Determine type of a variable whose value is NULL

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brian.vanheesch@gmail.com - 14 Sep 2007 13:46 GMT

I have a variable defined (local, global, parameter, field; it does
matter) such as:

MyClass abc=null;
MyClass def=new MyClass();

The variables above will house an object of type MyClass or any
subclass. Now in code, I would like to determine the specific type of
the variable. So I can do:

System.out.println(def.getClass().getName());

but what can I do for variable 'abc' ? Using the same syntax for
variable abc, will naturally result in an NullPointerException.

Is there a typeof syntax similar to instanceof?

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Lew - 14 Sep 2007 14:32 GMT

> I have a variable defined (local, global, parameter, field; it does
> matter) such as:
[quoted text clipped - 12 lines]
>
> Is there a typeof syntax similar to instanceof?

No. Why does instanceof not help you?

<http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2>

abc can be tested with the instanceof operator, in which case null will return
fallse for (abc instanceof MyClass), but more usefully just test it for null
first, as in
if ( abc == null )
{
// handle null
}
else ...

The literal null has only one type.
<http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.1>

> There is also a special /null/ type, the type of the expression null, which has no name.

[emph. orig.]

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Lew

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Mike Schilling - 14 Sep 2007 15:04 GMT

>I have a variable defined (local, global, parameter, field; it does
> matter) such as:
[quoted text clipped - 9 lines]
>
> but what can I do for variable 'abc' ?

System.out.println( abc == null ? "<null>" : abc.getClass().getName());

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Roedy Green - 15 Sep 2007 20:23 GMT

>Is there a typeof syntax similar to instanceof?

null variables don't have a class.

You can do something like this

Skurtlewax w = null;

System.out.println( Skurltewax.class );

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Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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Determine type of a variable whose value is NULL

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