Java Data Type Tutorial - Java Array Copy

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The following code shows how to use for loop to copy an array

import java.util.Arrays;
//from  w  w  w  .j  a va  2  s .com
public class Main {
  public static void main(String[] args) {
    int[] data = { 1, 2, 3, 4, 5 };

    // Expand the data array to 7 elements
    int[] eData = expandArray(data, 7);

    // Truncate the data array to 3 elements
    int[] tData = expandArray(data, 3);

    System.out.println("Original  Array: " + Arrays.toString(data));
    System.out.println("Expanded Array: " + Arrays.toString(eData));
    System.out.println("Truncated  Array: " + Arrays.toString(tData));

  }

  // Uses a for-loop to copy an array
  public static int[] expandArray(int[] oldArray, int newlength) {
    int originallength = oldArray.length;
    int[] newArray = new int[newlength];
    int elementsToCopy = 0;

    if (originallength > newlength) {
      elementsToCopy = newlength;
    } else {

      elementsToCopy = originallength;
    }
    for (int i = 0; i < elementsToCopy; i++) {
      newArray[i] = oldArray[i];
    }
    return newArray;
  }
}

The code above generates the following result.

System.arraycopy

We can also copy elements of an array to another array by using the arraycopy() method of the System class.

The signature of the arraycopy() method is as follows:

public static  void  arraycopy(Object sourceArray, 
                               int  sourceStartPosition, 
                               Object destinationArray,
                               int destinationStartPosition, 
                               int  lengthToBeCopied)

Here,

• sourceArray is the source array.
• sourceStartPosition is the starting index in the source array from where the copying of elements will start.
• destinationArray is the destination array.
• destinationStartPosition is the start index in the destination array from where new elements from source array will be copied.
• lengthToBeCopied is the number of elements to be copied.

We can replace the previous for loop with the following code:

System.arraycopy (ids,  0, tmyIDs, 0, elementsToCopy);

The following code shows how to use System.arraycopy to copy an array.

import java.util.Arrays;
/*from   w  ww. ja  va  2  s.  c  o m*/
public class Main {
  public static void main(String[] args) {
    int[] data = { 1, 2, 3, 4, 5 };

    // Copy data array to new arrays 
    int[] eData = new int[9];
    int[] tData = new int[2];
    System.arraycopy(data, 0, eData, 0, 5);
    System.arraycopy(data, 0, tData, 0, 2);

    System.out.println("Original  Array: " + Arrays.toString(data));
    System.out.println("Expanded Array: " + Arrays.toString(eData));
    System.out.println("Truncated  Array: " + Arrays.toString(tData));
  }
}

The code above generates the following result.

Array Clone

We can make a copy of array using array's clone() method. For primitive types, the cloned array will have a true copy of the original array.

However, for reference types, the reference of the object stored in each element is copied to the cloned array.

This is known as a shallow copy. In a shallow copy, elements of both arrays, the original and the cloned, refer to the same object in memory.

The following code illustrates the cloning of an int array and a String array.

public class Main {
  public static void main(String[] args) {
    // Create an array of 3 integers 1,2,3
    int[] ids = { 1, 2, 3 };
    int[] clonedIds = (int[]) ids.clone();
//from   ww  w. j  av a2  s  .  com
    // Create an array of 3 strings.
    String[] names = { "A", "B", "C" };
    String[] clonedNames = (String[]) names.clone();
  }
}

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