Groovy exposes an ExpandoMetaClass that allows you to dynamically add instance and class methods/properties to a POJO. I would like to use it to add an instance method to one of my Java classes:

public class Fizz {
    // ...etc.
}

Fizz fizz = new Fizz();
fizz.metaClass.doStuff = { String blah -> fizz.buzz(blah) }

This would be the equivalent to refactoring the Fizz class to have:

public class Fizz {
    // ctors, getters/setters, etc...

    public void doStuff(String blah) {
        buzz(blah);
    }
}

My question:

Does this add doStuff(String blah) to only this particular instance of Fizz? Or do all instances of Fizz now have a doStuff(String blah) instance method?

If the former, how do I get all instances of Fizz to have the doStuff instance method? I know that if I made the Groovy:

fizz.metaClass.doStuff << { String blah -> fizz.buzz(blah) }

Then that would add a static class method to Fizz, such as Fizz.doStuff(String blah), but that's not what I want. I just want all instances of Fizz to now have an instance method called doStuff. Ideas?