question regarding the java.lang.String design

[David Holmes - Sun Microsystems]

Hi Xiaobin,

As you've probably gleaned by now the count and offset fields are to 
allow sharing of the underlying char[] - which is a safe thing to do 
exactly because a string is immutable. I've often thought this 
particular optimization was under-utilized.

As others have said optimization of strings has been a recurring theme 
for many years now - there was even a paper on it at last year's ACM 
OOPSLA conference. IBM Research's Tokyo labs do a lot in this area - see 
for example "RT0750 A Quantitative Analysis of Space Waste from Java 
Strings and its Elimination at GC Time".

I've occasionally thought that with all the duplicate strings that 
readily occur in Java it might be an option to have a few large tables 
of "text" containing all the characters, and then to define a String as 
one or more pairs of indices into these tables. But that's as far as 
I've thought about it :)

Cheers,
David Holmes


Xiaobin Lu said the following on 01/31/09 04:42:
> Resend the email to hotspot-dev at openjdk.java.net.
> -Xiaobin
> 
> Xiaobin Lu wrote:
>> Hi folks,
>>
>> While I am looking at the java.lang.String implementation, I noticed 
>> that it has "offset" and "count" field in java.lang.String. For the 
>> offset field, I only found two places which set that field, but I 
>> believe they can be got rid of too. The two places are 
>> String(StringBuffer buffer) & String(StringBuilder builder).
>>
>> My question is that if String is immutable, why do we need to carry 
>> these two fields? String could be more compacted without these two 
>> fields. The equals to method can be more efficiently implemented as 
>> just calling java.util.Array.equals(v1, v2) which is intrinsified on 
>> x86 at least.
>>
>> Another crazy thought is that we can compact the character array to a 
>> byte array if we don't have any characters other than ASCII (which we 
>> might use a boolean flag to indicate that).
>>
>> I'd appreciate your insight on this.
>>
>> -Xiaobin
>>
>>
>>
>