(Un)boxing primitive arrays in Java
in the Android Java world, is there a straighforward (ideally one-call) way to convert an array of int
to an ArrayList<Integer>开发者_如何转开发;
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.
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.
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.
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 ) ;)
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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