The fact that Java had to add a whole extra set of keywords to control this indicates that this is a site of complexity. Since it isn't needed for composition, it's a site of unnecessary complexity.
What you lose by using composition is that the composing object is no longer a subtype of the constituent object, so you can't use it as a "decoration" of the original object in a program that expects an instance of the original.
It can be, if the composing object re-implements the constituent object's interface. This way, code reuse and polymorphism are orthogonal features, which I think is better. If you want both, you can do both, but inheritance pushes you toward using both even when you only need one.
