java optimization by compiler or JIT

Question

From time to time I see code like this:

if (id.split(":").length > 1) {
 sub_id = id.split(":")[1];
 parent_id = id.split(":")[0];
}

Wouldn't it be better (and faster) to do something like

String [] ids = id.split(":");
if (ids.length > 1) {
  sub_id = ids[1];
  parent_id = ids[0];
}

This way you don't have to call 'split()' multiple times, or will the compiler/JIT do such optimizations?

The optimization engine can improve performance of the algorithm used, but cannot (to my knowledge) replace your code with another algorithm. — Thorbjørn Ravn Andersen
– Thorbjørn Ravn Andersen, Commented Mar 11, 2013 at 13:21
If this is not just a theoretical question, but you have a real need to optimize the code, I would avoid to use the regexp-able split method at all. Using indexOf to find the ':' and two substring calls to extract each part of the id ought to be much faster than both alternatives. — jarnbjo
– jarnbjo, Commented Mar 11, 2013 at 14:00

Jon Skeet · Accepted Answer · 2013-03-11 13:13:20Z

10

I certainly wouldn't expect either the JIT or the compiler do perform such optimizations. It would have to know that:

The results don't "usefully" change between calls
Nothing was going to use the fact that each method call produces separate arrays
Nothing was going to use the fact that each method call produces different string objects

It seems very unlikely that either the JIT or the compiler would optimize for this.

Yes, it's definitely more efficient to use the second form - and I'd argue it's more readable too. When more readable code is also more efficient, that's a pretty clear indication of which code to use ;)

answered Mar 11, 2013 at 13:13

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

14 Comments

Thorbjørn Ravn Andersen Over a year ago

Measure first, then you have hard data instead of waving your arms.

OddBeck Over a year ago

... I sometimes get the impression that people expect the compiler to be smarter than it is.

Jon Skeet Over a year ago

@OddBeck: Why didn't you test the actual code you presented? Surely that would be simpler and more useful.

Jon Skeet Over a year ago

@jarnbjo: That would be surprising - and very easily observed, just by comparing references. Would love to see evidence that it's happening though.

Jon Skeet Over a year ago

@jarnbjo: Right, but it's also possible because the results really, really can't be distinguished from each other. That's not true with String.split.

|

Collectives™ on Stack Overflow

java optimization by compiler or JIT

1 Answer 1

14 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

14 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related