Double Dispatching

Double Dispatching In Java

Single Dispatch • Uses polymorphism between base and derived classes • Compiler registers container pointer to base class • At run-time a dispatch vector is used to determine what the container is actually pointing to

Single Dispatch • This can be either an object of base or derived type • But, if calling a method dynamically the signatures of the base and derived methods must be identical • If they are not the run-time system uses the static, not the dynamic type to determine the call.

An Example • My apartment in Yerevan has ants and cockroaches • I want to spray the ants with ant poison and the cockroaches with cockroach poison • The Public Health Department at the AUA comes up with the following hierarchy:

GenericPoison AntPoison CockroachPoison Insect Ant Cockroach

Single Dispatch • Poison is a kind of visitor to Insect • We want to have a method spray() in all three classes that is insect-specific • Start with the base class Poison • Use Insect as the parameter type of spray()

class Poison { public void spray(Insect a Bug){ …} } class AntPoison { public void spray( ???){…} }

Single Dispatch • Problem: we would like to spray ants with ant poison • Hence, if I have a declared Insect pointer that has a reference at run-time to an Ant object I would like to call the Ant’s spray() method • What is the parameter type for the Ant spray()?

First Attempt public class Poison { public void spray(Insect aBug){ System.out.println(“Generic Spray”); }; Public class AntPoison extends Poison { public void spray(Ant anAnt){ System.out.println(“Spraying ant with ant spray”); }; public class Insect{…}; public class Ant{…}; Try directly to apply ant poison to an ant:

Now try it • Make a container and try polymorphism: Poison container = new AntPoison(); container.spray(new Ant()); • Produces Generic Spray • !!!!! • What happened?

Just this • There is no method in Poison with the parameter of type Ant. • Therefore the Java run-time system reverts back to the static definition of the polymorphic container, i.e. Poison

Cheap way out: • Make the container point to AntPoison: AntPoison container = new AntPoison(); Container.spray(new Ant()); Container.spray(new Insect()); • Produces Spraying ant with ant spray Generic Spray

Caveat • There is no method in AntPoison to spray a generic insect • Hence the compiler reverts to the spray method in the super class, i.e. in poison • Way out: Need a spray method in Poison with an Ant parameter:

public class Poison{ public void spray(Insect aBug){ System.out.println(“Generic Spray”); } public void spray(Ant antBug){ System.out.println(“Generic Ant Spray”); } }; public class AntPoison extends Poison{ public void spray(Ant antBug){ System.out.println(“Spraying Ant with Ant spray”); } };

Test This Poison container = new AntPoison(); Insect bugType = new Ant(); container.spray(bugType); bugType = new Insect(); container.spray(bugType); Produces Generic Spray Generic Spray

Why? • Because there is no spray method in Ant with an insect parameter • But, calling Poison container = new AntPoison(); Container.spray(new Ant()); Container.spray(new Insect());

Single Dispatch • Produces Spraying ant with ant spray Generic spray • Which at least works on ants. • Doesn’t work properly on insects

Problem • We are only spraying with generic poison • In order to make use of a spray method in the AntPoison class we must repeat the spray() method with exactly the same signature in the AntPoison derived class • Furthermore, we need a beingSprayed() method of each poison type in both Insect and Ant

Double Dispatching