Java Program to Convert Integer List to Integer Array - GeeksforGeeks

17 Jun, 2021
There are many ways to convert integer List to ArrayList where in this article we will be discussing out 2 approaches as below:
Using concept od streams in Java8Using Apache Commons LangUsing Guava Library
Using concept od streams in Java8
Using Apache Commons Lang
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
// Java Program to Convert Integer List to Integer Array  // Importing Arrays and List classes// from java.util packageimport java.util.Arrays;import java.util.List;  // Main classclass GFG {      // Main driver method    public static void main(String[] args)    {          // Creating an List of integer type and        // customly inserting input elements a arguments        List<Integer> list = Arrays.asList(1, 3, 5, 7, 9);          // Converting Stream<Integer> 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:
 Use toPrimtive() method of Apache Common Lang’sUse List.toArray() method
 Use toPrimtive() method of Apache Common Lang’s
Use List.toArray() method
Example:
Java
// Java Program to Convert Integer List to Integer Array  // Importing ArrayUtis class// Importing Arrays and List class from java.util packageimport java.util.Arrays;import java.util.List;import org.apache.commons.lang3.ArrayUtils;  // Main classclass GFG {      // Main driver method    public static void main(String[] args)    {          // Creating an object of List class of integer type        List<Integer> 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
// Java Program to Convert List to Integer Array// Using Guava Library  // Importing required classesimport com.google.common.primitives.Ints;import java.util.Arrays;import java.util.List;  // Main classclass GFG {      // Main driver method    public static void main(String[] args)    {          // Creating an object of List class of integer type        List<Integer> 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
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