Table of Contents
1. Overview
In this quick tutorial, we’re going to learn how to convert between an Array and a List using core Java libraries, Guava and Apache Commons Collections.
2. Convert List to Array
2.1. Using Plain Java
Let’s start with the conversion from List to Array using plain Java:
@Test
public void givenUsingCoreJava_whenListConvertedToArray_thenCorrect() {
List<Integer> sourceList = Arrays.asList(0, 1, 2, 3, 4, 5);
Integer[] targetArray = sourceList.toArray(new Integer[0]);
}
Note that the preferred way for us to use the method is toArray(new T[0]) versus toArray(new T[size]). As Aleksey Shipilëv proves in his blog post, it seems faster, safer, and cleaner.
2.2. Using Guava
Now let’s use the Guava API for the same conversion:
@Test
public void givenUsingGuava_whenListConvertedToArray_thenCorrect() {
List<Integer> sourceList = Lists.newArrayList(0, 1, 2, 3, 4, 5);
int[] targetArray = Ints.toArray(sourceList);
}
3. Convert Array to List
3.1. Using Plain Java
Let’s start with the plain Java solution for converting the array to a List:
@Test
public void givenUsingCoreJava_whenArrayConvertedToList_thenCorrect() {
Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
List<Integer> targetList = Arrays.asList(sourceArray);
}
Note that this is a fixed-sized list that will still be backed by the array. If we want a standard ArrayList, we can simply instantiate one:
List<Integer> targetList = new ArrayList<Integer>(Arrays.asList(sourceArray));
3.2. Using Guava
Now let’s use the Guava API for the same conversion:
@Test
public void givenUsingGuava_whenArrayConvertedToList_thenCorrect() {
Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
List<Integer> targetList = Lists.newArrayList(sourceArray);
}
3.3. Using Commons Collections
Finally, let’s use the Apache Commons Collections CollectionUtils.addAll API to fill in the elements of the array in an empty List:
@Test
public void givenUsingCommonsCollections_whenArrayConvertedToList_thenCorrect() {
Integer[] sourceArray = { 0, 1, 2, 3, 4, 5 };
List<Integer> targetList = new ArrayList<>(6);
CollectionUtils.addAll(targetList, sourceArray);
}
4. Conclusion
The implementation of all of these examples and code snippets can be found over on GitHub. This is a Maven-based project, so it should be easy to import and run as it is.
