There are many ways to convert integer List to ArrayList where in this article we will be discussing out 2 approaches as below:

1. Using concept od streams in Java8
2. Using Apache Commons Lang
3. Using Guava Library

Method 1: Using concept od streams in Java8

So, first, we will get to know about how to convert integer list to array list using java 8. In java 8 there is a stream provided for converting a list of integer to an integer array.

Procedure:

• Convert List<Integer> to Stream<Integer> using List.stream() —> Here List is calling stream() method.
• Now we convert Stream<Integer> to int[].

Example:

// Java Program to Convert Integer List to Integer Array

// Importing Arrays and List classes
// from java.util package
import java.util.Arrays;
import java.util.List;

// Main class
class GFG {

	// Main driver method
	public static void main(String[] args)
	{

		// Creating an List of integer type and
		// customly inserting input elements a arguments
		List list = Arrays.asList(1, 3, 5, 7, 9);

		// Converting Stream to integer array
		// using stream() and mapToInt() methods, lately
		// storing them in an integer array
		int[] primitive = list.stream()
							.mapToInt(Integer::inValue)
							.toArray();

		// Print and display the integer array
		System.out.println(Arrays.toString(primitive));
	}
}

Output:

[1,3,5,7,9]

Method 2: Using Apache Commons Lang

Apache Commons Lang’s ArrayUtils class provides toPrimitive() method that can convert an Integer array to primitive ints. We need to convert a list of integers to an Integer array first. We can use List.toArray() for easy conversion.

Procedure:

1.  Use toPrimtive() method of Apache Common Lang’s
2. Use List.toArray() method

Example:

// Java Program to Convert Integer List to Integer Array

// Importing ArrayUtis class
// Importing Arrays and List class from java.util package
import java.util.Arrays;
import java.util.List;
import org.apache.commons.lang3.ArrayUtils;

// Main class
class GFG {

	// Main driver method
	public static void main(String[] args)
	{

		// Creating an object of List class of integer type
		List list = Arrays.asList(1, 2, 3, 4, 5);

		// Using toPrimtive() method of Apache Common Lang's
		int[] primitive = ArrayUtils.toPrimitive(
			list.toArray(new Integer[0]));

		// Print and display the integer array
		System.out.println(Arrays.toString(primitive));
	}
}

Output:

[1,2,3,4,5]

Method 3: Using Guava Library

Guava library is an open-source decentralized software development model which is a set of common libraries of java providing utility methods for collections, caching, primitives support, concurrency, string processing, and validations. Here we will be glancing at string processing in the guava library by converting integer list to integer array. It is depicted in below example as follows:

Example

// Java Program to Convert List to Integer Array
// Using Guava Library

// Importing required classes
import com.google.common.primitives.Ints;
import java.util.Arrays;
import java.util.List;

// Main class
class GFG {

	// Main driver method
	public static void main(String[] args)
	{

		// Creating an object of List class of integer type
		List ints = Arrays.asList(3, 7, 8, 6, 1);

		// Converting a list of Integer to primitive integer
		// array and storing in integer array
		int[] primitive = Ints.toArray(ints);

		// Print and display the integer array via Guava
		// by converting it to string
		// via standard toString() method
		System.out.println(Arrays.toString(primitive));
	}
}

Output:

3,7,8,6,1