Re: catching exceptions in subclass' constructor?

From:
Owen Jacobson <angrybaldguy@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Tue, 2 Sep 2008 01:03:29 -0700 (PDT)
Message-ID:
<270322d1-eaba-49d5-b8a0-37fbcc5efce6@l42g2000hsc.googlegroups.com>
On Sep 1, 6:07 am, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:

(in these examples my Exceptions are chosen arbitrarily
from those in java.lang)

Consider a class 'A'

class A {
     public A(int i) throws NoSuchFieldException {
     };

};

It (for reasons all its own) can throw an exception.

Now consider a sub-class 'B', that extends 'A'.

I would like to give it 2 constructors, one a simple
"clone" of the 'A' constructor, and a parameterless
constructor. I want this because in my real
app 'B' has many subclasses.

If a parameterless constructor that throws
no exceptions can be managed, I won't need to declare
a constructor at all in the (many) sub classes,
which will be helpful to me.

The first constructor is trivial.

class B extends A {
     public B(int i) throws NoSuchFieldException {
         super(i);
     };

}

Now, since I am going to (carefully) provide
a valid default argument in the B() constructor,
I do not want it to throw a checked exception,
so I map" it to a runtime exception.

So I tried:

    public B() {
         try {
             this(10);
         } catch(NoSuchFieldException e) {
             throw new IllegalStateException(e);
         }
     }

but the compiler says:

" call to this must be first statement in constructor"

I would welcome advice on how to achieve my goal (a parameterless
constructor for B that does not throw an exception).


Stop trying.

A subclass (B) may not change invariants of its superclass (A). An
exception from a constructor means that some invariant could not be
established, so the whole instance of 'A' is discarded and no further
attempt to construct the 'B' instance around it can possibly succeed.

Imagine if the inverse were true - you'd have to write each class on
the premise that some third party might subclass it and change any of
the assumptions it makes. It's certainly possible to code this way,
but I'd rather not assume all programmers (or even myself, three weeks
later) are perfect and well-intentioned.

-o

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