Creating Map using withDefault causing null when putting element - groovy

Question

I'm trying to use the Groovy way of creating a TreeMap<String, List<Data>> with default values so I easily add data to a new list if the key isn't already present.
TreeMap<String, List<Data>> myData = (TreeMap<String, List<Data>>) [:].withDefault { [] }
As you can see, I have the requirement to use a TreeMap and withDefault only returns a Map instance, so I need to cast.
When I attempt to add a new list to the map,
myData[newKey].add(newData)
myData[newKey] is null. However, if I change my Map initilization to remove the TreeMap cast (and change the type to just Map instead of TreeMap), myData[newKey].add(newData) works as expected.
What's the reasoning for this? Can I not use withDefault if I cast the map?

Answer 1

The problem isn't just about the cast. It also has to do with the declared type. The problem can be simplified to something like this:
def map1 = [:].withDefault { 0 }
TreeMap map2 = map1
When that is executed map1 is an instance of groovy.lang.MapWithDefault and map2 is an instance of java.util.TreeMap. They are 2 separate objects on the heap, not just 2 references pointing to the same object. map2 will not have any default behavior associated with it. It is as if you had done this:
def map1 = [:].withDefault { 0 }
TreeMap map2 = new TreeMap(map1)
That is what is happening with your code. The cast and the generics just makes it less clear with your code.
This:
TreeMap<String, List<Data>> myData = (TreeMap<String, List<Data>>) [:].withDefault { [] }
Can be broken down to this:
def tmpMap = [:].withDefault { [] }
TreeMap<String, List<Data>> myData = (TreeMap<String, List<Data>>)tmpMap
I hope that helps.
EDIT:
Another way to see the same thing happening is to do something like this:
Set names = new HashSet()
ArrayList namesList = names
When the second line executes a new ArrayList is created as if you had done ArrayList namesList = new ArrayList(names). That looks different than what you have in your code, but the same sort of thing is happening. You have a reference with a static type associated with it and are pointing that reference at an object of a different type and Groovy is creating an instance of your declared type. In this simple example above, that declared type is ArrayList. In your example that declared type is TreeMap<String, List<Data>>.