CODEWITHSUNDEEP
javaarraylistarrays
user2279869
user2279869

Reputation: 235

What consumes less, to use ArrayList or System.arraycopy

Maybe obvious but I want to know what and why is the best practice when keeping down the consumption of system resources.

When I want to dynamically expand an Array I can use either

System.arraycopy(arraystuff, 0, tempArray, 0, arraystuff.length);

or 

ArrayList

Easiest is of course ArrayList but is there any reason to use arraycopy ?

Upvotes: 2

Views: 304

Answers (2)

Andy Turner
Andy Turner

Reputation: 140484

Almost always, you should use the higher-level abstraction. You often don't need to care that ArrayList resizes itself internally, or even that it stores the data in an array (most of the time, you'll have a List reference, not an ArrayList); it "just works".

However, the resizing strategy is fixed with ArrayList: it always resizes by a multiplicative factor of 1.5 in order to amortize the cost of reallocating the array over many additions. This could lead to wasting a lot of space if, say, you only needed to add 1 more element to a full 1,000,000 element list.

True, you can use trimToSize to get rid of this wasted space, but only after the fact: you'd then be allocating a 1.5M-element array and a (1M+1)-element array. The 1.5M-element array would only be allocated transiently, and could then be GC'd, but that's still more effort than just allocating the (1M+1)-element array directly.

In that case, you might want to have something that you have more control over how it resizes, and in that situation, using System.arraycopy might be more appropriate.

But really, until you have profiled your code and found that ArrayList is using too much space, just stick with that.

Note also that you might find Arrays.copyOf easier to work with than System.arraycopy for the purposes of resizing an array, because it takes fewer parameters and is type-safe. (It uses System.arraycopy internally).

Upvotes: 3

Jai
Jai

Reputation: 8363

System.arraycopy() is a native method while ArrayList is a high-level Java implementation, which will eventually call System.arraycopy(). Therefore, in terms of performance, System.arraycopy() is definitely better. But in general, unless every bit of performance matters for your application, it is generally more favorable to use a List.

Upvotes: 1

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