When you call any constructor, you need the "new" construction:

 S = new Stack();

This will not compile in your particular case because you have
declared your Stack class to be abstract.  Abstract classes cannot be
instantiated, and the compiler will tell you so.

One way to initialise the stack member variable in Function is to
initialise it from a constructor parameter:

 public Function(Stack ss) {
   S = ss;
 }

This will compile.

Inside your program you could derive a non-abstract class from your
abstract Stack and you can pass an instance of that derived class into
your Function constructor:

 class ConcreteStack extends Stack { ... }

 ConcreteStack myCStack = new ConcreteStack();

 Function myFunction = new Function(myCStack);

rossum

Please post real code.  This is too far from real code. Please have a
look at http://mindprod.com/jgloss/codingconventions.html 

What I think your question might be is this:

I have a circular reference:

class F {
Stack s;
...
}

class Stack{
F f ;
...
}

If I compile F first javac can't find Stack.  If I compile Stack
first, it can't find F.  I'm hosed.  What do I do?

The answer is at http://mindprod.com/jgloss/javacexe.html#CIRCULAR