Sine and cosine are high school math, you should know this stuff (do they even let people on the internet without knowing it?)

Maybe you should store the velocity as a vector instead of an angle. That way, it takes less calculating to move the object, and you can give them different speeds.

class Bullet(float x, float y, float dx, float dy)

where dx^2 + dy^2 = speed^2 (Pythagoras)