Java Copy Array: How To Copy / Clone An Array In Java

This Tutorial on Copying and Cloning of Arrays Discusses the Various Methods to Copy an Array in Java:

Here we will discuss the copy operation of Java arrays. Java provides various ways in which you can make copies of array elements. As we know, in Java, arrays can contain elements either of primitive types or objects or references.

While making copies of primitive types, the task is rather easy but when it comes to objects or references, you need to pay attention as to whether the copy is deep or shallow.

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Arrays - Copying and Cloning

Shallow copy makes a copy of the element. It is not a problem when primitive data types are involved. But when references are involved, a shallow copy will just copy the value and not the underlying information.

Thus, even though you have made copies of elements, a change in one copy will reflect in other copy too as the memory locations are shared. To prevent this, you need to go for a deep copy in which the memory locations are not shared.

Copy And Clone Java Arrays

Java allows you to copy arrays using either direct copy method provided by java.util or System class. It also provides a clone method that is used to clone an entire array.

In this tutorial, we will discuss the following methods of Copying and Cloning Arrays.

  • Manual copying using for loop
  • Using System.arraycopy()
  • Using Arrays.copyOf()
  • Using  Arrays.copyOfRange()
  • Using Object.clone()

Let's Explore!!

Manual Copying Using For Loop

Normally when we copy variables, for example, a and b, we perform the copy operation as follows:

a=b;

It is not going to work correctly if we apply the same method to arrays.

Let's see a programming example.

public class Main 
{ 
     public static void main(String[] args) 
    { 
         int intArray[] = {12,15,17}; 

        //print original intArray
        System.out.println("Contents of intArray[] before assignment:"); 
        for (int i=0; i<intArray.length; i++) 
             System.out.print(intArray[i] + " ");

        // Create an array b[] of same size as a[] 
        int copyArray[] = new int[intArray.length]; 

        // intArray is assigned to copyArray; so references point to same location 
        copyArray = intArray;     

        // change element of copyArray
        copyArray[1]++; 

        //print both arrays
        System.out.println("\nContents of intArray[]:"); 
        for (int i=0; i<intArray.length; i++) 
              System.out.print(intArray[i] + " "); 

        System.out.println("\nContents of copyArray[]:"); 
        for (int i=0; i<copyArray.length; i++) 
              System.out.print(copyArray[i] + " "); 
    } 
}

Output:

Java Copy Array using for loop

In the above program, there are two arrays i.e. intArray and copyArray. The task is to copy the contents of the intArray to copyArray. To do this, the statement copyArray = intArray is introduced. What is done here is the references of the array are assigned. Hence this is not actual copying.

As a result of the above statement, the memory location of the intArray is shared by the copyArray as well. Now when the copyArray element is incremented, that change is reflected in the intArray too. This is shown in the output.

To overcome this problem, we employ a method of copying the array using for loop. Here, each element of the original array is copied to the new array using a for loop.

This program is shown below.

public class Main 
{ 
     public static void main(String[] args) 
    { 
         int intArray[] = {12,15, 17}; 

        // define an array copyArray to copy contents of intArray
        int copyArray[] = new int[intArray.length]; 

        // copy contents of intArray to copyArray
        for (int i=0; i<intArray.length; i++) 
           copyArray[i] = intArray[i]; 

        // update element of copyArray 
        copyArray[0]++; 

        //print both arrays
        System.out.println("intArray[] elements:"); 
        for (int i=0; i<intArray.length; i++) 
            System.out.print(intArray[i] + " "); 

        System.out.println("\n\ncopyArray[] elements:"); 
        for (int i=0; i<copyArray.length; i++) 
            System.out.print(copyArray[i] + " "); 
    } 
}

Output:

Copy Array Java - Output

Here we have modified the previous program to include for loop and inside for loop, we assign each element of intArray to the corresponding element of copyArray. This way, the elements are actually copied. So when one array is modified, the changes do not reflect in another array.

Using System.arraycopy()

Java’s System class has a method called “ArrayCOpy” that allows you to copy elements of one array to another array.

The general prototype of this method is as follows:

public static void arraycopy(  
	Object src_array, int src_Pos,Object dest_array, int dest_Pos, int length ) 

Here,

  • src_array => Source array from where the contents are to be copied.
  • src_Pos => The position in the source array from where copying will start.
  • dest_array => Destination array to which elements are to be copied.
  • dest_Pos => Starting position in the destination array for the elements to be copied.
  • length => Length of the array to be copied.

Let's understand this method with an example.

class Main {  
    public static void main(String[] args) {  
        //declaring a source array  
        char[] src_array = { 'S','o','f','t','w','a','r','e','T','e','s','t','i','n','g','H','e','l','p'}; 
 
        char[] dest_array = new char[19];  

        System.arraycopy(src_array, 0, dest_array, 0,19);
        System.out.println("Source array:" + String.valueOf(src_array));

        System.out.println("Destination array after arraycopy:"+ String.valueOf(dest_array));  
    }  
}  

Output:

Using System.arraycopy

In the above program, we use the ‘arraycopy’ method to copy an array to another array. You can see the call to the arraycopy method. We copy the source array from the beginning (0th location) and copy the entire array.

Lastly, we display the string equivalent of both the source as well as destination arrays.

With the arraycopy method, you can copy even partial arrays as it takes the start and end element positions as arguments. This method makes a shallow copy of array elements.

Using Arrays.copyOf()

The method Arrays.copyOf() internally makes use of the System.arraycopy () method. Though it is not as efficient as arraycopy, it can be used to copy full or partial array just like the arraycopy method.

‘copyOf()’ method is a part of the java.util package and belongs to the “Arrays” class.

The prototype of this method is as follows:

public static int[] copyOf(int[] original_array,int newLength)

Where,

  • original: The array to be copied to the new array.
  • newLength: The length of the copied array to be returned.

Thus, this method makes a copy of the array provided in the first argument to the specified length by truncating or padding the length with 0 to the new array. This means if the length of the copied array is more than the original array, 0s are replaced for the remaining elements.

The program given below shows an example of the copyOf method.

importjava.util.Arrays; 

public class Main 
{ 
public static void main(String args[]) 
    { 
        // define original array 
        int[] even_Array = new int[] {2,4,6,8}; 

         System.out.println("Original Array:" + Arrays.toString(even_Array));

        // copying array even_Array to copy_Array
        int[] copy_Array = Arrays.copyOf(even_Array,5); 

        System.out.println("Copied Array:" + Arrays.toString(copy_Array));
        // assign value to unassigned element of copied array 
        copy_Array[4] = 10; 

          System.out.println("Copied and modified Array:" + Arrays.toString(copy_Array));
    } 
}

Output:

Using Arrays.copyOf

In the above program, we copy the even_Array of length 4 by using the copyOf method. The second argument provided is 5. Hence, the new copied array has 5 elements in it. The first four are the same as the original array and the fifth element is 0 as copyOf pads it because the length of the original array is less than that of the new array.

Using Arrays.copyOfRange()

The method Arrays.copyOfRange() is specifically used when you want to copy partial arrays. Like copyOf() method, this method also internally makes use of System.arraycopy() method.

The prototype of Arrays.copyOfRange() method is as follows:

public static short[] copyOfRange(short[] original, int from, int to)

where,

  • original: The array from which a range is to be copied.
  • from: Initial index of the range to be copied, inclusive.
  • to: The final index of the range to be copied, exclusive.

An example implementation of the copyOfRange method is shown below.

import java.util.Arrays; 
class Main { 
     public static void main(String args[]) 
    { 
         int intArray[] = { 10,20,30,40,50 }; 

        // to index is within the range 
        int[] copyArray = Arrays.copyOfRange(intArray, 2, 6); 
             System.out.println("Array copy with both index within the range: " + 
Arrays.toString(copyArray));

       //to index is out of range
       int[] copy1 = Arrays.copyOfRange(intArray, 4, intArray.length + 3); 

       System.out.println("Array copy with to index out of range: " + Arrays.toString(copy1));

    } 
}

Output:

Using Arrays.copyOfRange

Using Object.clone()

Java array internally implements a Cloneable interface and thus it is easy to clone a Java array. You can clone one-dimensional as well as two-dimensional arrays. When you clone one-dimensional array, it makes a deep copy of array elements which is copying the values.

On the other hand, when you clone two dimensional or multi-dimensional arrays, a shallow copy of elements is made i.e. only references are copied. This cloning of arrays is done by the ‘Clone ()’ method provided by the arrays.

A deep copy of 1-D arrays as a result of cloning can be represented as below:

Using Object.clone

Now let's implement the 1-D array cloning in a Java program.

class Main 
{     
     public static void main(String args[])  
    { 
     int num_Array[] = {5,10,15,20,25,30}; 

     int clone_Array[] = num_Array.clone(); 

    System.out.println("Original num_Array:");
    for (int i = 0; i <num_Array.length; i++) { 
            System.out.print(num_Array[i]+" "); 
        } 
        System.out.println();

       System.out.println("Cloned num_Array:");
       for (int i = 0; i <clone_Array.length; i++) { 
            System.out.print(clone_Array[i]+" "); 
        } 
      System.out.println("\n");

      System.out.print("num_Array == clone_Array = "); 
      System.out.println(num_Array == clone_Array); 
    } 
}

Output:

1-D array cloning in a Java program

As you can see in the output, the expression to check for equality of both the arrays returns false. This is because the cloning of one-dimensional array results in deep copy wherein the values are copied to a new array and not merely references.

Frequently Asked Questions

Q #1) How to make a copy of an array in Java?

Answer: There are different methods to copy an array.

  • You can use a for loop and copy elements of one to another one by one.
  • Use the clone method to clone an array.
  • Use arraycopy() method of System class.
  • Use copyOf() or copyOfRange() methods of Arrays class.

Q #2) How do you assign one array to another?

Answer: You can assign the array to another using a simple assignment operator (=). You have to ensure that the two arrays are of the same data type and have an identical dimension.

Q #3) What is a Shallow copy and Deep copy?

Answer: In shallow copy, only the attributes of objects or arrays in question are copied. So any changes to the copied array will reflect in the original. Java cloning is an example of a shallow copy.

A deep copy is the one wherein we need a complete copy of the object so that when we clone or copy that object, it is an independent copy. When primitive or built-in types are involved, there is no difference between the shallow and deep copy.

Q #4) What does an Array Clone do?

Answer: The cloning method of arrays is used to copy the attributes of one object to another. It uses a shallow copy for doing this.

Q #5) Can you store an Array in an Array?

Answer: Arrays can contain arrays provided with the elements that are of the same type (primitive or object). This means you cannot store an integer array in a string array.

Conclusion

In this tutorial, we explored copy array and clone arrays in Java. We have seen various methods/approaches to copy and clone an array.

Note that most of these methods implement a shallow copy. For primitive data types, shallow and deep copy does not differ. But when an array contains objects or references, the programmer needs to implement a deep copy as per the requirements.

In our subsequent tutorials, we continue to explore more about Java arrays.

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