Creates a new javafx.beans.binding.LongBinding that contains the mapping of a specific key in an javafx.collections.ObservableMap. The LongBinding will hold 0L if the key cannot be found in the ObservableMap.

Parameters:

op		the ObservableMap
key		the key in the Map
<K>		type of the key elements of the Map

Returns:  the new LongBinding

Exceptions:

NullPointerException

if the ObservableMap is null

Since:  JavaFX 2.1