Re: can this be done with generics?

From:
Daniel Pitts <newsgroup.nospam@virtualinfinity.net>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 07 Dec 2013 09:04:20 -0800
Message-ID:
<pyIou.381562$5W.329013@fx11.iad>
On 11/24/13 4:55 PM, Andreas Leitgeb wrote:

I'd like to use the "call methods in a chain" pattern
across a class-subhierarchy, with most of the methods
defined in the base-class.

Suppose, I had two classes:

  class Foo {
     public Foo foo1() {
        /* do something very interesting ... */
        return this;
     }
     // assume there were like 100 such foo#() methods here
     // (in practice they of course have more diverse names)
  }
  class Bar extends Foo {
     public Bar bar() {
        /* do something very interesting */
        return this;
     }
  }

And then, somewhere else:

   // the single-class chain works just fine:
   new Foo().foo1().foo42().foo84();

   // but if I start with a Bar, and eventually after some foo#() calls
   // want to call method bar() which is not in Foo, then I'm out of luck:
   new Bar().foo1().foo42().foo84().bar(); // doesn't work, of course!

Is there a way to use *generics* for the methods of class Foo such that
each foo#() returns the *static* type on which the compiler saw it
applied?

Alternatives, that do NOT satisfy me:
   a) use an explicit cast to (Bar)
       ((Bar)(new Bar().foo1().foo42().foo84())).bar();
      Would turn into a nightmare, when more of Bar's
      methods got mixed into the chain.
   b) override each of the 100 foo#()s in Bar with a Bar return-type.
      Would turn into a nightmare, when more sub-classes like Bar
      appeared, or new methods added to Foo.
      (Note, that "bad performance" due to extra calls is NOT my concern here)
   c) add "Foo bar() { return this; }" in Foo.
      Would turn into a nightmare, when more sub-classes like Bar
      appeared (and Foo would have to reflect all subclasses' methods),
      or some chain accidentally called a subclass method on a chain
      started from some other class's instance, and compiler wouldn't
      detect the mistake.

I'm pretty sure, that this would be principially possible (as in: the
compiler has all the information that would be needed), but I can't seem
to find an approach for how to capture the static type of the expression
on which the (non-static) method is called.

Thanks in advance!


Yes, this *can* be done with generics, but its relatively ugly and hard
to maintain. I've been doing a lot of work in PHP lately, and they have
a return type hint "@return $this", which makes my IDE of choice
recognize the intent. Java, though superior in many ways, is lacking
that kind of ability.

To answer your question though:

abstract class Foo<T extends Foo<T>> {

     public T foo1() { return getThis(); }

     protected abstract T getThis();

}

class Bar extends Foo<Bar> {

    public Bar bar1() { /* something interesting */ return this; }
    protected T getThis() { return this; }
}

It gets uglier if you need multiple levels of inheritance, or if you
have two classes which refer to each other.

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