Name: dgC58589 Date: 01/28/98


Java is unnecessarily strict in its requirement that a
superclass constructor must be called before any other
operations can occur in a subclass constructor. For
example, the following code causes a compiler error
(actually two compiler errors) because the if() statement
appears before the call to the superclass constructor:

   import java.awt.Dialog;
   import java.awt.Frame;

   public class MyDialog extends Dialog
   {
      private static final Frame dummyFrame = new Frame();

      // This constructor will not compile!
      public MyDialog(Frame f, String title, boolean isModal)
      {
         if(f == null)
            f = dummyFrame;

         // The call to the superclass constructor must occur
         // as the very first operation in the subclass constructor.
         super(f, title, isModal);
      }

      public static void main(String[] args)
      {
         MyDialog md = new MyDialog(null, "ABC", true);
      }
   }

In my opinion, it is extremely common to perform some form
of initialization or checking prior to invoking a superclass
constructor—the initialization may in fact determine which
superclass constructor is appropriate to invoke.

I would propose the following change to Java's requirement
that the superclass constructor call be the very first
operation in a subclass constructor:

      A subclass constructor must call a version of its
      superclass constructor before the subclass
      constructor attempts any of the following:

            1) Making a reference to a field (method or
            variable) defined in the superclass. Note
            that such a reference includes any
            mention of the this object.
            
            2) Returning control from the subclass
            constructor.

      If the subclass constructor does not explicitly
      call a superclass constructor, the superclass
      default constructor is implicitly called before
      any statements in the subclass constructor are
      performed.
(Review ID: 23960)
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