Dynamically add a property or method to an object in groovy - groovy

Question

Is it possible to add a property or a method to an object dynamically in Groovy? This is what I have tried so far:
class Greet {
def name
Greet(who) { name = who[0].toUpperCase() + [1..-1] }
def salute() { println "Hello $name!" }
}
g = new Greet('world') // create object
g.salute() // Output "Hello World!"
g.bye = { println "Goodbye, $name" }
g.bye()
But I get the following exception:
Hello World!
Caught: groovy.lang.MissingPropertyException: No such property: bye for class: Greet
Possible solutions: name
at test.run(greet.groovy:11)

Answer 1

If you just want to add the bye() method to the single instance g of the class Greet, you need to do:
g.metaClass.bye = { println "Goodbye, $name" }
g.bye()
Otherwise, to add bye() to all instance of Greet (from now on), call
Greet.metaClass.bye = { println "Goodbye, $name" }
But you'd need to do this before you create an instance of the Greet class
Here is a page on the per-instance metaClass
And here is the page on MetaClasses in general
Also, there's a bug in your constructor. You're missing who from infront of your [1..-1] and if the constructor is passed a String of less than 2 characters in length, it will throw an exception
A better version might be:
Greet( String who ) {
name = who.inject( '' ) { String s, String c ->
s += s ? c.toLowerCase() : c.toUpperCase()
}
}
As metioned in the comments,
Greet( String who ) {
name = who.capitalize()
}
is the proper way