Does Calling Clone() on an Array Also Clone Its Contents

How to clone ArrayList and also clone its contents?

You will need to iterate on the items, and clone them one by one, putting the clones in your result array as you go.

public static List<Dog> cloneList(List<Dog> list) {
    List<Dog> clone = new ArrayList<Dog>(list.size());
    for (Dog item : list) clone.add(item.clone());
    return clone;
}

For that to work, obviously, you will have to get your Dog class to implement the Cloneable interface and override the clone() method.

Clone method for Java arrays

When the clone method is invoked upon an array, it returns a reference to a new array which contains (or references) the same elements as the source array.

So in your example, int[] a is a separate object instance created on the heap and int[] b is a separate object instance created on the heap. (Remember all arrays are objects).

    int[] a = {1,2,3};
    int[] b = a.clone();

    System.out.println(a == b ? "Same Instance":"Different Instance");
    //Outputs different instance

If were to modify int[] b the changes would not be reflected on int[] a since the two are separate object instances.

    b[0] = 5;
    System.out.println(a[0]);
    System.out.println(b[0]);
    //Outputs: 1
    //         5

This becomes slightly more complicated when the source array contains objects. The clone method will return a reference to a new array, which references the same objects as the source array.

So if we have the class Dog...

    class Dog{

        private String name;

        public Dog(String name) {
            super();
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

    }

and I create and populate an array of type Dog...

    Dog[] myDogs = new Dog[4];

    myDogs[0] = new Dog("Wolf");
    myDogs[1] = new Dog("Pepper");
    myDogs[2] = new Dog("Bullet");
    myDogs[3] = new Dog("Sadie");

then clone dog...

    Dog[] myDogsClone = myDogs.clone();

the arrays refer to the same elements...

    System.out.println(myDogs[0] == myDogsClone[0] ? "Same":"Different");
    System.out.println(myDogs[1] == myDogsClone[1] ? "Same":"Different");
    System.out.println(myDogs[2] == myDogsClone[2] ? "Same":"Different");
    System.out.println(myDogs[3] == myDogsClone[3] ? "Same":"Different");
    //Outputs Same (4 Times)

This means if we modify an object accessed through the cloned array, the changes will be reflected when we access the same object in the source array, since they point to the same reference.

    myDogsClone[0].setName("Ruff"); 
    System.out.println(myDogs[0].getName());
    //Outputs Ruff

However, changes to the array itself will only affect that array.

    myDogsClone[1] = new Dog("Spot");
    System.out.println(myDogsClone[1].getName());
    System.out.println(myDogs[1].getName());
    //Outputs Spot
    //        Pepper

If you generally understand how object references work, it is easy to understand how arrays of objects are impacted by cloning and modifications. To gain further insight into references and primitives I would suggest reading this excellent article.

Gist of Source Code

Why clone() is the best way for copying arrays?

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()).

clone(): ArrayList.clone() I thought does a shallow copy

Shallow copy does not mean that they point to the same memory location. That would be just an assignment:List b = a;.

Cloning creates a new instance, holding the same elements. This means you have 2 different lists, but their contents are the same. If you change the state of an object inside the first list, it will change in the second list. (Since you are using an immutable type - Integer - you can't observe this)

However, you should consider not using clone(). It works fine with collections, but generally it's considered broken. Use the copy-constructors - new ArrayList(originalList)

How do you clone an array of objects in JavaScript?

Creating a deep copy with structuredClone

The modern way to deep copy an array in JavaScript is to use structuredClone:

array2 = structuredClone(array1);

However, this function is relatively new (Chrome 98, Firefox 94) and is currently only available to about 85% of users, so it's not ready for production yet without a polyfill.

As an alternative, you can use one of the well-supported JSON-based solutions below.

Creating a deep copy with JSON.parse

A general solution, that accounts for all possible objects inside an Array of objects may not be possible.
That said, if your array contains objects that have JSON-serializable content (no functions, no Number.POSITIVE_INFINITY, etc.) one simple way to avoid loops, at a performance cost, is this pure vanilla one-line solution.

let clonedArray = JSON.parse(JSON.stringify(nodesArray))

To summarize the comments below, the primary advantage of this approach is that it also clones the contents of the array, not just the array itself. The primary downsides are its limit of only working on JSON-serializable content, and it's performance is ~30 times slower than the spread method.

If you have shallow objects in the array, and IE6 is acceptable, a better approach is to use the spread operator combined with the .map array operator. For a two levels deep situation (like the array in the Appendix below):

clonedArray = nodesArray.map(a => {return {...a}})

The reasons are two fold: 1) It is much, much faster (see below for a benchmark comparison) and it will also allow any valid object in your array.

*Appendix:
The performance quantification is based on cloning this array of objects a million times:

 [{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic1.jpg?raw=true', id: '1', isFavorite: false}, {url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic2.jpg?raw=true', id: '2', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic3.jpg?raw=true', id: '3', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic4.jpg?raw=true', id: '4', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic5.jpg?raw=true', id: '5', isFavorite: true},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic6.jpg?raw=true', id: '6', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic7.jpg?raw=true', id: '7', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic8.jpg?raw=true', id: '8', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic9.jpg?raw=true', id: '9', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic10.jpg?raw=true', id: '10', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic11.jpg?raw=true', id: '11', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic12.jpg?raw=true', id: '12', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic13.jpg?raw=true', id: '13', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic14.jpg?raw=true', id: '14', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic15.jpg?raw=true', id: '15', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic16.jpg?raw=true', id: '16', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic17.jpg?raw=true', id: '17', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic18.jpg?raw=true', id: '18', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic19.jpg?raw=true', id: '19', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic20.jpg?raw=true', id: '20', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic21.jpg?raw=true', id: '21', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic22.jpg?raw=true', id: '22', isFavorite: false},{url: 'https://github.com/bobziroll/scrimba-react-bootcamp-images/blob/master/pic23.jpg?raw=true', id: '23', isFavorite: false}]

either using:

let clonedArray = JSON.parse(JSON.stringify(nodesArray))

or:

clonedArray = nodesArray.map(a => {return {...a}})

The map/spread approach took 0.000466 ms per pass and the JSON.parse and JSON.stringify 0.014771 ms per pass.*

Is there any reason to prefer System.arraycopy() over clone()?

No. If you're really microbenchmarking, then maybe, depending on what JVM you're running. But in actuality, no.

.clone() or Arrays.copyOf()?

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];
}

Difference between the System.Array.CopyTo() and System.Array.Clone()

The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element's object, resulting in a different, yet identical object.

So the difference are :

1- CopyTo require to have a destination array when Clone return a new array.
2- CopyTo let you specify an index (if required) to the destination array.

Edit:

Remove the wrong example.

Right way to clone objects / arrays during setState in React

I personally rely on this deep copy strategy.
JSON.parse(JSON.stringify(object)) rather than spread operator because it got me into weird bugs while dealing with nested objects or multi dimensional arrays.

spread operator does not do a deep copy if I am correct and will lead to state mutations with NESTED objects in React.

Please run through the code to get a better understanding of what is happening between the two. Imagine that is the state variable that you mutate using spread operator.

const obj = {Dogs: [{name: "Snoopy"}, {name: "Lola"}, {name: "Sprinkles"}], Cats: [{name: "Felidae"}, {name: "Garfiled"}, {name: "Cat in the Hat"}] };
const newObj = {...obj};console.log("BEFORE SPREAD COPY MUTATION")
console.log("NEW OBJ: " + newObj.Dogs[0].name); //Snoopyconsole.log("OLD OBJ: " + obj.Dogs[0].name); //Snoopy
newObj.Dogs[0].name = "CLONED Snoopy";
console.log("AFTER SPREAD COPY MUTATION")
console.log("NEW OBJ: " + newObj.Dogs[0].name); // CLONED Snoopyconsole.log("OLD OBJ: " + obj.Dogs[0].name); // CLONED Snoopy
// Even after using the spread operator the changed on the cloned object are affected to the old object. This happens always in cases of nested objects.
// My personal reliable deep copy
console.log("*********DEEP COPY***********");
console.log("BEFORE DEEP COPY MUTATION")deepCopyObj = JSON.parse(JSON.stringify(obj));

console.log("NEW OBJ: " + newObj.Dogs[0].name); //CLONED Snoopyconsole.log("OLD OBJ: " + obj.Dogs[0].name); // CLONED Snoopyconsole.log("DEEP OBJ: " + deepCopyObj.Dogs[0].name); //CLONED Snoopy

deepCopyObj.Dogs[0].name = "DEEP CLONED Snoopy";
console.log("AFTER DEEP COPY MUTATION")console.log("NEW OBJ: " + newObj.Dogs[0].name); // CLONED Snoopyconsole.log("OLD OBJ: " + obj.Dogs[0].name); // CLONED Snoopyconsole.log("DEEP OBJ: " + deepCopyObj.Dogs[0].name); // DEEP CLONED Snoopy

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