map
  Creates a MapProperty implementation to hold a Map of the given key type K and value type V. The property has an empty map as its initial value. 
The implementation will return immutable Map values from its query methods.
Return
the property. Never returns null.
Since
5.1
Parameters
key
the type of key.
value
the type of value.
<K>
the type of key.
<V>
the type of value.