public class A {
}

public class B {
    private List lista;
    private A a;
   
    public B() {
        lista = new ArrayList();
        lista.add(new A());
        lista.add(new A());
        lista.add(new A());
        lista.add(new A());
    }

    public void pegaA() {
        a = lista.getIndex(2);
    }

    public void alteraA() {
        ...
    }

}