Is there a way to have callable objects in Groovy?

Question

If for example I have a class named A. Can I make an object be callable, just like Python does? For example :

def myObject = new A()
myObject()

and that would call some object method. Can it be done?

Answer 1

In Groovy only closures are callable by default. E.g. Classes are not callable out of the box. If necessary you can dynamically add a call method to a type's ExpandoMetaClass to make all instances of that type callable.

Hint: you can try out all code sample using the GroovyConsole

Closures are callable by default in Groovy:

// A closure
def doSomething = { println 'do something'}
doSomething()

// A closure with arguments
def sum = {x, y -> x + y}
sum(5,3)
sum.call(5,3)

// Currying
def sum5 = sum.curry(5)
sum5(3)

To make all instances of a specific type callable you can dynamically add a call method to its meta class:

MyObject.metaClass.call = { prinlnt 'I was called' }
def myObject = new MyObject()
myObject()

If you rather only make a specific instance callable you can dynamically add a call method to its meta class:

def myObject = new MyObject()
myObject.metaClass.call = { println 'Called up on' }
myObject()