Re: Interface with implied Constructor

Eric Sosman <esosman@comcast-dot-net.invalid>
Wed, 24 Jul 2013 17:42:11 -0400
On 7/24/2013 5:18 PM, Martin Gregorie wrote:

On Wed, 24 Jul 2013 13:27:57 -0700, Gene Wirchenko wrote:

On Tue, 23 Jul 2013 14:43:48 -0700, Jim Gibson <>

Classes can implement more than one interface. What should the compiler
demand in the case of multiple, mutually-exclusive constructor

      If they are really mutually exclusive, then that is an error.

I suspect he meant "mutually disjoint", i.e. have no arguments in common,
which is a distinct possibility. To me that sounds like a good argument
for excluding constructors from interface definitions.

     "Ayeh." It would be even more confusing, I think, if two
interfaces specified constructors with identical signatures but
different intent:

    interface Flexible {
        Flexible(double stiffness); // constructor spec.
        ... methods ...

    interface Edible {
        Edible(double caloriesPerGram); // constructor
        ... methods ...

    class LicoriceStick implements Flexible, Edible {
        // Constructor mandated by both interfaces:
        LicoriceStick(double whatAttributeDoesThisGovern) {

In any case, you can always lock down the whole interface by declaring
an abstract class that implements the methods in the interface(s) as
abstract methods and declares concrete constructor(s).

     That still doesn't mandate any particular form of constructor
for concrete subclasses. It does, however, require that each
subclass constructor have enough information available to use in
an invocation of one of the abstract class' constructors, so a
certain amount of influence is present.

Eric Sosman

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