.clone() or Arrays.copyOf()?

Question

In an effort to reduce mutability, should we rather use

public void setValues(String[] newVals) {

     this.vals = ( newVals == null ? null : newVals.clone() );
}

or

public void setValues(String[] newVals) {

     this.vals = ( newVals == null ? null : Arrays.copyOf(newVals, newVals.length) );
}

Answer 1

Update using jmh

Using jmh, I get similar results, except that clone seems to be marginally better.

Original post

I ran a quick test for performance: clone, System.arrayCopy and Arrays.copyOf have very similar performance (jdk 1.7.06, server vm).

For details (in ms), after JIT:

clone: 68
arrayCopy: 68
Arrays.copyOf: 68

Test code:

public static void main(String[] args) throws InterruptedException,
        IOException {
    int sum = 0;
    int[] warmup = new int[1];
    warmup[0] = 1;
    for (int i = 0; i < 15000; i++) { // triggers JIT
        sum += copyClone(warmup);
        sum += copyArrayCopy(warmup);
        sum += copyCopyOf(warmup);
    }

    int count = 10_000_000;
    int[] array = new int[count];
    for (int i = 0; i < count; i++) {
        array[i] = i;
    }

    // additional warmup for main
    for (int i = 0; i < 10; i++) {
        sum += copyArrayCopy(array);
    }
    System.gc();
    // copyClone
    long start = System.nanoTime();
    for (int i = 0; i < 10; i++) {
        sum += copyClone(array);
    }
    long end = System.nanoTime();
    System.out.println("clone: " + (end - start) / 1000000);
    System.gc();
    // copyArrayCopy
    start = System.nanoTime();
    for (int i = 0; i < 10; i++) {
        sum += copyArrayCopy(array);
    }
    end = System.nanoTime();
    System.out.println("arrayCopy: " + (end - start) / 1000000);
    System.gc();
    // copyCopyOf
    start = System.nanoTime();
    for (int i = 0; i < 10; i++) {
        sum += copyCopyOf(array);
    }
    end = System.nanoTime();
    System.out.println("Arrays.copyOf: " + (end - start) / 1000000);
    // sum
    System.out.println(sum);
}

private static int copyClone(int[] array) {
    int[] copy = array.clone();
    return copy[copy.length - 1];
}

private static int copyArrayCopy(int[] array) {
    int[] copy = new int[array.length];
    System.arraycopy(array, 0, copy, 0, array.length);
    return copy[copy.length - 1];
}

private static int copyCopyOf(int[] array) {
    int[] copy = Arrays.copyOf(array, array.length);
    return copy[copy.length - 1];
}

Answer 2

Please also consider the security of using "clone()". A class of well-known attacks use classes that override objects' "clone()" methods with malicious code. For example, CVE-2012-0507 (the "Flashback" attack on Mac OS) was addressed by basically replacing a ".clone()" call with a ".copyOf" call.

Additional discussion on the obsolete-ness of "clone()" can be found on StackOverflow here: object cloning with out implementing cloneable interface

Answer 3

I written a simple program to check the difference.

public static void main(String[] args) throws IOException, InterruptedException,
        PrinterException
{
  //Verify array remains immutable.

  String[] str =  {"a","b","c"};
  String[] strings  = str.clone();
  //change returned array
  strings[2]= "d";
  System.out.println(Arrays.toString(str));
  System.out.println(Arrays.toString(strings));

  String[] stringsCopy = Arrays.copyOf(str, str.length);
  stringsCopy[2]= "d";
  System.out.println(Arrays.toString(str));
  System.out.println(Arrays.toString(stringsCopy));

  //peformance
  long before = System.currentTimeMillis();
  for(int i=0;i<Integer.MAX_VALUE;i++)
  {
      str.clone();
  }
  System.out.println("Time Required for Clone: "+ (System.currentTimeMillis()-before));

  //peformance
  long beforeCopy = System.currentTimeMillis();
  for(int i=0;i<Integer.MAX_VALUE;i++)
  {
      Arrays.copyOf(str, str.length);
  }
  System.out.println("Time Required for Copy of: "+ (System.currentTimeMillis()-beforeCopy));

}

And it outputs

[a, b, c]
[a, b, d]
[a, b, c]
[a, b, d]
Time Required for Clone: 26288
Time Required for Copy of: 25413

So if you see in both case String[] is immutable and performance is almost same thought Arrays.copyOf() is slightly faster on my machine.

Update

I changed program to create large array[100 strings] rather than small array.

  String[] str =  new String[100];

  for(int i= 0; i<str.length;i++)
  {
      str[i]= Integer.toString(i);
  }

And moved copy of method before clone method. With below results.

 Time Required for Copy of: 415095
 Time Required for Clone: 428501

Which are again more of same. Please do not ask me to run the test again as it takes a while :(

Update 2

For String array 1000000 and for number of iterations 10000

Time Required for Copy of: 32825
Time Required for Clone: 30138

copy of takes more time than clone

Answer 4

In terms of mutability, they will provide exactly the same - shallow copy of data.