The description for the progress member of the Worker interface reads as follows:

ReadOnlyDoubleProperty progress Gets the ReadOnlyLongProperty representing the progress.


ReadOnlyDoubleProperty progressProperty()
Gets the ReadOnlyLongProperty representing the progress.

As the signature of the method and properties shows, the progress property is a DoubleProperty, not a LongProperty. The description needs to be fixed. This appears as well in the Task class, as the javadoc is probably inherited from Worker.