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The dangers of auto-unboxing

JDK 1.5 provides a collection of very nice language additions and I've become an early adopter of many of them. I recently began having my doubts about auto-unboxing of reference variables though.

Consider the following class:

class AutoUnbox<T>
    private T   mResult;
    T getResult()
        /* do some stuff and return a result */
        return mResult;

and then some code that uses it:

AutoUnbox<Boolean> a = new AutoUnbox<Boolean>();
if (true == a.getResult())

This looks quite harmless, doesn't it? Well you have a totally invisible NullPointerException that is lurking in this code. If the getResult() method returns null, the conversion to a primitive will throw a NullPointerException. The biggest issue with this is that it happens behind the scenes. I'm used to thinking about references that might be null when I call a method upon them. As soon as I type the dot operator, my mind thinks about the context and considers if I shouldn't perform a nullcheck first. With auto-unboxing, there is nothing that indicates that you should be careful. It could literally happen anywhere.

Imho JDK 1.5 is wrong about this and instead of throwing the NPE it should return an unintialized primitive, just as null is the uninitialized value for a reference.

I mean, look at this code:

public class Uninit
    int test;
    boolean bool;

    public static void main(String[] args)
        new Uninit().method();

    public void method()

It returns 0 and false which means that there are default values defined for unitialized primitives. To me, when thinking about unboxing a null reference to a primitive I always expected to get this 'empty' value, not a NPE.

I think I'm going to submit a bugreport to Sun about this, what do you think?

posted by Geert Bevin in Java on Mar 11, 2004 1:16 PM : 8 comments [permalink]


Re: The dangers of auto-unboxing

I think you are right. Please do submit a bug report.

Re: The dangers of auto-unboxing

As I understand it, this was something that the people working on JSR-201 spent quite a bit of time discussing and working on.

See, for example, Joshua Bloch's preview of 1.5 features from almost a year ago:

"One thing worth noting: this program assumes that when you auto-unbox null, you get zero. It's still an open issue whether this will be the case. The alternative is to throw NullPointerException. Both alternatives have their advantages. Unboxing null to zero beautifies applications like the one above, but it can also sweep real errors under the rug. If anyone has any strong opinions, or better yet, convincing arguments on this issue, please pass them along to the JSR-201 expert group."
At this point, I don't think that you'll get them to consider NPE to be a "bug".

Re: The dangers of auto-unboxing

Uwyn, you should definitely send an email about this to the appropriate email address, which is . They do read comments and take them seriously. I get the impression the JSR201 folks couldn't find strong reasons for or against the unboxing null question, so your example might tip the balance.

Re: The dangers of auto-unboxing

Yeah Josh, I got a better suggestion it's just that you are not interested in listening to it! Autoboxing is a bandage over the real problem. The real problem is obvious. primitives need to be first class objects and not primitive. The design anti-patterns to prove this are littered all throughout the language and the JDK libraries. For example, I point to java.lang.Math. This abstract final class acts as a shelter to behavior that would otherwise be homeless. If primitives were represented as first class objects, this class would not be needed.

Re: The dangers of auto-unboxing

And what about throwing something like UnintializedPrimitiveException which would be a SUBCLASS of NullPointerException? It's a compromise, but it would give you more information about the cause of error.

Re: The dangers of auto-unboxing

same behavior in c#

auto-boxing / unboxing turns out to be obnoxious. For example, consider what your code must look like if you're extracting from a Map.... ugly

Re: The dangers of auto-unboxing

Autoboxing for a null value should always throw an exception? Why? null has no meaning for primitives, be they boolean, int, or anything else. By using autoboxing you are saying to the compiler "please convert this primitive from a primitive to an object and back". Since there is no mapping from null back to a primitive it always indicates you have done something wrong. Autoboxing is there simply to save you typing, not to change the rules of type casting.

Re: The dangers of auto-unboxing
You should not return an uninitialized value because in many case there is not any value like that. Think about int, boolean, ...
If the domain you're working in does no have a usable value meaning "this is not a value", you're screwed.
null should be a value for primitive types too or primitive types should be thrown away or something else...
In fact throwing a NPE is not a bad idea. It is juste not clear when one checks the code visually.

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