As usual, an excellent idea Jason. I'll probably run an internal test/benchmark with both this and the CharSequence inner class implementation to see what breaks and where there are performance differences between the two. I was planning to also test a version of the CharSequence implementation which implemented equals() and hashCode() compatible with String and see if this produced fewer (or different) test failures. Good idea. Mike On Jun 24 2012, at 12:06 , Jason Mehrens wrote:
Mike,
Why not implement subSequence as 'java.nio.CharBuffer.wrap(data, beginIndex, endIndex).asReadOnlyBuffer()' ? Easy to implement and test. The nice thing is that parsers would know what a 'CharBuffer' vs. a sub sequence String internal class.
Jason
Subject: String.subSequence and CR#6924259: Remove offset and count fields from java.lang.String From: mike.duigou@oracle.com Date: Fri, 22 Jun 2012 15:15:40 -0700 To: peter.levart@gmail.com CC: core-libs-dev@openjdk.java.net
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.
Mike
On Jun 3 2012, at 13:44 , Peter Levart wrote:
On Thursday, May 31, 2012 03:22:35 AM mike.duigou@oracle.com wrote:
Changeset: 2c773daa825d Author: mduigou Date: 2012-05-17 10:06 -0700 URL: http://hg.openjdk.java.net/jdk8/tl/jdk/rev/2c773daa825d
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?