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How to resize an array in Java

Tarun Telang

Overview

The size of an array is the number of elements it can hold. In Java, the array size is fixed when it is created.

The easiest way to resize an array is to create a new array with the desired size and copy the contents of the old array into it. We can delete the old array by setting it to null.

For this, we use the arraycopy() method of the System class or copyOf() method of the java.util.Arrays class.

The system.arraycopy() function

The arraycopy() function is used to copy elements from one array to another.

Syntax

void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
Syntax of `arraycopy()` function

Parameters

  • src: This is the source array.
  • srcPos: This is the starting position in the source array.
  • dest: This is the destination array.
  • destPos: This is the starting position in the destination array.
  • length: This is the number of elements to be copied.

Return value

This function returns no value. Its return type is void.

Resize an array to a larger size

Below is the code example of resizing an array to a larger size. For example, if we have an array of size 3 and we want to resize it to size 6, we can do it as follows:

Code example 1

import java.util.Arrays;

class HelloWorld {
    public static void main( String args[] ) {
      int[] oldArray = {1, 2, 3};
      int[] newArray = new int[6];
      System.arraycopy(oldArray, 0, newArray, 0, 3);
      oldArray = null;
      System.out.println(Arrays.toString(newArray));
    }
}
resizing an array to ar larger size

Explanation

  • Line 5: We declare an integer array of three elements, named oldArray , with values 1, 2, and 3.
  • Line 6: We declare an integer array of six elements, named newArray. It is double the size of oldArray.
  • Line 7: We copy the elements of oldArray into newArray using the arraycopy() method.
  • Line 8: We set the oldArray to null, freeing up the memory it was using.
  • Line 10: We print out the contents of newArray.

Note: This will only work if the new array is larger than the old one. If we want to resize an array to a smaller size, we'll have to create a new array and copy only the desired elements into it.

Resize an array to a smaller size

The code example below shows how to resize an array to a smaller size. For example, consider an array of size 10 and we want to resize it to size 5 by copying the last 5 elements:

Code example 2

import java.util.Arrays;

class HelloWorld {
    public static void main( String args[] ) {
      int[] oldArray = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
      int[] newArray = new int[5];
      System.arraycopy(oldArray, 5, newArray, 0, 5);
      oldArray = null;
      System.out.println(Arrays.toString(newArray));
    }
}
resizing an array to smaller size

Explanation

  • Line 5: We declare an array oldArray of size 10.
  • Line 6: We create a new array named newArray, that is half the size of oldArray.
  • Line 7: We copy the elements of oldArray starting from index 5 into newArray using the arraycopy() method. We have copied only the last 5 elements from oldArray into newArray.
  • Line 8: We set oldArray to null, freeing up the memory it was using.
  • Line 10: We print out the contents of newArray.

The java.util.Arrays class has a method named copyOf(), which can also be used to increase or decrease the size of an array. We can also use this method to resize an Array.

The arrays.copyOf() function

The copyOf() method is a utility function that creates a new array with the same contents as an existing array. The new array is always of the same type as the original array. This method is useful for creating a copy of an array that will not be modified.

Syntax

public static int[] copyOf(int[] array,int length)
Syntax of `copyOf()` function

Parameters

  • array: This is the array that we want to resize or copy.
  • length: This is the size of the new array. If the new size is greater than the size of the original array, extra elements will be filled with the default values. If the new size is less than the old size, extra elements will be truncated. This function is also overloaded for other types.

Example

import java.util.Arrays;
class ArrayResize {
    public static void main(String[] args) {
        int[] oldArray = {1,2,3,4,5};
        int newSize = 10; // resize to 10 elements
        int[] newArray = Arrays.copyOf(oldArray, newSize);
        oldArray = null;
        //printing elements of new array
        System.out.println(Arrays.toString(newArray));
    }
}
Resizing an array using Arrays.copyOf() method

Explanation

  • Line 4: We declare an integer array of five elements, named oldArray , with values 1, 2, 3, 4, and 5.
  • Line 5: We create a new array named newArray. It is larger in size than oldArray.
  • Line 6: We copy the elements of oldArray into newArray using the Arrays.copyOf() method.
  • Line 7: We set oldArray to null, freeing up the memory it was using.
  • Line 9: We print out the contents of newArray.

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