Arrays.copyOf in Java With Example

Arrays.copyOf in Java With Example | The copyOf() method of java.util.Arrays class copies the specified array, truncating or padding with zeros/nulls (if necessary). Internally, this method calls the System.arraycopy() method of Java.

There is a total of 10 overloaded forms of Arrays.copyOf() method. Those methods are listed here,

  • public static byte[] copyOf(byte[] original, int newLength)
  • public static short[] copyOf(short[] original, int newLength)
  • public static int[] copyOf(int[] original, int newLength)
  • public static long[] copyOf(long[] original, int newLength)
  • public static char[] copyOf(char[] original, int newLength)
  • public static float[] copyOf(float[] original, int newLength)
  • public static double[] copyOf(double[] original, int newLength)
  • public static boolean[] copyOf(boolean[] original, int newLength)
  • public static <T> T[] copyOf(T[] original, int newLength)
  • public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType)

In these methods the arguments are,

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

Let us see demonstration of the copyOf() method of Arrays class.

import java.util.Arrays;

public class ArrayTest {
  public static void main(String[] args) {
    
    // original array
    int[] arr = {10, 20, 30, 40, 50};
    System.out.println("Original array = " +
                        Arrays.toString(arr));
    
    // Example1
    int[] newArray1 = Arrays.copyOf(arr, 3);
    System.out.println("NewArray1 = " + 
                        Arrays.toString(newArray1));
    
    // Example2
    int[] newArray2 = Arrays.copyOf(arr, 5);
    System.out.println("NewArray2 = " + 
                        Arrays.toString(newArray2));
    
    // Example3
    int[] newArray3 = Arrays.copyOf(arr, 7);
    System.out.println("NewArray3 = " + 
                        Arrays.toString(newArray3));

  }
}

Output:-

Original array = [10, 20, 30, 40, 50]
NewArray1 = [10, 20, 30]
NewArray2 = [10, 20, 30, 40, 50]
NewArray3 = [10, 20, 30, 40, 50, 0, 0]

How these different outputs are coming? To understand these, we need to see how copyOf() method of the Arrays class is implemented?

How Arrays.copyOf() is implemented?

Whenever we call Arrays.copyOf(arr, newLength) then,

PrepBytes
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a) One local array variable is created with the size of passed length “newLength”
b) Then it internally calls System.arraycopy() method
c) Return the copied array

The source code for the copyOf() method with int[] array is given as,

public static int[] copyOf(int[] original, int newLength) {
     int[] copy = new int[newLength];
     System.arraycopy(original, 0, copy, 0,
                  Math.min(original.length, newLength));
     return copy;
 }

All the overloaded forms of Arrays.copyOf() method is implemented in a similar way. In System.arraycopy() method we have seen that the last argument was a length. The Arrays.copyOf() method internally finds the minimum value between original array length and newLength.

If newLength is minimum compared to the length of the original array then, only that portion of array elements will be copied from the original array. Example:-

// original array
int[] arr = {10, 20, 30, 40, 50};
// new array
int[] newArray = Arrays.copyOf(arr, 3);
System.out.println(Arrays.toString(newArray));

Output:-

[10, 20, 30]

The length of the original array arr is 5, see how to find the length of the Java array. We want to copy only 3 elements therefore we are calling Arrays.copyOf(arr, 3); Internally this method we will find Math.min(5, 3) => 3, and since 3 is lesser between 5 and 3, therefore, it will call System.arraycopy(arr, 0, copy, 0, 3); Only three-element will copied to the temporary array, and returned to the caller method.

But if the original array length is minimum compared to the newLength then the remaining elements will be filled by default values. Let us see it through an example,

// original array
int[] arr = {10, 20, 30, 40, 50};
// new array
int[] newArray = Arrays.copyOf(arr, 7);
System.out.println(Arrays.toString(newArray));

Output:-

[10, 20, 30, 40, 50, 0, 0]

In this example, we have passed newLength as 7. The length of the original array is 5 but we are passing 7. Internally after calling Arrays.copyOf(arr, 7);

  • One copy[] will be created with length 7. Since it is array creation with default value therefore the copy array will holds [0, 0, 0, 0, 0, 0, 0] because 0 is default value of int data type.
  • Now, it calls System.arraycopy(arr, 0, copy, 0, 5), and all five elements of the original array will be copied to copy[].
  • The remaining 2 elements will remain as it is.

Examples to Copy Double and Char array using Arrays.copyOf()

// double array
double[] doubleArr = {10.5, 15.9, 500, -88888, 0.9};
System.out.println("Double Array = " + Arrays.toString(doubleArr));

// copy array
double[] arr1 = Arrays.copyOf(doubleArr, 3);
System.out.println("Arr1 = " + Arrays.toString(arr1));

double[] arr2 = Arrays.copyOf(doubleArr, doubleArr.length);
System.out.println("Arr2 = " + Arrays.toString(arr2));

double[] arr3 = Arrays.copyOf(doubleArr, 7);
System.out.println("Arr3 = " + Arrays.toString(arr3));

Output:-

Double Array = [10.5, 15.9, 500.0, -88888.0, 0.9]
Arr1 = [10.5, 15.9, 500.0]
Arr2 = [10.5, 15.9, 500.0, -88888.0, 0.9]
Arr3 = [10.5, 15.9, 500.0, -88888.0, 0.9, 0.0, 0.0]

// char array
char[] charArr = {'K','n','o','w','P','r','o','g','r','a','m'};
System.out.println("Char Array = " + Arrays.toString(charArr));

// copy array
char[] arr1 = Arrays.copyOf(charArr, 4);
System.out.println("Arr1 = " + Arrays.toString(arr1));

char[] arr2 = Arrays.copyOf(charArr, charArr.length);
System.out.println("Arr2 = " + Arrays.toString(arr2));

char[] arr3 = Arrays.copyOf(charArr, 15);
System.out.println("Arr3 = " + Arrays.toString(arr3));

Output:-

Char Array = [K, n, o, w, P, r, o, g, r, a, m]
Arr1 = [K, n, o, w]
Arr2 = [K, n, o, w, P, r, o, g, r, a, m]
Arr3 = [K, n, o, w, P, r, o, g, r, a, m,

Additional Points

  • The copyOf() method with all its overloaded forms was introduced in Java 1.6 version.
  • This method throws NullPointerException if the passed original array is null. Therefore, it is better to check if(original != null) and then call copyOf().
  • It also throws NegativeArraySizeException if the passed length (i.e. newLength) is negative.
import java.util.Arrays;
public class ArrayTest {
  public static void main(String[] args) {
    int[] arr = null;
    int[] newArray1 = Arrays.copyOf(arr, 3);
  }
}

Exception in thread “main” java.lang.NullPointerException: Cannot read the array length because “original” is null
at java.base/java.util.Arrays.copyOf(Arrays.java:3585)
at ArrayTest.main(ArrayTest.java:5)

import java.util.Arrays;
public class ArrayTest {
  public static void main(String[] args) {
    int[] arr = null;
    int[] newArray1 = Arrays.copyOf(arr, -3);
  }
}

Exception in thread “main” java.lang.NegativeArraySizeException: -3
at java.base/java.util.Arrays.copyOf(Arrays.java:3584)
at ArrayTest.main(ArrayTest.java:5)

Copy Multidimensional Array using copyOf() of Java Arrays

The Arrays.copyOf() method also can be used to copy the Java multidimensional array like 2D Java array and 3D Java array.

Java program to find the copy of two dimensional (2D) Java array using Arrays.copyof() method,

import java.util.Arrays;

public class ArrayTest {
  public static void main(String[] args) {
    
    // original 2D array 3x2 
    int matrix[][] = { { 1, 2 }, { 4, 5 }, { 7, 8 } };
    System.out.println("Original = " + 
               Arrays.deepToString(matrix));

    // Example1
    int[][] newMatrix1 = Arrays.copyOf(matrix, 1);
    System.out.println("newMatrix1 = " + 
               Arrays.deepToString(newMatrix1));
    
    // Example2
    int[][] newMatrix2 = Arrays.copyOf(matrix, 3);
    System.out.println("newMatrix2 = " + 
               Arrays.deepToString(newMatrix2));
    
    // Example3
    int[][] newMatrix3 = Arrays.copyOf(matrix, 5);
    System.out.println("newMatrix3 = " + 
               Arrays.deepToString(newMatrix3));

  }
}

Output:-

Original = [[1, 2], [4, 5], [7, 8]]
newMatrix1 = [[1, 2]]
newMatrix2 = [[1, 2], [4, 5], [7, 8]]
newMatrix3 = [[1, 2], [4, 5], [7, 8], null, null]

Java program to find the copy of three dimensional (3D) Java array using Arrays.copyof() method,

import java.util.Arrays;

public class ArrayTest {
  public static void main(String[] args) {
    
    // 3D array 2x3x2
    int[][][] arr = {{{1,2},{3,4},{5,6}},{{7,8},{9,1},{2,3}}};
    System.out.println("Original = " + 
               Arrays.deepToString(arr));

    // Example1
    int[][][] newArr1 = Arrays.copyOf(arr, 1);
    System.out.println("newArr1 = " + 
               Arrays.deepToString(newArr1));
    
    // Example2
    int[][][] newArr2 = Arrays.copyOf(arr, 2);
    System.out.println("newArr2 = " + 
               Arrays.deepToString(newArr2));
    
    // Example3
    int[][][] newArr3 = Arrays.copyOf(arr, 4);
    System.out.println("newArr3 = " + 
               Arrays.deepToString(newArr3));

  }
}

Output:-

Original = [[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 1], [2, 3]]]
newArr1 = [[[1, 2], [3, 4], [5, 6]]]
newArr2 = [[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 1], [2, 3]]]
newArr3 = [[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 1], [2, 3]], null, null]

Copy Array of Objects

Using the Arrays.copyOf() method, we can also copy the array of objects. It is very similar to the previous programs. In the Arrays.toString() method, we have discussed how to display an array of objects we will use that concept.

class Student {
private int idNum;
private String name;

// constructor
public Student(int idNum, String name) {
this.idNum = idNum;
this.name = name;
}

@Override
public String toString() {
return "[" + idNum + ", " + name + "]";
}
}
import java.util.Arrays;

public class Collage {
  public static void main(String[] args) {

    // array of Student objects
    Student[] st = new Student[3];

    // initialize Student object
    st[0] = new Student(100, "Emma");
    st[1] = new Student(115, "Alex");
    st[2] = new Student(225, "Amelia");

    // display Student details (Original)
    System.out.println("Array = " + Arrays.toString(st));

    // Example1:- copy only first Student objects
    Student[] st1 = Arrays.copyOf(st, 1);
    // display new array
    System.out.println("St1 = " + Arrays.toString(st1));
    
    // Example2:- copy all Student objects
    Student[] st2 = Arrays.copyOf(st, st.length);
    // display new array
    System.out.println("St2 = " + Arrays.toString(st2));
    
    // Example3:- copy more then 3 Student objects
    Student[] st3 = Arrays.copyOf(st, 5);
    // display new array
    System.out.println("St3 = " + Arrays.toString(st3));
  }
}

Output:-

Array = [[100, Emma], [115, Alex], [225, Amelia]]
St1 = [[100, Emma]]
St2 = [[100, Emma], [115, Alex], [225, Amelia]]
St3 = [[100, Emma], [115, Alex], [225, Amelia], null, null]

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