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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.

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4 Answers 4

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16

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.

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2 Comments

What about byte and Byte? It seems there is no mapToByte.
@Anakhand I haven't been doing Java dev anymore for the past few years, but you might find this helpful: stackoverflow.com/questions/32459683/…
13

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.

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I'm not introducing a new dependency into the project for the sake of a three line function I could write in my sleep.
4

If you create a utility method, that would be a one call conversion. Perhaps, in your case, that would be better than adding a whole library (of which you might just use one function), although that should be too much burden for your app.

Here's the source code for the h3xStream answer (glad it uses Apache license) ;)

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1

I managed to find a solution in Java 8 :

import java.util.Arrays;
...
double[] columnsValue = {...};
Double[] values = Arrays.stream(columnsValue).boxed().toArray(Double[]::new);

I can't explain what Double[]::new means exactly, because I didn't know this feature of Java 8 before (in fact, IntelliJ IDEA wrote this for me), but it looks like it is doing new Double(double) on every element of the array.

Please note : I tested this only on desktop, I don't know if it works in Android.

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1 Comment

What you're doing there is actually only boxing (converting a primitive to its corresponding wrapper class), not unboxing (converting from a wrapper class to its corresponding primitive).

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