How to initialise an array in Java with a constant value efficiently

Question

Is there a way to initialise an array of integers (or possibly any array) to a constant value other than zero (or null) which are the defaults, without a for loop?

Ideally I am looking for a function like "ones" in matlab, which is not only neater but also more efficient.

Such methods almost always use loops. A reason it could be faster in Matlab is that its an interpreted language whereas Java can be compiled to native code. Ironically, some loops in java can be so heavily optimised they are faster than using native calls. e.g. a loop to copy long[] can be faster using a loop than using System.arrayCopy — Peter Lawrey
– Peter Lawrey, Commented Feb 5, 2012 at 15:43

biziclop · Accepted Answer · 2012-02-05 15:06:23Z

9

Arrays.fill() is the method you're after. (Although internally it still uses a for loop, so unlike System.arrayCopy(), it isn't any faster.)

P.s.: Arrays, and its collection-based counterpart Collections are two extremely useful classes in general.

edited Feb 5, 2012 at 15:06

answered Feb 5, 2012 at 14:57

biziclop

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5 Comments

Hot Licks Over a year ago

It may be faster in some implementations since the JITC knows it can bypass bounds checks.

biziclop Over a year ago

It may be, but I don't know of any JIT implementations out there that treat the same code in a different way just because it's in `java.util. Do you know a specific JIT that does this?

Hot Licks Over a year ago

Well, I once worked on a JITC that did so, and I think the more advanced Sun/Oracle JITCs do something similar, based on indirect evidence.

BeeOnRope Over a year ago

Many low-level routines in java. packages are intrinsified by the JIT, so that the "reference" implementation you see in the JDK source may never be executed. You can check the list of intrinsics here (look for Here are all the intrinsics). It doesn't seem to include Arrays.fill currently, but does include other methods from Arrays.

biziclop Over a year ago

Thanks, that's useful knowledge.

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