JLS string concatenation clarification

[Martin Buchholz]

So, the context is that some of us have noticed that compiling
x + y
where x and y are both Strings, to
(x == null ? "null" : x).concat(y)
is much more efficient than creating a StringBuilder, and the vast majority
of our users would thank us for implementing that optimization in javac.
 The fact that Strings even have an object identity is essentially a bug.



On Tue, Jan 21, 2014 at 4:45 PM, Alex Buckley <alex.buckley at oracle.com>wrote:

> No, we can't. There are three levels to consider:
>
> - "Real" constant expressions like 3+"". They can avoid creating a new
> String object per https://bugs.openjdk.java.net/browse/JDK-4836025.
>
> - "Semi" constant expressions like s+"". Concatenating the empty string
> literal "" with a String expression is mentioned in
> https://bugs.openjdk.java.net/browse/JDK-4036535. The expectation (per
> JLS1) that a new String object is always created in this case is misplaced
> (per the JDK's actual behavior). But, the JLS never clarified this.
>
> - "Not" constant expressions: s+empty. Not detectable in general. Always
> creates a new String object.
>
> Alex
>
>
> On 1/21/2014 10:41 AM, Martin Buchholz wrote:
>
>> Can we agree that when
>> String empty = "";
>> then
>> s + empty
>> should be permitted to be the same object as s.
>>
>>
>> On Tue, Jan 21, 2014 at 9:59 AM, Louis Wasserman <lowasser at google.com
>> <mailto:lowasser at google.com>> wrote:
>>
>>     JLS 15.18.1 states:
>>
>>         The |String| object is newly created (§12.5
>>         <http://docs.oracle.com/javase/specs/jls/se7/html/jls-
>> 12.html#jls-12.5>)
>>
>>         unless the expression is a compile-time constant expression
>>         (§15.28
>>         <http://docs.oracle.com/javase/specs/jls/se7/html/jls-
>> 15.html#jls-15.28>).
>>
>>
>>     JLS 12.5 states:
>>
>>         Execution of a string concatenation operator (§15.18.1
>>         <http://docs.oracle.com/javase/specs/jls/se7/html/jls-
>> 15.html#jls-15.18.1>)
>>
>>         that is not part of a constant expression (§15.28
>>         <http://docs.oracle.com/javase/specs/jls/se7/html/jls-
>> 15.html#jls-15.28>)
>>
>>         sometimes creates a new |String| object to represent the result.
>>
>>
>>     It's not clear to me, from reading this, whether string
>>     concatenation with + /always/ creates a new String object or
>>     /sometimes /creates a new String object.  15.18.1 seems to imply
>>
>>     always, but 12.5 says "sometimes."
>>
>>     My understanding is that String.concat used to be used for string
>>     concatenation, and String.concat currently returns its receiver if
>>     its argument is the empty string.  This seems to be evidence for the
>>     "sometimes" interpretation.
>>
>>     --
>>     Louis Wasserman
>>
>>
>>
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