Java 8 - Convert Array to List Using Stream

Introduction

In Java 8, converting an array to a list is a common task when working with collections. While the traditional approach uses Arrays.asList(), the Stream API offers a functional and flexible way to convert an array into a list using streams. By converting the array to a stream, you can easily perform additional operations like filtering, mapping, or sorting before collecting the elements into a list.

In this guide, we will learn how to convert an array to a list using Java 8's Stream API.

Solution Steps

1. Define the Array: Create an array of elements (e.g., strings or integers) that you want to convert to a list.
2. Convert the Array to a Stream: Use Arrays.stream() or Stream.of() to convert the array to a stream.
3. Collect the Stream to a List: Use the collect(Collectors.toList()) method to collect the stream elements into a list.
4. Display the Result: Print or use the resulting list.

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Java Program

Example 1: Convert an Array of Strings to a List

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ArrayToListUsingStream {
    public static void main(String[] args) {
        // Step 1: Define the array of strings
        String[] fruits = {"Apple", "Banana", "Orange", "Mango"};

        // Step 2: Convert the array to a Stream and collect to a List
        List<String> fruitList = Arrays.stream(fruits)
                                       .collect(Collectors.toList());

        // Step 3: Display the List
        System.out.println(fruitList);
    }
}

Output

[Apple, Banana, Orange, Mango]

Explanation

Step 1: Define the Array

We define an array of strings:

String[] fruits = {"Apple", "Banana", "Orange", "Mango"};

Step 2: Convert the Array to a Stream and Collect to a List

We use Arrays.stream() to convert the array into a stream, and then collect the stream elements into a list using Collectors.toList():

List<String> fruitList = Arrays.stream(fruits)
                               .collect(Collectors.toList());

Step 3: Display the List

We print the resulting list:

System.out.println(fruitList);

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Example 2: Convert an Array of Integers to a List

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ArrayToListOfIntegers {
    public static void main(String[] args) {
        // Step 1: Define the array of integers
        Integer[] numbers = {1, 2, 3, 4, 5};

        // Step 2: Convert the array to a Stream and collect to a List
        List<Integer> numberList = Arrays.stream(numbers)
                                         .collect(Collectors.toList());

        // Step 3: Display the List
        System.out.println(numberList);
    }
}

Output

[1, 2, 3, 4, 5]

Explanation

Step 1: Define the Array

We define an array of integers:

Integer[] numbers = {1, 2, 3, 4, 5};

Step 2: Convert the Array to a Stream and Collect to a List

We use Arrays.stream() to convert the array to a stream and collect the elements into a list:

List<Integer> numberList = Arrays.stream(numbers)
                                 .collect(Collectors.toList());

Step 3: Display the List

We print the resulting list:

System.out.println(numberList);

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Example 3: Convert a Primitive Array to a List (Using boxed())

For primitive arrays like int[], you need to box the primitive values into their wrapper class (Integer) before collecting them into a list.

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class PrimitiveArrayToList {
    public static void main(String[] args) {
        // Step 1: Define the primitive int array
        int[] numbers = {10, 20, 30, 40, 50};

        // Step 2: Convert the primitive int array to a Stream and collect to a List
        List<Integer> numberList = Arrays.stream(numbers)
                                         .boxed()  // Convert from int to Integer
                                         .collect(Collectors.toList());

        // Step 3: Display the List
        System.out.println(numberList);
    }
}

Output

[10, 20, 30, 40, 50]

Explanation

Step 1: Define the Primitive Array

We define a primitive int array:

int[] numbers = {10, 20, 30, 40, 50};

Step 2: Convert the Array to a Stream and Box the Primitives

We use Arrays.stream() to create an IntStream from the primitive array. The boxed() method converts the int values to their wrapper class Integer:

List<Integer> numberList = Arrays.stream(numbers)
                                 .boxed()
                                 .collect(Collectors.toList());

Step 3: Display the List

We print the resulting list:

System.out.println(numberList);

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Conclusion

In Java 8, converting an array to a list is easily achieved using the Stream API. By using Arrays.stream() or Stream.of(), you can convert an array into a stream and collect the elements into a list using Collectors.toList(). For primitive arrays, the boxed() method is required to convert primitive types to their wrapper classes. This approach provides a flexible and efficient way to process arrays in a functional style.