(Un)boxing primitive arrays in Java

问题

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.

回答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.

回答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.

回答3:

If you create an 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 ) ;)

回答4:

Common.Lang from Apache has a Utility class that does it: ArrayUtils

回答5:

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.

来源：https://stackoverflow.com/questions/6064522/unboxing-primitive-arrays-in-java

标签

java

android

collections

boxing