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Autoboxing and Nullpointer [message #524083]

Tue, 30 March 2010 11:44

Eclipse User

Hello,

could someone please explain me, why the following snippet produces a NullPointerException:

public class Main {

public static void main(String[] args) {

System.out.println(getDouble());

Double d1 = true ? getDouble() : new Double(0);
System.out.println(d1);

Double d2 = true ? getDouble() : 0;
System.out.println(d2);
}

public static Double getDouble() {

return null;
}
}

The output is:

null
null
Exception in thread "main" java.lang.NullPointerException
at Main.main(Main.java:9)

Apparently it has to do with autoboxing, but is this the right behaviour?
I'm using Eclipse Galileo for RCP/Plug-in Developers, Build id: 20090920-1017

Thank you in advance,
Rudi

Re: Autoboxing and Nullpointer [message #524183 is a reply to message #524083]

Tue, 30 March 2010 21:22

Eclipse User

Rudi wrote:
> Hello,
>
> could someone please explain me, why the following snippet produces a
> NullPointerException:
>
> public class Main {
>
> public static void main(String[] args) {
>
> System.out.println(getDouble());
>
> Double d1 = true ? getDouble() : new Double(0);
> System.out.println(d1);
>
> Double d2 = true ? getDouble() : 0;
> System.out.println(d2);
> }
>
> public static Double getDouble() {
>
> return null;
> }
> }
>
> The output is:
>
> null
> null
> Exception in thread "main" java.lang.NullPointerException
> at Main.main(Main.java:9)
>
>
> Apparently it has to do with autoboxing, but is this the right behaviour?
> I'm using Eclipse Galileo for RCP/Plug-in Developers, Build id:
> 20090920-1017
>
>
> Thank you in advance,
> Rudi

The ternary operator (?:) in Java is notorious for having problems with mixed or
ambiguous types. In this case the problem is that 0 is not a Double, it's an
int; the type of the expression "true ? getDouble() : 0" is ambiguous, it's not
clear whether 0 should be cast to a Double or the Double should be cast to an int.

You can read more about this issue in the Java Puzzlers book, info at
http://www.javapuzzlers.com/ .

Re: Autoboxing and Nullpointer [message #524192 is a reply to message #524183]

Wed, 31 March 2010 02:54

Eclipse User

Hi Walter,

thank you for the link to the book, i know Joshua Bloch from his excellent book "Effective Java".

Best regards,
Rudi

Re: Autoboxing and Nullpointer [message #524320 is a reply to message #524083]

Wed, 31 March 2010 05:26

Eclipse User

On 3/30/2010 9:44 AM, Rudi wrote:
> Hello,
>
> could someone please explain me, why the following snippet produces a
> NullPointerException:
>
> public class Main {
>
> public static void main(String[] args) {
>
> System.out.println(getDouble());
>
> Double d1 = true ? getDouble() : new Double(0);
> System.out.println(d1);
>
> Double d2 = true ? getDouble() : 0;
> System.out.println(d2);
> }
>
> public static Double getDouble() {
>
> return null;
> }
> }
>
> The output is:
>
> null
> null
> Exception in thread "main" java.lang.NullPointerException
> at Main.main(Main.java:9)
>
>
> Apparently it has to do with autoboxing, but is this the right behaviour?
> I'm using Eclipse Galileo for RCP/Plug-in Developers, Build id:
> 20090920-1017
>
>
> Thank you in advance,
> Rudi

While the ternary operator in Java has understandable problems for which
I lodge no particular complaint (despite being an old C guy who uses
this construct copiously), in this case, it's the null returned from
getDouble() that leads to the exception and not the ternary operation's
innate grumpiness, no?

Russ

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