What is the fastest way to copy a double[] in Java? [duplicate]

Question

Simple question, what's the fastest way of copying an array of doubles in Java. I currently do this...

public static double[] clone_doubles(double[] from) { double[] to = new double[from.length]; for (int i = 0; i < from.length; i++) to[i] = from[i]; return to; }

which also does the allocation to avoid overflows, but if there is a quicker way I will separate the allocation from the copy.

I have looked at Arrays.copyOf() and System.arraycopy() but I'm wondering if anyone has any neat tricks.

Edit: How about copying a double[][]?

What do you mean by "separate the allocation from the copy"? Do you mean that you are able to re-use destination arrays instead of allocating a new one? I'd guess that System.arraycopy() is fastest, much faster than your code. Have you profiled it? — erickson
– erickson, Commented Oct 12, 2011 at 16:57
Why not add arr.clone() to your list, benchmark all four methods, and post the results here? :-) — NPE
– NPE, Commented Oct 12, 2011 at 16:57

Community · Accepted Answer · 2020-06-20 09:12:55Z

14

Java Practices did a comparison of different copy methods on arrays of int:

Here are the results from their site:

java -cp . -Xint ArrayCopier performance 250000

Using clone: 93 ms
Using System.arraycopy: 110 ms
Using Arrays.copyOf: 187 ms
Using for loop: 422 ms

Looks to me like a tie between System.arraycopy() and clone().

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Oct 12, 2011 at 17:04

dtyler

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7 Comments

Simon Over a year ago

interesting, the same site says avoid clone... javapractices.com/topic/TopicAction.do?Id=71

monksy Over a year ago

This is the kind of answer I like to see! :)

Naveen Babu Over a year ago

@Simon the site says "avoid implementing clone" but not to avoid clone

Vasu Over a year ago

Why there is so difference between System.arraycopy() (110 ms) and Arrays.copyOf() (187 ms) ? Is not Arrays.copyOf() uses System.arraycopy() internally ?

assylias Over a year ago

Their benchmark is significantly flawed. I would not trust their results (which BTW I know are inaccurate).

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Mike Samuel · Accepted Answer · 2011-10-12 16:55:18Z

System.arraycopy is probably your best bet if you just want to copy from one array to another.

Otherwise

public static double[] clone_doubles(double[] from) { return (double[]) from.clone(); }

will give you a clone.

Community · Accepted Answer · 2017-05-23 12:09:39Z

System.arraycopy() is your best bet. Generally, it's implemented with native instructions, probably involving direct calls to the operating system's memory manager.

In Java 6 the performance of arraycopy was improved as "hand-coded assembly stubs are now used for each type size when no overlap occurs".

Read this other post for further details.

Community · Accepted Answer · 2023-11-13 19:27:07Z

Arrays.copy, [].clone and System.arraycopy w/ new object are using the same code path when properly JIT.

Here is the corresponding bug: https://bugs.java.com/bugdatabase/view_bug?bug_id=6428387

Sean Patrick Floyd · Accepted Answer · 2011-10-12 16:57:15Z

public static double[] clone_doubles(double[] from) { return Arrays.copyOf(from, from.length); }

This is internally implemented using System.arrayCopy():

public static double[] copyOf(double[] original, int newLength) { double[] copy = new double[newLength]; System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength)); return copy; }

this is irrelevant actually, it uses intrinsics, the code is not executed by the JIT like that

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