ExpandoMetaClass - Adding & Overriding instance methods

Once you have an ExpandoMetaClass to add new methods to it is trivial:

class Book {
   String title
}

Book.metaClass.titleInUpperCase << {-> title.toUpperCase() }

def b = new Book(title:"The Stand")

assert "THE STAND" == b.titleInUpperCase()

Note that in this case the left shift << operator is used to "append" the new method. If the method already exists an exception will be thrown. If you want to replace an instance method you can use the = operator:

Book.metaClass.toString = {-> title.toUpperCase() }