If you tried it and it didn't work then I don't know. If you didn't try it then you should.

class GameMechanics <------------------------------------ Not main class. GameMechanics class
{
    const int SCREEN_WIDTH = 640;
    const int SCREEN_HEIGHT = 480;
 
    GameItem target;
    public void Bounce() <---------------------------Method inside GameMechanics class
    {

That says otherwise.