JLS string concatenation clarification

Sergey Bylokhov Sergey.Bylokhov at oracle.com
Fri Feb 7 13:28:12 PST 2014

Hi, Vicente.
For a long time I ask a question related to the implementation of 
concatenations of the string in javac:
1 Why "" + expression is compiled to
         new StringBuilder()).append("").append(expression).toString()
         instead of String.valueOf(expression)? see JDK-4947460
2 Why expression1 + expression2 is compiled to
         instead of new 
StringBuilder(expression1)).append(expression2).toString()?  see JDK-4059189
3 Why JDK-6709423 is closed as not a defect, it could at least save a 
few bytes in the class files. Generated code currently is not perfect:
         s += a;
         s += b;
         is compiled to:
         s = new StringBuilder()).append(s).append(a).toString();
         s = new StringBuilder()).append(s).append(b).toString();
         instead of new StringBuilder(s).append(a).append(b).toString();

I suppose, these optimizations will not affect a JIT (will not make 
result worse) and will save the size of class file.

On 29.01.2014 1:00, Vicente-Arturo Romero-Zaldivar wrote:
> Hi Alex,
> On 27/01/14 19:29, Alex Buckley wrote:
>> Hi Vicente,
>> Generated code. Also, I realize that s+"" may generate different code 
>> than ""+s - please indicate if that's the case.
> Yes the generated code is slightly different. But at the end javac 
> only indicates what should be done. A string concatenation implies the 
> creation of a StringBuilder with calls to it's append() method. Later 
> the VM should these instructions only as indications.
> Vicente
>> Alex
>> On 1/27/2014 9:33 AM, Vicente-Arturo Romero-Zaldivar wrote:
>>> Hi Alex,
>>> Are you interested in the generated code or in the compiler's internal
>>> representation?
>>> Thanks,
>>> Vicente
>>> On 22/01/14 22:14, Alex Buckley wrote:
>>>> To compiler-dev and others,
>>>> Since the original question asked when the JLS allows compilers to
>>>> deviate from "always" creating a new String object, I would like to
>>>> know more about the following case:
>>>> On 1/21/2014 4:45 PM, Alex Buckley wrote:
>>>>> - "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.
>>>> If compiler authors can confirm (on-list or privately) that indeed
>>>> "concatenation with empty string does not create the new instance of
>>>> String", then I will clarify the JLS.
>>>> Alex

Best regards, Sergey.

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