It seems that your A class extends MIDlet. If this is the case you should not try to make a new instance of it.
You should add an A attribute to your B class and receive the instance as a constructor parameter or have a setter method.
With this you can call the getFormA() method from the attribute.
Update
public class A extends MIDlet {
B b;
public A() {
b = new B(this);
}
}
class B {
A a;
B(A a) {
this.a = a;
}
public void commandAction(Command c, Displayable d) {
switchDisplayable(null, a.getFormA());
}
}