CODEWITHSUNDEEP
javaarraysperformance
wfbarksdale
wfbarksdale

Reputation: 7606

Is there a difference in performance for calling .length on an array versus saving a size variable?

I am creating a simulation program, and I want the code to be very optimized. Right now I have an array that gets cycled through a lot and in the various for loops I use

 for(int i = 0; i<array.length; i++){
       //do stuff with the array
 }

I was wondering if it would be faster if I saved a variable in the class to specify this array length, and used that instead. Or if it matters at all.

Upvotes: 8

Views: 1443

Answers (3)

Mike Dunlavey
Mike Dunlavey

Reputation: 40659

It's unlikely to make a difference.

The broader issue is you want your simulation to run as fast as possible. This sourceforge project shows how you can use the running program to tell you what to optimize, as opposed to wondering about little things like.length. Most code, as first written (other than little toy programs), has huge speedup potential, in ways that elude being guessed.

The biggest opportunities for speedup are usually diffuse, like your choice of data structure. They aren't localized to particular routines, and measuring tools don't point them out. The method demonstrated in that project, random pausing, does identify them. The changes you need to make may not be easy to do, but they will yield speedup.

Upvotes: 1

duffymo
duffymo

Reputation: 308743

Accessing the length attribute on an array is as fast as it gets.

You'll see people recommending that you save a data structure size before entering the loop because it means a method all for each and every iteration.

But this is the kind of micro-optimization that seldom matters. Don't worry much about this kind of thing until you have data that tells you it's the reason for a performance issue.

You should be spending more time thinking about the algorithms you're embedding in that loop, possible parallelism, etc. That'll be far more meaningful in your quest for an optimized solution.

Upvotes: 15

D3_JMultiply
D3_JMultiply

Reputation: 1080

it really doesn't matter, when you make an array with [#] it stores that number, and just returns that.

If you're using []{object1, object2, object3} then it will calculate the size, in either case, the value is calculated once only, and stored.

length is a variable, not a method, when you access it it has a value, and it just retrieves that value, it's not like it's going to calculate the length of the array each time you use the length attribute. array.length is no slower, in fact, having to set an extra variable will actually take more time then just using length.

array.length is faster, although by fractions of a milisecond, but in loops and such you can save a ton of time if cycleing through let's say 1,000,000 items (might be 0.1 seconds).

Upvotes: 2

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