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Date: Fri, 08 Apr 2011 00:14:48 -0700
From: Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com>
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Subject: Re: Why only public methods on interfaces?
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On 4/7/11 10:37 PM, Patricia Shanahan wrote:
> [...]
>> As Stefan pointed out, interfaces themselves don't have to be public.
>> This is legal:
>>
>> package com.example;
>>
>> interface SomeInternalAbstraction {
>> public void flog(Horse horse);
>> }
>>
>> -o
>>
>
> Yes, but then the implementing methods do have to be public, which is
> undesirable if they should not be used outside the package.

The implementing type doesn't have to be public either.

Perhaps a future version of Java will include a feature C# offers: 
explicit implementation of an interface.  In C#, one can implement an 
interface implicitly as in Java, in which case if the implementing type 
is visible, then yes…one has access to the public members of that type 
that implement the interface.

But one can also implement interfaces explicitly, necessarily implying 
that a member is private, used only to implement an interface:

   interface IA { void A(); }

   class A : IA { void IA.A(); }

In that case, one can only call IA.A through an interface reference, and 
not a reference typed as A:

   A a = new A();
   IA ia = a;

   ia.A(); // legal
   a.A(); // not legal

In the meantime, in Java if you don't want the implemented methods 
called, just keep the implementing type non-public as well.

Pete