JLS8 section 4.2.5 conversion of integer/float expressions and references to boolean values

[Alex Buckley]

On 11/19/2015 12:35 AM, Florian.Wiedenmann at grob.de wrote:
> Hello!
>
> I have a question regarding chapter 4.2.5 of the current
> specification JLS8.
>
> The last passage of page 51 and the first one on page 52 refer to the
> conversion of float and int expressions as well as object references
> to a boolean value. I have never heard or seen that before and I'm
> not able to reproduce that behavior with jdk1.8.0_25 (out of date - I
> know ;) ).

I can only assume you mean this text, but that you missed the last 
clause in each sentence:

---
An integer or floating-point expression x can be converted to a boolean 
value, following the C language convention that any nonzero value is 
true, by the expression x!=0.

An object reference obj can be converted to a boolean value, following 
the C language convention that any reference other than null is true, by 
the expression obj!=null.
---

Alex

> I have tried the following statements:
>
> // the float expression 0.55f - 0.33f  is not converted to a boolean
> value - i.e. the entire expression cannot be evaluated // compiler
> message: // error: incompatible types: float cannot be converted to
> boolean boolean b = 0.55f - 0.33f ? true : false;
>
> // trying to assign a float value to a boolean variable // compiler
> message: // error: incompatible types: float cannot be converted to
> boolean boolean b = 0.55f;
>
> // trying to assign a float expression to a boolean variable: //
> error: incompatible types: float cannot be converted to boolean
> boolean b = 0.55f - 0.33f;
>
>
> // trying to convert a new reference to boolean: // incompatible
> types: Object cannot be converted to boolean boolean b = new
> Object();
>
> // trying reference conversion in expression: // error: incompatible
> types: Object cannot be converted to boolean boolean b = new Object()
> ? true : false;
>
> Now I'm wondering if I misunderstood the passage (which is likely -
> I'm not exactly an expert) or whether it's implementation specific
> (maybe it works with OpenJDK?). In any case I think that behaviour is
> not very clear - any of the conversions can easily be written as a
> Boolean expression which tells the reader exactly what it does (e.g.
> new Object() != null ? true : false). I guess it has been added as a
> concession to C developers.
>
> I'd be really happy to hear from you!
>
> Greetings from Germany ;)
>
> Best regards, Florian Wiedenmann
>
>
>
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