Null Object

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Moon2 - 20 Mar 2007 21:41 GMT

Can anyone please tell me why the following doesnt create a runtime error.

Object s = null;
System.out.println(s);

The output is null. Why doesnt it call the Object's tostring method and
throw a null pointer exception. Any comments will be appreciated.

Moon2

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Stefan Ram - 20 Mar 2007 21:49 GMT

"Moon2" <none@none.com> writes (with minor modifications):

>Why doesnt it call the object's tostring method and
>throws a null pointer exception?

This is the result of a a design decisions.

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Stefan Ram - 20 Mar 2007 21:58 GMT

>This is the result of a a design decisions.

I meant »This is the result of a design decision.«.

Usually, when null is not a valid value, one wants a receiver
of the value to mark failure as early as possible, e.g., by
throwing.

But it makes sense for some receivers to accept null as a
possible value. This depends on the context.

In the case of »println«, it is often more helpful to show the
null than to abort the action, e.g., when used for debugging.

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Mike Schilling - 20 Mar 2007 22:01 GMT

> Can anyone please tell me why the following doesnt create a runtime error.
>
[quoted text clipped - 3 lines]
> The output is null. Why doesnt it call the Object's tostring method and
> throw a null pointer exception. Any comments will be appreciated.

Because the engineers who defined the method deemed that to be less useful
behavior that what it actually does. Note that this isn't a general
language issue, it's specific to OutputStream.println(Object ). If you'd
asked about

Object s = null;
String msg = "The answer is " + s;

Now it's a language issue, though the answer remains pretty much the same.

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Oliver Wong - 20 Mar 2007 22:03 GMT

> Can anyone please tell me why the following doesnt create a runtime
> error.
[quoted text clipped - 4 lines]
> The output is null. Why doesnt it call the Object's tostring method and
> throw a null pointer exception. Any comments will be appreciated.

System.out.println() doesn't actually directly call the passed-in
object's toString() method. Instead, it calls String.valueOf(s). The
implementation of the String.valoeOf(Object) method is:

public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}

That is why you see the output "null" instead of an NPE.

- Oliver

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Moon2 - 20 Mar 2007 22:55 GMT

Thanks very much for clearing things up

Moon2

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Hendrik Maryns - 21 Mar 2007 10:56 GMT

Moon2 schreef:

> Thanks very much for clearing things up

By the way, note that your subject is misleading: this is not about a
Null Object (there is non in the standard Java API), but about the null
*reference*.

H.
- --
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
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Null Object

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