How do I do a deep copy of a 2d array in Java?

Question

I just got bit by using .clone() on my 2d boolean array, thinking that this was a deep copy.

How can I perform a deep copy of my boolean[][] array?

Should I loop through it and do a series of System.arraycopy's?

Taliped

Answer 1

Yes, you should iterate over 2D boolean array in order to deep copy it. Also look at java.util.Arrays#copyOf methods if you are on Java 6.

I would suggest the next code for Java 6:

public static boolean[][] deepCopy(boolean[][] original) {
    if (original == null) {
        return null;
    }

    final boolean[][] result = new boolean[original.length][];
    for (int i = 0; i < original.length; i++) {
        result[i] = Arrays.copyOf(original[i], original[i].length);
        // For Java versions prior to Java 6 use the next:
        // System.arraycopy(original[i], 0, result[i], 0, original[i].length);
    }
    return result;
}

Backbreaker

Note that this does not seem to work for Objects. See #15135604 – Newsstand

This works because you have 2D array of boolean, a primitive type. But when you have a 2D array of objects, this will not copy or clone the objects. Note that Arrays.copyOf() by itself does a shallow copy. – Worthless

Answer 2

In Java 8 this can be accomplished as a one-liner using lambdas:

<T> T[][] deepCopy(T[][] matrix) {
    return java.util.Arrays.stream(matrix).map(el -> el.clone()).toArray($ -> matrix.clone());
}

Electrophilic

Also, consider the code here: https://mcmap.net/q/37099/-java-how-to-make-a-copy-of-an-array-of-object – Detect

I don't believe that code adopts itself easily to 2d arrays but I'd be curious if you managed to do it since I can't get it to work. – Elmer

@Elmer What do you mean? The given example deep copies a 2d array. – Electrophilic

was referring to JIm G's link. I still don't know how to @JimG Edit: looks like that worked – Elmer

Answer 3

I'm a fan of the Arrays utility. It has a copyOf method that will do a deep copy of a 1-D array for you, so you'd want something like this:

//say you have boolean[][] foo;
boolean[][] nv = new boolean[foo.length][foo[0].length];
for (int i = 0; i < nv.length; i++)
     nv[i] = Arrays.copyOf(foo[i], foo[i].length);

Weismannism

Note, that this only creates a "deep copy" for primitive types! Arrays.copyOf() itself only creates shallow copies. – Schell

Note too that this only works if the columns of all the rows are the same size as the first. – Gulf

@NilSuria That is not true. See it working here. Though an argument could be made for the code not working when the original array is empty. – Moneychanger

Answer 4

I've managed to come up with a recursive array deep copy. It seems to work pretty well even for multi dimensional arrays with varying dimension lengths e.g.

private static final int[][][] INT_3D_ARRAY = {
        {
                {1}
        },
        {
                {2, 3},
                {4, 5}
        },
        {
                {6, 7, 8},
                {9, 10, 11},
                {12, 13, 14}
        }
};

Here is the utility method.

@SuppressWarnings("unchecked")
public static <T> T[] deepCopyOf(T[] array) {

    if (0 >= array.length) return array;

    return (T[]) deepCopyOf(
            array, 
            Array.newInstance(array[0].getClass(), array.length), 
            0);
}

private static Object deepCopyOf(Object array, Object copiedArray, int index) {

    if (index >= Array.getLength(array)) return copiedArray;

    Object element = Array.get(array, index);

    if (element.getClass().isArray()) {

        Array.set(copiedArray, index, deepCopyOf(
                element,
                Array.newInstance(
                        element.getClass().getComponentType(),
                        Array.getLength(element)),
                0));

    } else {

        Array.set(copiedArray, index, element);
    }

    return deepCopyOf(array, copiedArray, ++index);
}

EDIT: Updated the code to work with primitive arrays.

Mcghee

Edit: Found out myself: import java.lang.reflect.Array; – Ursola

This answer has a good idea, but the code unfortunately has some problems that make it not a very good example. (1) array[0].getClass() makes the method unsuitable for arrays of mixed objects such as e.g. new Number[] {1, 2.5}. It should use array.getClass().getComponentType() instead. (2) Using recursion to iterate each array index makes the method unsuitable/unpredictable for large arrays. (3) Uses ++index instead of index+1 for no reason. – Bernardo

@Bernardo why does the use of recursion make it unsuitable/unpredictable for large arrays? – Sib

Answer 5

Yes, that's the only way to do it. Neither java.util.Arrays not commons-lang offer deep copy for arrays.

Rosario

Answer 6

You can iterate over this array and perform a series of calls of Arrays.copyOf method:

boolean[][] arr1 = {{true, true}, {false, true}};    // original array
boolean[][] arr2 = Arrays.copyOf(arr1, arr1.length); // shallow copy
boolean[][] arr3 = Arrays.stream(arr1)               // deep copy
        .map(arr -> Arrays.copyOf(arr, arr.length))
        .toArray(boolean[][]::new);

arr1[0][0] = false;

System.out.println(Arrays.deepToString(arr1)); // [[false, true], [false, true]]
System.out.println(Arrays.deepToString(arr2)); // [[false, true], [false, true]]
System.out.println(Arrays.deepToString(arr3)); // [[true, true], [false, true]]

Or you can call Object.clone method:

boolean[][] arr3 = Arrays.stream(arr1)
        .map(boolean[]::clone)
        .toArray(boolean[][]::new);

Or you can create a generic method for this purpose:

static <T> T[][] deepCopy(T[][] matrix) {
    return Arrays.stream(matrix)
            .map(arr -> arr.clone())
            .toArray(s -> matrix.clone());
}

---

^{See also: Why does Array.copyOf() mutate the original array in case of 2D Arrays?}

Rachaba

The object.clone method was key -- someone posted the generic method but it was hard to make a one liner since the variable used needed to be effectively final if you tried to call it outside of a method) – Elmer

Answer 7

Here's a reflective example using java.lang.reflect.Array which is more robust and a bit easier to follow. This method will copy any array, and deeply copies multidimensional arrays.

package mcve.util;

import java.lang.reflect.*;

public final class Tools {
    private Tools() {}
    /**
     * Returns a copy of the specified array object, deeply copying
     * multidimensional arrays. If the specified object is null, the
     * return value is null. Note: if the array object has an element
     * type which is a reference type that is not an array type, the
     * elements themselves are not deep copied. This method only copies
     * array objects.
     *
     * @param  array the array object to deep copy
     * @param  <T>   the type of the array to deep copy
     * @return a copy of the specified array object, deeply copying
     *         multidimensional arrays, or null if the object is null
     * @throws IllegalArgumentException if the specified object is not
     *                                  an array
     */
    public static <T> T deepArrayCopy(T array) {
        if (array == null)
            return null;

        Class<?> arrayType = array.getClass();
        if (!arrayType.isArray())
            throw new IllegalArgumentException(arrayType.toString());

        int length = Array.getLength(array);
        Class<?> componentType = arrayType.getComponentType();

        @SuppressWarnings("unchecked")
        T copy = (T) Array.newInstance(componentType, length);

        if (componentType.isArray()) {
            for (int i = 0; i < length; ++i)
                Array.set(copy, i, deepArrayCopy(Array.get(array, i)));
        } else {
            System.arraycopy(array, 0, copy, 0, length);
        }

        return copy;
    }
}

Bernardo