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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.java.programmer
Subject: Re: it's Closeable, but I don't want to close() it yet.
Date: Fri, 1 Mar 2019 09:31:46 -0000 (UTC)
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bursejan@gmail.com <bursejan@gmail.com> wrote:
> Could the OP please give the full test case,
> including its outcome?

Note ahead: I don't really have a problem with the code. I just
wanted to improve my understanding of details, and I wanted to
know that for (at least almost) every warning, there is a way
to do it properly without warning. (An exception to this are
generics-warnings on dynamically "born" instances.)

The original code is "long and boring", but this "sligthly shorter
and still boring" code comes pretty close to my real situation:
(I've added a theoretical solution behind a spoiler-marker,
but in my real code I just added the @Suppress... anno)

interface Entity {
      public void a();
}
class EntityImpl implements Entity, java.io.Closeable {
      public void a() { /* ... */ }
      public void b() { /* ... */ } // semantically doesn't fit in Entity.
      public void close() { /* ... */ }
}
class Context {
   private final Entity m_ent;
   public Context(Entity ent) { m_ent = ent; }
   public Entity getEnt() { return m_ent; }
}
public class Foo {
   public static void main(String[] args) {
      EntityImpl ei = new EntityImpl();
      Context ctx = new Context( ei );
      helper1(ctx);
      helper2(ctx);
      ei.close();
   }
   private static void helper1(Context ctx) {
      ctx.getEnt().a();
   }
   private static void helper2(Context ctx) {
       Entity ent = ctx.getEnt();
       if (ent instanceof EntityImpl) {
          // the following line gets tagged by eclipse:
          //  "Resource leak: 'entImp' is never closed"
          EntityImpl entImp = (EntityImpl)ent;
          entImp.b();
       }
    }
}



The proper(tm) solution here is to define a second interface EntityB
that declares method b(). Add EntityB to EntityImpl's "implements"
list and in helper2, use EntityB instead of EntityImpl.