Best match

In the last section we defined our first multimethod of dimension two. In this section we will see how the best matching method is selected. Let’s redefine our multimethod by adding a match() method for (A, C):

Method2<Float> foo2 = new Method2<Float>().add(new Cases() {
			
	float match(A x, A y, String s) {
		return x.id;
	}
	
	float match(A x, B y, String s) {
		if (s.equals("multiply")) {
			return x.id * y.fd;
		}
		return 0;
	}
	
	float match(A x, C y, String s) {
		if (s.equals("add")) {
			return x.id + y.fd;
		}
		return 0;
	}
});

We apply it to the objects (we do not put the necessary try/catch here):

print(foo2, a, b); // 0
print(foo2, a, c); // 12.3

For the couple (A, B), there is only one compatible method which is match(A, B, String), however for (A, C) the method match(A, C, String) is also compatible meaning that there are two compatible methods. The multimethod selects the best match, that is match(A, C, String) which is in fact the perfect match for (A, C).

Let’s modify the class hierarchy by adding a D class that inherits A and instantiated by the d variable:

public class D extends A {

}

I d = new D();

Let’s modify the multimethod to take into account D:

Method2<Float> foo2 = new Method2<Float>().add(new Cases() {
			
	float match(A x, A y, String s) {
		return x.id;
	}
	
	float match(A x, B y, String s) {
		if (s.equals("multiply")) {
			return x.id * y.fd;
		}
		return 0;
	}
	
	float match(A x, C y, String s) {
		if (s.equals("add")) {
			return x.id + y.fd;
		}
		return 0;
	}
	
	float match(D x, B y, String s) {
		if (s.equals("add")) {
			return 2 * x.id + 3 * y.fd;
		}
		return 0;
	}
});

Let’s apply it to a new couple (D, C):

print(foo2, d, c); // 31.9

This time, we still have two compatible methods match(A, C, String) and match(D, B, String) but none of them is the perfect match. In this case, the multimethod selects match(D, B, String) because by default it selects the best match dimension by dimension from left to right. Indeed by default the multimethod is configured with a method comparator using the lexicographic order.