Creating Map using withDefault causing null when putting element

groovy

Solution

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>>`.

Problem

Original source