1. Introduction

In this post we will see how to convert List<E> to array in 4 different ways. The following methods acts as a bridge between arrays and collections.

2. How to convert List to array?

The 4 ways to convert the List to arrays are as follows:

1. Using List interface’s toArray(T[]) method,
2. For loop on array
3. Stream API
4. Google guava library

2.1 Using List interface’s toArray(T[]) method.

Let us say that List<E> consist of Object, more specifically String objects. So now we will try to convert List<String> to String[]. We will use toArray(T[]) method.

toArray method signature.

      <T> T[] toArray(T[] a)

List<String> names = Arrays.asList("John", "Jane", "Robb", "Emily");
String[] str = names.toArray(new String[names.size()]);
Assert.assertTrue(Arrays.asList(str).equals(names));

2.2 Pull mechanism to array

Second, let’s assume that we have a List<Integer> that we want to convert to int[].

List<Integer> numbers = Arrays.asList(3, 65, 7, 6, 324, 23);
int[] arr = new int[numbers.size()];
for (int i = 0; i < numbers.size(); i++) {
    arr[i] = numbers.get(i).intValue(); // No autoboxing.
}

2.3 Stream API

Stream API was added in Java 8 to support sequential and parallel aggregate operations. This operations can be done declaratively and hence code looks more readable.

List<Integer> numbers = Arrays.asList(3, 65, 7, 6, 324, 23);
int[] arr = numbers.stream().mapToInt(Integer::intValue).toArray();

2.4 Using Google Guava library.

We can use Ints class’s static method toArray to convert List<Integer> to int[].

List<Integer> numbers = Arrays.asList(3, 65, 7, 6, 324, 23);
int[] arr = Ints.toArray(numbers);

3. Conclusion

In this article, we saw few different ways to convert List to array. This article determines how to establish a bridge between Collection and an array.