Is there a built-in Java method to box an array?

Question

Is there a standard method I can use in place of this custom method?

public static Byte[] box(byte[] byteArray) {
    Byte[] box = new Byte[byteArray.length];
    for (int i = 0; i < box.length; i++) {
        box[i] = byteArray[i];
    }
    return box;
}

Out of curiosity, why do you want to create an array of boxed values? — Klitos Kyriacou, Nov 12 '15 at 18:14
Probably to use them in a list or another data structure, because java does not permit primitive generics. — Dragas, Nov 25 '18 at 09:08

score 36 · Accepted Answer · edited May 07 '13 at 20:05

36

No, there is no such method in the JDK.

As it's often the case, however, Apache Commons Lang provides such a method.

edited May 07 '13 at 20:05

Ian Roberts

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answered Apr 06 '10 at 15:04

Joachim Sauer

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YoYo · Answer 2 · 2015-06-06T08:52:43.203

25

Enter Java 8, and you can do following (boxing):

int [] ints = ...
Integer[] boxedInts = IntStream.of(ints).boxed().toArray(Integer[]::new);

However, this only works for int[], long[], and double[]. This will not work for byte[].

You can also easily accomplish the reverse (unboxing)

Integer [] boxedInts = ...
int [] ints = Stream.of(boxedInts).mapToInt(Integer::intValue).toArray();

edited Jun 06 '15 at 08:52

answered Jun 04 '15 at 05:49

YoYo

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you can also: `List boxedList = IntStream.of(ints).boxed().collect(Collectors.toList())` – juanmf Jul 01 '17 at 16:09
@juanmf correct, but that does not answer the post directly: a `List` is not an `Integer []` as is requested for a boxed _array_. – YoYo Oct 02 '17 at 02:28

score 3 · Answer 3 · edited Apr 01 '16 at 18:58

3

In addition to YoYo's answer, you can do this for any primitive type; let primArray be an identifier of type PrimType[], then you can do the following:

BoxedType[] boxedArray = IntStream.range(0, primArray.length).mapToObj(i -> primArray[i]).toArray(BoxedType[] :: new);

edited Apr 01 '16 at 18:58

Nacho

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answered Apr 01 '16 at 18:02

bodmas

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I have seen this solution several times - and in my guts it does not seem like a clean solution (the way you bring in a non-enclosed variable). However I do not seem to find a real good argument against it. – YoYo Jan 10 '17 at 22:01

score 1 · Answer 4 · answered Nov 25 '18 at 09:21

When looking into Apache Commons Lang source code, we can see that it just calls Byte#valueOf(byte) on each array element.

    final Byte[] result = new Byte[array.length];
    for (int i = 0; i < array.length; i++) {
        result[i] = Byte.valueOf(array[i]);
    }
    return result;

Meanwhile, regular java lint tools suggest that boxing is unnecessary and you can just assign elements as is.

So essentially you're doing the same thing apache commons does.

Is there a built-in Java method to box an array?

4 Answers4

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