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Copying Arrays

Use System's arraycopy method to efficiently copy data from one array into another. The arraycopy method requires five arguments:

public static
    void arraycopy(Object source,
                   int srcIndex,
                   Object dest,
                   int destIndex,
                   int length,

The two Object arguments indicate the array to copy from and the array to copy to. The three integer arguments indicate the starting location in each the source and the destination array, and the number of elements to copy. The following figure illustrates how the copy takes place:

The following program, ArrayCopyDemo, uses arraycopy to copy some elements from the copyFrom array to the copyTo array.

public class ArrayCopyDemo {
    public static void main(String[] args) {
        char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',
			    'i', 'n', 'a', 't', 'e', 'd' };
        char[] copyTo = new char[7];

        System.arraycopy(copyFrom, 2, copyTo, 0, 7);
        System.out.println(new String(copyTo));
    }
}

The output from this program is:

caffein

The arraycopy method call in this example program begins the copy at element number 2 in the source array. Recall that array indices start at 0, so that the copy begins at the array element 'c'. The arraycopy method call puts the copied elements into the destination array beginning at the first element (element 0) in the destination array copyTo. The copy copies 7 elements: 'c', 'a', 'f', 'f', 'e', 'i', and 'n'. Effectively, the arraycopy method takes the "caffein" out of "decaffeinated", like this:

Note that the destination array must be allocated before you call arraycopy and must be large enough to contain the data being copied.

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