Re: StringBuffer/StringBuilder efficiency

From:
Tom Anderson <twic@urchin.earth.li>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 9 Oct 2009 18:24:23 +0100
Message-ID:
<alpine.DEB.1.10.0910091817370.5274@urchin.earth.li>
On Thu, 8 Oct 2009, Roedy Green wrote:

On Thu, 08 Oct 2009 20:56:22 -0700, Roedy Green
<see_website@mindprod.com.invalid> wrote, quoted or indirectly quoted
someone who said :

I wrote and posted FastCat, code that saves addresses of Strings, and
concatentate them when you call toString. It needs a new String
constructor to avoid the final copy, but as it is, it does not buy me
anything noticeable over regular StringBuilders with optimised buffer
sizes. I benchmarked it in my app that expands static macros for my
website.


One of the things I did not appreciate, is each chunk requires a
4-byte overhead, If your chunks are only 2 to 4 bytes long, that is
quite a huge percentage.

I did not convert over StringBuffers that were working a char at a
time.


Okay, so maybe we can have a hybrid approach that accumulates large
strings in the list, but when fed short strings or individual characters,
puts them into a buffer - which it then has to push onto the list when the
next large thing comes along, of course.

final static int SMALL_THRESHOLD = 8;
StringBuilder smallStrings;
List<CharSequence> pieces;

void append(String s) {
  if (s.length() < SMALL_THRESHOLD) {
  if (smallStrings == null) smallStrings = new StringBuilder();
  smallStrings.append(s);
  }
  else {
  if (smallStrings != null) {
  pieces.add(smallStrings);
  smallStrings = null;
  }
  pieces.add(s);
  }
}

You'd want a bit more finesse than this, to avoid ever growing
smallStrings - if it fills up, you want to push it onto pieces and make a
new one. StringBuilder is probably the wrong choice; you could roll a
little FixedSizeStringBuffer thing.

tom

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