Re: Interface with implied Constructor

Eric Sosman <esosman@comcast-dot-net.invalid>
Tue, 09 Jul 2013 21:08:56 -0400
On 7/9/2013 6:46 PM, Richard Maher wrote:

On 7/9/2013 7:08 PM, Robert Klemme wrote:

Why do you want to do that?

For me it's personal preference, but for some technical rationale: -

If you don't provide sufficient information to the constructor:
1) You get half-baked objects
2) Every real (non-init()) method now has to check "Has init() run? Am I
3) If using setters, how do you enforce that they are called in the
correct order?

Most importantly, Why the hell do you think Java (and all other OO
languages) support arguments to constructors and overloading in the
first place??? Do you think the authors have been wasting their time or
providing redundant technology all these years?

     Somebody's losing his cool ...

So let me ask you "Why do you want to use setters or an init() method?"

     Because it's more flexible, perhaps. Do you feel that
HashSet should have an (int,boolean,boolean,int,int,Throwable)
constructor, just because DataTruncation also has one and both
classes implement Iterable?

     Interfaces are about "contracts," about the "promises" an
implementing class makes. Constructors are about initialization
of particular class implementations. If interfaces were able to
legislate the existence of specified constructors, and if the
related "promise" was that an instance so initialized would be
complete and functional, then how could you ever implement a
class that needed just one little smidgen of additional state?

     For example: In a recent thread somebody was talking about
a ColoredPoint class that carried the notion of a location and
an associated color. Let us suppose that this ColoredPoint class
implemented a Point interface, and that the interface demanded
the existence of an (int x, int y) constructor. Well then, how
can a ColoredPoint be sure of getting a color? You could write
the required constructor and default the color to mauve, but would
that be satisfactory? I don't think so: The eventual user of the
class would have to use a constructor not mentioned by the interface,
or would have to use a setter also not mentioned by the interface.
What benefit would the constructor-mandated-by-interface confer?

     Granted, the line between "contract" and "promise" can be a
bit blurry sometimes. An interface can require the existence of
a method with a certain name and parameter list and can influence
the return type and visibility, but it cannot really say much
about what the function does: An Iterable must have a public
hasNext() method of no arguments returning a boolean, but as far
as Java is concerned `public boolean hasNext() { return false; }'
is entirely satisfactory. On the other side, conventions like
JavaBeans require the existence of no-argument constructors and of
methods whose names and signatures aren't specified in any
interface. I think it's best to think of these as awkwardnesses
in the language, shortcomings that should not be imitated except
when the need is pressing -- and definitely not as practice to be
formalized and sanctified.

     Finally: If you want abstract classes, ...

Eric Sosman

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