How to box double array to Double array when adding to Map?

Question

I want to fill up a HashMap<Integer,Double[]>

Map<Integer,Double[]> cached_weights = new HashMap<Integer,Double[]>();

with just regular int and double[], what's the best way to do that?

I see this question, but it answers the opposite question.

If you want to use `int` type as key instead of `Integer` then this is impossible. Also `Double[]` and `double[]` are not covariant, and `double[]` will not be automatically boxed to `Double[]`. — Pshemo, Feb 24 '15 at 03:47
Also, a `Double[]` is *not* a boxed `double[]` (boxing doesn't apply to arrays), and you can store a `double[]` in a hashmap already anyway. — user253751, Feb 24 '15 at 03:47
An `int` will "auto box" to an `Integer`. That is not the case for a `double[]`, however. You will have to manually change the `double[]` into a `Double[]`. — Brett Okken, Feb 24 '15 at 03:48
yeah that's right about double[] but still, what about int to Integer? — smatthewenglish, Feb 24 '15 at 03:49
Auto boxing is syntactic sugar where the compiler automatically calls Integer.valueOf for you. — Brett Okken, Feb 24 '15 at 03:50
"so int just automatically becomes Integer" yes, that is how boxing works. If you have `int` you can store it in `Integer` variable (code handling conversion from primitive `int` to object `Integer` - `Integer.valueOf` - will be added by compiler for you). — Pshemo, Feb 24 '15 at 03:51
See http://stackoverflow.com/questions/880581/how-to-convert-int-to-integer-in-java for how to convert double[] to Double[] — Adrian Leonhard, Feb 24 '15 at 04:04

Jean-François Savard · Accepted Answer · 2015-02-24T04:38:16.747

For the key (Integer) the compiler will handle this automatically for you and you can pass directly an int value.

For the Boolean array, you could handle this way with Java 8

Map<Integer, Double[]> foo = new HashMap<Integer, Double[]>();
double[] bar = new double[10];
//As you can see, 1 is passed directly and will be converted to Integer object.
foo.put(1, Arrays.stream(bar)
            .boxed()
            .toArray(Double[]::new));

the boxed method of DoubleStream returns a Stream consisting of the elements of this stream, boxed to Double.

Then you get a Stream on which you can easily call toArray to convert to a Double[].

How to box double array to Double array when adding to Map?

1 Answers1