Josh Bloch states in this blog that
You should use clone to copy arrays, because that's generally the fastest way to do it.
I always used System.arraycopy(...).
Both approaches are native, so probably without going deeper into the sources of libraries I cannot figure out, why it is so.
Why is it the fastest way?
The difference with System.arraycopy is explained here, but it does not answer the question why Josh Bloch considers clone() as the fastest way.
I would like to make some points about why clone() is the fastest way to copy an array than System.arraycopy(..) or others:
1. clone() doesn't have to do the typechecking before copying a source array to the destination one as provided here. It just simple allocates new memory space and assigns the objects to it. On the other hand, System.arraycopy(..) checks for the type and then copies an array.
2. clone() also breaks the optimization to eliminate redundant zeroing. As you know, every allocated array in Java must be initialized with 0s or respective default values. However, JIT can avoid zeroing this array if it sees that the array is filled right after creation. That makes it definitely faster compared to changing the copy values with existing 0s or respective default values. While using System.arraycopy(..) spends significant amount of time clearing and copying the initialized array. To do so I have performed some of the benchmark tests.
@BenchmarkMode(Mode.Throughput)
@Fork(1)
@State(Scope.Thread)
@Warmup(iterations = 10, time = 1, batchSize = 1000)
@Measurement(iterations = 10, time = 1, batchSize = 1000)
public class BenchmarkTests {
@Param({"1000","100","10","5", "1"})
private int size;
private int[] original;
@Setup
public void setup() {
original = new int[size];
for (int i = 0; i < size; i++) {
original[i] = i;
}
}
@Benchmark
public int[] SystemArrayCopy() {
final int length = size;
int[] destination = new int[length];
System.arraycopy(original, 0, destination, 0, length);
return destination;
}
@Benchmark
public int[] arrayClone() {
return original.clone();
}
}
Output:
Benchmark (size) Mode Cnt Score Error Units
ArrayCopy.SystemArrayCopy 1 thrpt 10 26324.251 ± 1532.265 ops/s
ArrayCopy.SystemArrayCopy 5 thrpt 10 26435.562 ± 2537.114 ops/s
ArrayCopy.SystemArrayCopy 10 thrpt 10 27262.200 ± 2145.334 ops/s
ArrayCopy.SystemArrayCopy 100 thrpt 10 10524.117 ± 474.325 ops/s
ArrayCopy.SystemArrayCopy 1000 thrpt 10 984.213 ± 121.934 ops/s
ArrayCopy.arrayClone 1 thrpt 10 55832.672 ± 4521.112 ops/s
ArrayCopy.arrayClone 5 thrpt 10 48174.496 ± 2728.928 ops/s
ArrayCopy.arrayClone 10 thrpt 10 46267.482 ± 4641.747 ops/s
ArrayCopy.arrayClone 100 thrpt 10 19837.480 ± 364.156 ops/s
ArrayCopy.arrayClone 1000 thrpt 10 1841.145 ± 110.322 ops/s
According to the outputs I am getting that clone is almost twice fast from System.arraycopy(..)
3. Also, using manual copying method like clone() results into faster ouput because it doesn't have to make any VM calls (unlike System.arraycopy()).