {
        return PI * number * number;
    }

    public double run(int methodIndex, int number) {
        return methods[methodIndex].execute(aNumber);
    }
}