String.subSequence and CR#6924259: Remove offset and count fields from java.lang.String

Mike Duigou mike.duigou at
Fri Jun 22 22:15:40 UTC 2012

I've made a test implementation of subSequence() utilizing an inner class with offset and count fields to try to understand all the parts that would be impacted. My observations thus far:

- The specification of the subSequence() method is currently too specific. It says that the result is a subString(). This would no longer be true. Hopefully nobody assumed that this meant they could cast the result to String. I know, why would you if you can just call subString() instead? I've learned to assume that somebody somewhere does always does the most unexpected thing.
- The CharSequences returned by subSequence would follow only the general CharSequence rules for equals()/hashCode(). Any current usages of the result of subSequence for equals() or hashing, even though it's not advised, would break. We could add equals() and hashCode() implementations to the CharSequence returned but they would probably be expensive.
- In general I wonder if parsers will be satisfied with a CharSequence that only implements identity equals().
- I also worry about applications that currently do use subSequence currently and which will fail when the result is not a String instance as String.equals() will return false for all CharSequences that aren't Strings. ie. CharSequence token = line.subSequence(line, start, end); if (keyword.equals(token)) ... This would now fail.

At this point I wonder if this is a feature worth pursuing.


On Jun 3 2012, at 13:44 , Peter Levart wrote:

> On Thursday, May 31, 2012 03:22:35 AM mike.duigou at wrote:
>> Changeset: 2c773daa825d
>> Author:    mduigou
>> Date:      2012-05-17 10:06 -0700
>> URL:
>> 6924259: Remove offset and count fields from java.lang.String
>> Summary: Removes the use of shared character array buffers by String along
>> with the two fields needed to support the use of shared buffers.
> Wow, that's quite a change.
> So .substring() is not O(1) any more?
> Doesn't this have impact on the performance of parsers and such that rely on 
> the performance caracteristics of the .substring() ?
> Have you considered then implementing .subSequence() not in terms of just 
> delegating to .substring() but returning a special CharSequence view over the 
> chars of the sub-sequence?

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