Failed to add element to new ArrayList in Groovy - groovy

Question

I am trying to simulate a multimap, each value of langVarMap is a list. When I add a new String to the list, I get the following error:
No signature of method: java.lang.Boolean.add() is applicable for argument types: (java.lang.String) values: [mm]
Here is the code snippet:
def langs = engine.languages as Set
def langVarMap = [:]
engine.models.each { model ->
def lang = (model.#language.text()) // String
def variant = (model.#variant.text()) // String
langs.add(lang)
if (langVarMap.get(lang)) {
def a = langVarMap.get(lang) //ArrayList
langVarMap.put(lang, a.add(variant))
}
else {
langVarMap.put(lang, [variant])
}
Thanks in advance.

Answer 1

Problem is with this line:
langVarMap.put(lang, a.add(variant))
ArrayList.add(E e) returns boolean not the list. Adding result of add action adds a boolean value of TRUE to the map after which cannot call add method on it. Need to rewrite as following:
if (langVarMap.get(lang)) {
def a = langVarMap.get(lang) //ArrayList
// a is already in langVarMap so don't need to put into ma again
a.add(variant)
} else {
langVarMap.put(lang, [variant])
}
And can further refine with this to remove the redundant lookup.
def a = langVarMap.get(lang) //ArrayList
if (a) {
a.add(variant)
} else {
langVarMap.put(lang, [variant])
}