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September 22, 2009

Groovy Goodness: MultiMethods or Multiple Dispatch

Multimethods or multiple dispatch is a mechanism where at runtime the correct method is choosen based on the runtime type of the arguments of the method. Dynamic languages can do this at runtime and because Groovy is a dynamic language we can use multimethods in Groovy. We define overloaded methods with differently typed arguments and let Groovy decided at runtime which method to invoke. This subject is best explained by an example:

// We start off by some very simple class definitions
// with a one-level hierarchy amongst them.
abstract class Person {
    String name
class Parent extends Person {}
class Child extends Person {}

// Now we define methods to return the name with some extra info.
def printName(Person person) { "printName(Person): $" }
def printName(Child child) { "printName(Child): $" }
def printName(p) { "printName(p): $" }

// Create new Parent and Child objects but use Person type reference.
Person parent1 = new Parent(name: 'parent1')
Person child1 = new Child(name: 'child1')

assert 'printName(Person): parent1' == printName(parent1)
assert 'printName(Child): child1' == printName(child1)  // This is not what Java would do!! 
assert 'printName(Person): child1' == printName(child1 as Person)  // Same as what Java would do with printName(child1)

// Create objects with type reference is equal to object.
Parent parent2 = new Parent(name: 'parent2')
Child child2 = new Child(name: 'child2')

assert 'printName(Person): parent2' == printName(parent2)
assert 'printName(Child): child2' == printName(child2) 

// Use class outside Person hierarchy.
class Dog { 
    String name 
assert 'printName(p): buck' == printName(new Dog(name: 'buck'))