To avoid suprises for the user:
"I solved the construction heuristic problem. Someone it didn't dawn on me
right away that primitive types don't work properly as planning variables,
because they're always initialized, thus the construction heuristic quits
right away, thinking there's nothing to do."
http://drools.46999.n3.nabble.com/OptaPlanner-best-strategy-for-variable-
number-of-planning-entities-tp4028470p4028555.html

Solution proposal A: Construction heuristics should consider variables of primitive types uninitialized if they are their default value.

Solution proposal B: Planning variables of primitive types should be illegal.