Array Copy in Java

26 Oct, 2021
Given an array, we need to copy its elements in a different array, to a naive user below way comes into mind which is however incorrect as depicted below as follows:
// Java Program to Illustrate Wrong Way Of Copying an Array

// Input array
int a[] = { 1, 8, 3 };

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

// Doesn't copy elements of a[] to b[], only makes
// b refer to same location
b = a;
Output: 
Output Explanation: When we do “b = a”, we are actually assigning a reference to the array. Hence, if we make any change to one array, it would be reflected in other arrays as well because both a and b refer to the same location. We can also verify it with code as shown below as follows:
Example:
Java
// A Java program to demonstrate that simply// assigning one array reference is incorrectpublic class Test {    public static void main(String[] args)    {        int a[] = { 1, 8, 3 };         // Create an array b[] of same size as a[]        int b[] = new int[a.length];         // Doesn't copy elements of a[] to b[],        // only makes b refer to same location        b = a;         // Change to b[] will also reflect in a[]        // as 'a' and 'b' refer to same location.        b[0]++;         System.out.println("Contents of a[] ");        for (int i = 0; i < a.length; i++)            System.out.print(a[i] + " ");         System.out.println("\n\nContents of b[] ");        for (int i = 0; i < b.length; i++)            System.out.print(b[i] + " ");    }}
Contents of a[] 
2 8 3 

Contents of b[] 
2 8 3 
Methods:
We have seen internal working while copying elements and edge cases to be taken into consideration after getting through errors as generated above, so now we can propose out correct ways to copy array as listed below as follows:
Iterating each element of the given original array and copy one element at a timeUsing clone() methodUsing arraycopy() methodUsing copyOf() method of Arrays classUsing copyOfRange() method of Arrays class
Iterating each element of the given original array and copy one element at a time
Using clone() method
Using arraycopy() method
Using copyOf() method of Arrays class
Using copyOfRange() method of Arrays class
Method 1: Iterating each element of the given original array and copy one element at a time. With the usage of this method, it guarantees that any modifications to b, will not alter the original array a, as shown in below example as follows:
Example:
Java
// Java program to demonstrate copying by// one by one assigning elements between arrays // Main classpublic class GFG {     // Main driver method    public static void main(String[] args)    {        // Input array a[]        int a[] = { 1, 8, 3 };         // Create an array b[] of same size as a[]        int b[] = new int[a.length];         // Copying elements of a[] to b[]        for (int i = 0; i < a.length; i++)            b[i] = a[i];         // Changing b[] to verify that        // b[] is different from a[]        b[0]++;         // Display message only        System.out.println("Contents of a[] ");         for (int i = 0; i < a.length; i++)            System.out.print(a[i] + " ");         // Display message only        System.out.println("\n\nContents of b[] ");         for (int i = 0; i < b.length; i++)            System.out.print(b[i] + " ");    }}
Contents of a[] 
1 8 3 

Contents of b[] 
2 8 3 
Method 2: Using Clone() method 
In the previous method we had to iterate over the entire array to make a copy, can we do better? Yes, we can use the clone method in Java. 
Example:
Java
// Java program to demonstrate Copying of Array// using clone() method // Main classpublic class GFG {     // Main driver method    public static void main(String[] args)    {        // Input array a[]        int a[] = { 1, 8, 3 };         // Copying elements of a[] to b[]        int b[] = a.clone();         // Changing b[] to verify that        // b[] is different from a[]        b[0]++;         // Display message for better readability        System.out.println("Contents of a[] ");         for (int i = 0; i < a.length; i++)            System.out.print(a[i] + " ");         // Display message for better readability        System.out.println("\n\nContents of b[] ");         for (int i = 0; i < b.length; i++)            System.out.print(b[i] + " ");    }}
Contents of a[] 
1 8 3 

Contents of b[] 
2 8 3 
Method 3: Using arraycopy() method
We can also use System.arraycopy() Method. The system is present in java.lang package. Its signature is as : 
public static void arraycopy(Object src, int srcPos, Object dest, 
                             int destPos, int length)
Parameters:
src denotes the source array.
srcPos is the index from which copying starts.
dest denotes the destination array
destPos is the index from which the copied elements are placed in the destination array.
length is the length of the subarray to be copied.
Example:
Java
// Java program to demonstrate array// copy using System.arraycopy() // Main classpublic class GFG {     // Main driver method    public static void main(String[] args)    {        // Custom input array        int a[] = { 1, 8, 3 };         // Creating an array b[] of same size as a[]        int b[] = new int[a.length];         // Copying elements of a[] to b[]        System.arraycopy(a, 0, b, 0, 3);         // Changing b[] to verify that        // b[] is different from a[]        b[0]++;         // Display message only        System.out.println("Contents of a[] ");         for (int i = 0; i < a.length; i++)            System.out.print(a[i] + " ");         // Display message only        System.out.println("\n\nContents of b[] ");         for (int i = 0; i < b.length; i++)            System.out.print(b[i] + " ");    }}
Contents of a[] 
1 8 3 

Contents of b[] 
2 8 3 
Method 4: Using copyOf() method of Arrays class 
If we want to copy the first few elements of an array or a full copy of the array, you can use this method.
Syntax: 
public static int[] copyOf​(int[] original, int newLength) 
Parameters:
Original array
Length of the array to get copied.
Example:
Java
// Java program to demonstrate array// copy using Arrays.copyOf() // Importing Arrays class from utility classimport java.util.Arrays; // Main classclass GFG {     // Main driver method    public static void main(String[] args)    {        // Custom input array        int a[] = { 1, 8, 3 };         // Create an array b[] of same size as a[]        // Copy elements of a[] to b[]        int b[] = Arrays.copyOf(a, 3);         // Change b[] to verify that        // b[] is different from a[]        b[0]++;         System.out.println("Contents of a[] ");         // Iterating over array. a[]        for (int i = 0; i < a.length; i++)            System.out.print(a[i] + " ");         System.out.println("\n\nContents of b[] ");         // Iterating over array b[]        for (int i = 0; i < b.length; i++)            System.out.print(b[i] + " ");    }}
Contents of a[] 
1 8 3 

Contents of b[] 
2 8 3 
Method 5: Using copyOfRange() method of Arrays class
This method copies the specified range of the specified array into a new array.
public static int[] copyOfRange​(int[] original, int from, int to)
Parameters:
Original array from which a range is to be copied
Initial index of the range to be copied
Final index of the range to be copied, exclusive
Example:
Java
// Java program to demonstrate array// copy using Arrays.copyOfRange() // Importing Arrays class from utility packageimport java.util.Arrays; // Main classclass GFG {     // Main driver method    public static void main(String[] args)    {        // Custom input array        int a[] = { 1, 8, 3, 5, 9, 10 };         // Creating an array b[] and        // copying elements of a[] to b[]        int b[] = Arrays.copyOfRange(a, 2, 6);         // Changing b[] to verify that        // b[] is different from a[]         // Iterating over array a[]        System.out.println("Contents of a[] ");        for (int i = 0; i < a.length; i++)            System.out.print(a[i] + " ");         // Iterating over array b[]        System.out.println("\n\nContents of b[] ");        for (int i = 0; i < b.length; i++)            System.out.print(b[i] + " ");    }}
Contents of a[] 
1 8 3 5 9 10 

Contents of b[] 
3 5 9 10 
Lastly, let us do discuss the overview of the above methods: 
Simply assigning references is wrong
The array can be copied by iterating over an array, and one by one assigning elements.
We can avoid iteration over elements using clone() or System.arraycopy()
clone() creates a new array of the same size, but System.arraycopy() can be used to copy from a source range to a destination range.
System.arraycopy() is faster than clone() as it uses Java Native Interface
If you want to copy the first few elements of an array or a full copy of an array, you can use Arrays.copyOf() method.
Arrays.copyOfRange() is used to copy a specified range of an array. If the starting index is not 0, you can use this method to copy a partial array.
This article is contributed by Ashutosh Kumar. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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