Re: Define Type at runtime

Lew <>
Thu, 8 Sep 2011 14:35:54 -0700 (PDT)
BGB wrote:

Lew wrote:

BGB wrote:

Lew wrote:

John wrote:

If I have a method:
public void someMethod(Object o){
       Entity<T> entity;

How could I define at runtime the type T if I have an Object?

You can do an illegal cast with the help of '@SuppressWarnings("unchec=

ked")' and a 'catch ( ClassCastException exc )'.

You can under certain circumstances use a run-time type token of type =


You can write your method to be type-safe in the first place and avoid=

 the problem.

Without some context we can't know what you really want. Your questio=

n is far too non-specific.

and, probably, not to forget "instanceof".

That's pretty useless in this context.

eg, [sic] rather than, say:
Object obja;
try {
      SomeObject objb=(SomeObject)obja;
}catch(ClassCastException ex)

one could be like:
if(obja instanceof SomeObject)
      SomeObject objb=(SomeObject)obja;

which could be more useful if one wants to do the types N-way...

}else if(...)
}else ...

or such...

That's bad advice on several fronts. First of all, the OP was asking ab=

out restoring generics information, for which 'instanceof' is not useful, a=
nd second, multi-'if' constructs on type are a red flag of bad design. Don=
't recommend bad design.

AFAICT, the OP was asking about dynamic type checking, which is what
instanceof is for.

They were asking about the generic type, which most emphatically is *not* w=
hat 'instanceof' is for. You even cited the post: "How could I define at r=
untime the type T if I have an Object?" where "T" was the generic type para=
meter in the OP's example. 'instanceof' won't help there. In fact, it wil=
l not even compile.

JLS =A715.20.2
"It is a compile-time error if the ReferenceType mentioned after the instan=
ceof operator does not denote a reifiable type (=A74.7)."

the bigger issue though would be, why the OP was dealing with the case
in the first case (like, yes, does a common base-class exist besides
"Object", or is there a common interface, ...), but failing this, nested=


if/else and instanceof does seem like a valid solution (and is probably=



still better than, say, a series of try/catch blocks intermixed with
return statements).

Except for that failure-to-compile thing.

try {
}catch(...) { ... }
try {
}catch(...) { ... }
but, whatever works...

By which I assume you at least mean compiles.

'instanceof Entity<T>' will not compile.

if-then chains of type comparisons are a major antipattern, a point you glo=
ss over. It means that you aren't using polymorphism and type-safe code. =
It's a Bad Thing.

if (x instanceof Foo)
  // do one thing
else if (x instanceof Bar)
  // do another thing
else if (x instanceof Baz)
  // OK, this code is far too stupid. Refactor intelligently.

You're supposed to have

 Super foo = obtainASubtypeInstance();


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