Java toArray() method: primitive result type

Question

I am trying to do the following:

double[][] ret = new double[res.size()][columnSize];
for(int i = 0; i < res.size(); i++){
     ret[i] = res.get(i).toArray(new double[columnSize]);
}

where res is declared as List<List<Double>>. The above does not work because toArray() method wants a parametrized array to infer resulting type and that cannot be primitive...

Now, I could just change return type of my method to Double[][] but later on I have other functions from different APIs that expect double[][] 9primitives). That means there would be a lot of Upcasting, doesn't it?

ANy solutions, advices?

Eugene Retunsky · Accepted Answer · 2012-03-31 20:07:40Z

3

You cannot keep primitives in collections and you need to convert collections to array of primive types like this:

double[] toArray(Collection<Double> collection) {
    double[] arr = new double[collection.size()];
    int i = 0;
    for (Double d : collection) {
        arr[i++] = d;
    }
    return arr;
}

answered Mar 31, 2012 at 20:07

Eugene Retunsky

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3 Comments

Bober02 Over a year ago

That was quite and easy solution... But not as nice :)

Eugene Retunsky Over a year ago

Unfortunately, no nicer solution is available.

Chloe Over a year ago

Wow, it's unbelievable that Java still doesn't have array auto-unboxing. BTW this answer is for a single dimension. The question is for 2D array, so it requires nested for loops. Crazy for something so simple.

Louis Wasserman · Accepted Answer · 2012-03-31 23:09:54Z

2

With Guava: double[] Doubles.toArray(Collection<? extends Number) does the trick.

answered Mar 31, 2012 at 23:09

Louis Wasserman

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6 Comments

Bober02 Over a year ago

Wow, this is really useful mate! Just looked at it briefly and at first glance it seems like a library that makes up for java's mistakes and missing implementation. Anything similar you might know about btw?

Louis Wasserman Over a year ago

I work on Guava. If you're curious about all the things Guava offers, the best place to start is the wiki.

Bober02 Over a year ago

Yeah looks good. Do you reckon this is the most complex suite ou there available?

Louis Wasserman Over a year ago

Um. Not sure what you mean, but not using Guava would be like having a hand tied behind my back.

Bober02 Over a year ago

I was wondering if there are similar packages, perhaps targetting other issues with Java as well, I should know about

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Chloe · Accepted Answer · 2013-08-15 20:45:08Z

0

For 2D arrays.

private static int[][] convertListArray(ArrayList<Integer[]> al) {
    int[][] ret = new int[al.size()][];
    for (int i=0; i < al.size(); i++) {
        Integer[] row = al.get(i);
        ret[i] = new int[row.length];
        for (int j = 0; j < row.length; j++) {
            ret[i][j] = row[j];
        }
    }
    return ret;
}

answered Aug 15, 2013 at 20:45

Chloe

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