Re: Interface with implied Constructor

Steven Simpson <ss@domain.invalid>
Sat, 27 Jul 2013 04:06:19 +0100
On 23/07/13 22:32, Gene Wirchenko wrote:

I would like to have
Java enforce constructor signatures.

In following this thread, I'm finding this desire for interface
constructors totally bizarre. I wouldn't be surprised if I once
(briefly) wanted them, but now I immediately think 'factory interface',
concoct something, and find it totally satisfactory. If you want to be
able to create a Foo from an argument list, define an interface type to
express it:

   interface FooFactory {
     Foo createFoo(int x, int y);

It totally decouples construction from the functionality of a Foo. The
FooFactory implementation just acts as an adapter between the available
arguments and the configuration required by a particular Foo
implementation's constructors. Not all of the configuration needs to
come from the arguments; fill in missing parts with constants or get
them from the environment. Not all arguments need to be used. The
factory could choose different constructors depending on argument
values, or return cached/pooled objects if that's compatible with the
factory type's informal contract, or pass in some arguments via the
constructor and others via reconfiguration methods. Multiple FooFactory
implementations could use the same Foo implementation. Multiple factory
types could be used when there are multiple contexts (with varying
available arguments) that all, nevertheless, demand the creation of a Foo.

Coding to use a factory in the dynamic case (where a factory class's
name is given as a string) is simple:

   Foo foo = Class.forName(clazzName)
                  .createFoo(x, y);

This is type-safe and easy to refactor the argument list.

If you had a way to impose a constructor Foo(int x, int y) on all Foo
implementations, what would you write instead?

   Foo foo = Class.forName(clazzName)
                  .getConstructor(new Class<?>[] { Integer.CLASS, Integer.CLASS })
                  .newInstance(new Object[] { x, y });

I think this is identical to what you'd write according to the OP's
preferred option, where you assume that the named class has a compatible
constructor. It still lacks the type safety and refactorability of the
factory code, and all you gain is the assurance that getConstructor
won't fail. But is that very valuable, when either approach could fail
at run time simply by naming a class that doesn't implement
Foo/FooFactory, or by naming a non-existent class?

ss at comp dot lancs dot ac dot uk

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