(Un)boxing primitive arrays in Java

Question

in the Android Java world, is there a straighforward (ideally one-call) way to convert an array of int to an ArrayList<Integer> and back? Calling toArray() on the ArrayList returns an array of Integer - not quite what I want.

I can easily do that by hand with a loop (already did, in fact). I'm wondering if the library/language supports the same.

EDIT: thanks all, I already wrote my own boxer and unboxer. I'm just surprised the language/RTL designers didn't think of it themselves, especially with primitive types being by design ineligible for collections.

Answer 1

Using Guava (Ints.asList(int...) and Ints.toArray(Collection<Integer>)):

int[] intArray = ...
List<Integer> intArrayAsList = Ints.asList(intArray);
int[] intArray2 = Ints.toArray(intArrayAsList);

Note that like Arrays.asList, Ints.asList returns a list that is a view of the given array. You can't add to or remove from it, but you can set the value at a specific index. You can also copy it to a new ArrayList if you want:

List<Integer> arrayList = Lists.newArrayList(Ints.asList(intArray));

Guava has the same methods for all primitive types.

Answer 2

Boxing and unboxing (without special libraries):

// Necessary Imports:
import java.util.*;
import java.util.stream.*;

// Integers:
int[] primitives = {1, 2, 3};
Integer[] boxed = Arrays.stream(primitives).boxed().toArray(Integer[]::new);
int[] unboxed = Arrays.stream(boxed).mapToInt(Integer::intValue).toArray();

// Doubles:
double[] primitives = {1.0, 2.0, 3.0};
Double[] boxed = Arrays.stream(primitives).boxed().toArray(Double[]::new);
double[] unboxed = Arrays.stream(boxed).mapToDouble(Double::doubleValue).toArray();

Note that this requires Java 8 (Stream Documentation).

For anyone unaware of what the :: means, it's used to make a Method Reference.