RTTI vs Compiler Type Identification (Casting?)

Question

Given:
Class Super and two subclasses of Super, SubOne and SubTwo.
Super has one method, doBeDooBeDoo()
SubOne has one method, doThis()
SubTwo has one method, doThat()
Super a = null;
if(foo) {
    a = new SubOne();
    a.doThis();
} else if(bar) {
    a = new SubTwo();
    a.doThat();
a.doBeeDoBeeDo();When I attempt to compile code like this, I receive a compiler error message indicating that class Super does not have a doThis method or a doThat method.
Conversely, when I run the following code...
Super a = null;
a = new SubOne();
System.out.println(a.getClass().getName());
a = new SubTwo();
System.out.println(a.getClass().getName());... I obtain the desired output of 'SubOne.class' and 'SubTwo.class'.
My confusion stems from the fact that when a = new SubOne() fires, object 'a' is an instance of SubOne, not an instance of Super. It appears that the compiler isn't picking up on this fact (understandable, since it only happens at runtime).
My questions:
1) Am I understanding the problem correctly or am I off-track?
1a) Am I misunderstanding the rules of casting in thinking that I can declare an object as being of one type and then instantiate it as a subtype?
2) Is there a way to force the compiler to recognize the casting that takes place when Super s = new SubOne()?

Answer 1

Thank you both for your replies - that helps to clear
up some of my confusion. I settled with...
Super a = null;
if(foo) {
a = new SubOne();
SubOne sub = (SubOne)a;
sub.doThis();
} else if(bar) {
a = new SubTwo();
SubTwo sub = (SubTwo)a;
sub.doThat();
a.doBeeDoBeeDo();...which seems equivalent to guifrei's response, but
somewhat more concise when the subclasses have a
number of unique methods.what is the point of having variable a in type of superclass, if you are not using it?
notiece that you are defining instances of your own subclass...
instead of a = new SubTwo();SubTwo sub = (SubTwo);sub.doThat(); you would most likely like to use the one with casting...
there is no point to assign value to a if you will end up using sub instead?!
ok, there is that a.doobidoobido fing outside if's... that maybe...
well.. actually i ment to write also about overloading your doBiDooBiDoo() method.
if you have in SubOne and SubTwo methods that do different thing, but have the same name as their superclass, then you can call a.doBiDooBiDoo() outside if and else-if and the result will depend on values of foo and bar.
and notiece that if you have line:
super a = null;
before these if statements, then in case foo and bar are false, you'll get nullpointerexception because a had no value...
so consider valuateing a with not-null value (super a = super();)
i hope i got some ideas from that, allthough i lost my point somewhere right before i started typing this...