[jdjlist] Re: Exception thrown from the constructor of a cla ss

[Greg Nudelman]

Title: RE: [jdjlist] Re: Exception thrown from the constructor of a class

Oh! one more thing: if you have many such constructors to validate for some very different things:

 

SomObjectFactory.getInstaceOfIncomingObject(String OopIsCool);

SomObjectFactory.getInstaceOfOutgoingObject(String butItsHardToReadAndTypeSomeLongVariableNames);

//some other constructor methods

 

This will make it _absolutely_ clear what you're trying to do.

 

OK, that's $0.05 more. I'll shut up for now and back to work.

 

Greg

 

-----Original Message-----
From: Greg Nudelman
Sent: Friday, October 04, 2002 4:40 PM
To: JDJList
Subject: [jdjlist] Re: Exception thrown from the constructor of a cla ss

How about a static object factory instead of a constructor?

Static factory can validate all the incoming data and create the object if and only if the data is valid.  This approach is especially useful (though by no means limited to) if you combine it with a private constructor.  It is invaluable if want to construct immutable objects: objects wrapping some parsed data (like DOM), enum types, value objects in EJB, etc.

That's my $0.25

Greg N.

-----Original Message-----
From: Blair Wyman [mailto:[EMAIL PROTECTED]]
Sent: Thursday, October 03, 2002 1:50 PM
To: JDJList
Subject: [jdjlist] Re: Exception thrown from the constructor of a class

According the "Practical Java" (a great book by Peter Haggar), the
developer is on logically solid ground.  In fact, "Praxis 26" is titled
"Throw exceptions from constructors."

A quick paraphrase:

Constructors cannot return error codes.  Presuming you want to ensure that
only "valid" objects can be constructed, consider the alternatives to
throwing exceptions from the constructor:

One alternative is what Haggar calls "two-stage construction."  Pieces of
the construction process that can cause errors are moved into a separate
routine.  The user constructs an instance, then calls the "finishing"
routine and checks the return code or exception from that secondary
routine.  Yuk.

Another alternative is having an "i_am_validly_constructed" flag in every
object.  Either the user checks the flag before every (or at least the
first) method call, or each instance method internally checks the flag as
the method is entered (Yuk times Yuk).

Neither of these is as simple and robust as simply throwing exceptions
directly from the constructor.

Sure, in simple cases the argument is often made that "the poor class user
has to wrap all his constructor calls with try/catch," but that isn't
necessarily so.  If the constructor throws exceptions that are subclasses
of java.lang.RuntimeException, no try/catch wrapping is required.  That's
why "IllegalArgumentException" or "UnsupportedOperationException" (or even
"NullPointerException") make nice candidates to be thrown from
constructors.

Just my $0.02 added to Haggar's $64K.

On Thursday, 10/03/2002 at 08:14 MST, "Repine, Burt" <[EMAIL PROTECTED]>
wrote:
> A developer on our team just introduced a new set of classes the project
> that throw an exception from the constructor.
> The classes follow the Value Object pattern so they're just data
containers.
> The exception is thrown if a null value is passed in.
>
> This seems like a really bad idea to me with no inherent benefit but the
> developer is holding fast to his implementation.
> Are there any philosophies out there on this subject?

-blair

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