Error converting from ArrayList to double[]?

Question

I have an ArrayList called out, and I need to convert it to a double[]. The examples I've found online have said two things:

First, try:

double[] d = new double[out.size()];
out.toArray(d);

However, this produces the error (eclipse):

The method toArray(T[]) in the type List<Double> is not applicable for the arguments (double[]).

The second solution I found was on StackOverflow, and was:

double[] dx = Arrays.copyOf(out.toArray(), out.toArray().length, double[].class);

However, this produces the error:

The method copyOf(U[], int, Class<? extends T[]>) in the type Arrays is not applicable for the arguments (Object[], int, Class<double[]>)

What is causing these errors, and how do I convert out to double[] without creating these problems? out indeed holds only double values.

Thanks!

how you define `out`? must it be double[] or Double[] is acceptable? — Nikolay Kuznetsov, Jan 03 '13 at 07:14
Uh... downvote, why? Research was made, multiple things were attempted, and I still had a question. — , Jan 03 '13 at 07:20
@Emrakul I did not vote up or down, but a SSCCE would make this question much better IMO. — The111, May 12 '16 at 03:02

Rahul · Accepted Answer · 2016-05-12T02:35:21.950

12

I think you are trying to convert ArrayList containing Double objects to primitive double[]

public static double[] convertDoubles(List<Double> doubles)
{
    double[] ret = new double[doubles.size()];
    Iterator<Double> iterator = doubles.iterator();
    int i = 0;
    while(iterator.hasNext())
    {
        ret[i] = iterator.next();
        i++;
    }
    return ret;
}

ALternately, Apache Commons has a ArrayUtils class, which has a method toPrimitive()

 ArrayUtils.toPrimitive(out.toArray(new Double[out.size()]));

but i feel it is pretty easy to do this by yourself as shown above instead of using external libraries.

edited May 12 '16 at 02:35

answered Jan 03 '13 at 07:17

Rahul

15,979
4
42
63

Also works with `for(int i = 0; i < ret.length; i++) ret[i] = out.get(i).doubleValue();`, no need for an iterator. Thank you! – Jan 03 '13 at 15:39
This won't work. i=i++ results in i always being 0, put simply i++; Also, explicit unboxing (.doubleValue()) is unnecessary. – Aleksandra Zalcman May 11 '16 at 20:18
incorporated the suggested changes. Thanks – Rahul May 12 '16 at 02:35

score 2 · Answer 2 · answered Jan 03 '13 at 07:15

2

Have you tried

Double[] d = new Double[out.size()];
out.toArray(d);

i.e use the class Double and not the primitive type double

The error messages seem to imply that this is the issue. After all, since Double is a wrapper class around the primitive type double it is essentially a different type, and compiler will treat it as such.

answered Jan 03 '13 at 07:15

Karthik T

31,456
5
68
87

@Telthien in that case could you redefine the list as `List` – Karthik T Jan 03 '13 at 07:17
@Telthien or you could use [ArrayUtils](http://stackoverflow.com/questions/3770289/converting-array-of-primitives-to-array-of-containers-in-java) as shown in the link to convert from `Double[]` to `double[]` – Karthik T Jan 03 '13 at 07:18

score 1 · Answer 3 · edited Jan 03 '13 at 07:22

1

Generics does not work with primitive types that's why you are getting an error. Use Double array instead of primitive double. Try this -

Double[] d = new Double[out.size()];
out.toArray(d);
double[] d1 = ArrayUtils.toPrimitive(d);

edited Jan 03 '13 at 07:22

Nandkumar Tekale

16,024
8
58
85

answered Jan 03 '13 at 07:18

Avinash T.

2,280
2
16
23

what is the package for ArrayUtils – Hussain Akhtar Wahid 'Ghouri' Jan 03 '13 at 07:22

Error converting from ArrayList to double[]?

3 Answers3

Linked