I wanted to try the Document.setPreferJavaDates today and see how it worked.
I called Document.setPreferJavaDates( true ); to ensure that no (Notes)DateTime objects were created as I didn't need them/didn't want to deal with recycling.
First I tried:
Document.getItemValueDateTimeArray( "fieldname" );
This returned a Vector of DateTime.
Then I tried:
Document.getItemValue( "fieldname" );
This also returned a Vector of DateTime.
The way that finally worked as expected (returned a Vector of java.util.Date) was this:
Document.firstItem( "fieldname" ).getValues();
I would imagine this is the least used method among developers wanting to get a date value from a field. This code was executed on a server running Domino 8.5.3FP2