This article aims to guide developers how to convert a collection of Integer type into an array. Each element in the array is twice the corresponding element in the collection. We'll explore two ways to implement this conversion using the Java Stream API, producing arrays of type int[] and Integer[] respectively, emphasizing the impact of collection unordering on the result.

Double and convert collection elements to array using Java Stream API

The Java Stream API provides powerful data processing capabilities and can easily convert collections. Here are two ways how to use the Stream API to convert a collection of type Integer into an array and double the value of each element.

Method 1: Convert to int[] array

This method uses IntStream to handle integers, which can avoid boxing and unboxing operations and improve performance.

 import java.util.Set;
import java.util.HashSet;

public class ArrayDoubling {

    public static void main(String[] args) {
        Set<integer> itemSet = new HashSet();
        itemSet.add(1);
        itemSet.add(3);
        itemSet.add(5);

        int[] arr = itemSet.stream().mapToInt(i -> i * 2).toArray();

        //Print the result for (int num : arr) {
            System.out.print(num " ");
        }
        System.out.println();
    }
}</integer>

Code explanation:

1. itemSet.stream(): Convert Set to a Stream.
2. mapToInt(i -> i * 2): Convert Stream to IntStream using mapToInt method and multiply each element by 2. i -> i * 2 is a lambda expression that defines a function that doubles each integer.
3. toArray(): Convert IntStream to an int[] array.

Things to note:

• HashSet is an unordered set. This means that the order of elements in the converted array may be different from the order in which the elements were added. If you need to maintain order, you can use LinkedHashSet.

Method 2: Convert to Integer[] array

This method uses a Stream directly and converts it to an Integer[] array using toArray(Integer[]::new).

 import java.util.Set;
import java.util.HashSet;

public class ArrayDoubling {

    public static void main(String[] args) {
        Set<integer> itemSet = new HashSet();
        itemSet.add(1);
        itemSet.add(3);
        itemSet.add(5);

        Integer[] arr = itemSet.stream().map(i -> i * 2).toArray(Integer[]::new);

        //Print the result for (Integer num : arr) {
            System.out.print(num " ");
        }
        System.out.println();
    }
}</integer>

Code explanation:

1. itemSet.stream(): Convert Set to a Stream.
2. map(i -> i * 2): Use the map method to multiply each element in the Stream by 2. i -> i * 2 is a lambda expression that defines a function that doubles each integer.
3. toArray(Integer[]::new): Convert Stream to Integer[] array. Integer[]::new is a constructor reference used to create an array of the specified type.

Things to note:

• Similar to the first method, a HashSet is an unordered set, so the order of elements in the converted array may be different from the order in which the elements were added.
• Using Integer[] requires boxing operation, and the performance is slightly lower than int[].

Summarize:

Both methods can achieve the function of doubling the elements of the collection and converting it to an array. Which method you choose depends on your specific needs. If you have high performance requirements, it is recommended to use the first method and convert to an int[] array. If you need an array of type Integer[], you can use the second method. You need to pay attention to the disorder of the set when using it. If you need to maintain the order, you can use LinkedHashSet.

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