Why are only final variables accessible in anonymous class?

a can only be final here. Why? How can I reassign a in onClick() method without keeping it as private member?

private void f(Button b, final int a){
    b.addClickHandler(new ClickHandler() {

        @Override
        public void onClick(ClickEvent event) {
            int b = a*5;

        }
    });
}

How can I return the 5 * a when it clicked? I mean,

private void f(Button b, final int a){
    b.addClickHandler(new ClickHandler() {

        @Override
        public void onClick(ClickEvent event) {
             int b = a*5;
             return b; // but return type is void 
        }
    });
}