In my game, entities are rectangles, but are able to rotate, I need a way to tell collision between them taking into account the rotation. I have a double x and y and a double rotation

What you're looking for is OBB (Oriented Bounding Box) Collisions. For collisions, I personally use a custom CollisionBox class that has 4 Vector2D's as the vertices. When I rotate the box around it's center, I rotate each point around that center point. If I were you, I would look into separated axis theorem.