Fast String.indexOf()

Another string optimization PMD rule!  If you see code calling String.indexOf() like this:

String str = "hello world";
int location = str.indexOf("w"); // using indexOf(String) with a one character String!

That works, but a faster way is to do:

String str = "hello world"
int location = str.indexOf('w'); // use indexOf(char) instead

I did a bit of benchmarking, and using String.indexOf(char) is about 10-20% faster depending on the String length and the location of the target character within the String.  If you're wondering why it's faster, take a peek at the source code for String.indexOf(char) and String.indexOf(String); you'll see the difference just in the amount of code in the implementation.   It's a minor optimization, but inside a tight loop, it could make a difference - can't hurt, anyhow!

This new rule is called UseIndexOfChar and it's in the strings ruleset in PMD's CVS repository.

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Comments

[simonsays]

now you have one for
sb.append('c')
and
str.indexOf('c')

and possible more with a single character.

Wouldn't it be possible to make a general rule for all places where you use a single string character and make it to a char?
I think its always better to that.. or did i overlook some cases where it's not possible?

[simonsays]

nevermind.. it ofcourse have to accept a char.
but what about concatenation with a single letter or is there a rule for that already?

str+'c'

[Allan Caplan]

Interesting rule Tom!

Is there the same performance benefit (and if so, does your new rule catch) this:

int iSearchPos = 5;
String string = ....;
string.indexOf('c', iSearchPos);

(searching from a given position)

[tomcopeland]

Hi Allan -
Yup, it catches that case... although I hadn't thought about it when coding that rule up, thanks for the pointer! I've added a test case for it...
Yours,
Tom

[tomcopeland]

Hi simonsays -
Hm, I suppose we could combine these two rules... hm. I'm not sure. I kind of feel like it might be trying to do too much, since AppendCharacterWithChar deals with StringBuffers and UseIndexOfChar deals with strings.
Yours,
Tom

[Tom Hawtin]

Looking at the code for String.indexOf(String) may be misleading. A good JVM will treat the method as an intrinsic.

Incidentally, String.concat may be faster than using the String + operator. Particularly from 1.5.

[tomcopeland]

Tom -
Hm, good point. I've been using JDK 1.4 for benchmarking... perhaps I should give it a whirl on something newer.
Great stuff on your blog, by the way; thanks for posting those. I like the builder suggestions here especially:
http://jroller.com/page/tackline?entry=static_creation_methods
Tom