Inference variable has incompatible bounds when trying to build 1D array

Question

I've reviewed the "may be related" questions but they don't seem to help; I'm also a beginner, particularly with Java8, so please bear with me!

The problem - I want to read a file containing a list of values, in this case double numerics, into an array. I'm doing these as methods in a helper class as I expect to need to do this numerous times in my project.

I've already created a simple method to convert a file into a 2d double array -

public static double[][] Double2DMatrixFromFile(String input) throws IOException 
    {

        Path path = Paths.get(input);
        return Files.lines(path)
                .map((x) -> Stream.of(x).mapToDouble(Double::parseDouble).toArray())
                .toArray(double[][]::new);

    }

This works, and I've been using it successfully. I then wanted to try reading a file containing a long list of big numbers into a 1D array so tried to quickly modify the method above to look like this -

public static double[] DoubleArrayFromFile(String input) throws IOException
    {
        Path path = Paths.get(input);
        return Files.lines(path)
                .map((y) -> Stream.of(y).mapToDouble(Double::parseDouble).toArray())
                .toArray(double[]::new);
    }

However this refuses to compile and it's not clear to me why. The exact error is

Error:(57, 17) java: method toArray in interface java.util.stream.Stream cannot be applied to given types;
required: java.util.function.IntFunction found: double[]::new
reason: inference variable A has incompatible bounds equality constraints: double upper bounds: java.lang.Object

I'm not clear where the incompatibility arises, given it works in the first case.

Apologies if this seems obvious, however I'm very new to coding.

I can’t go into specifics but each line is a minimum 100 characters long, in the first example a 100+ digit number. — david_c
While trying to do this by hard coding some examples I've also stumbled on the issue that even 50 digit integers are proving hard to manipulate. Might need to rethink my overall plan here! — david_c

Eran Eran · Accepted Answer · 2018-08-16T10:30:32

Your first method starts with a Stream<String>, maps it to a Stream<double[]> and then converts it to a 2 dimensional array (double[][]).

Your second method attempts to convert a Stream<double[]> to a 1 dimensional array. toArray can only convert a Stream<SomeType> to a SomeType[]. Therefore, if SomeType is already a 1-dimensional array, toArray can only produce a 2 dimensional array.

Since your second method should create a 1-dimensional array, you should map the Stream<String> to a DoubleStream and then convert to a 1-dimensional array:

public static double[] DoubleArrayFromFile(String input) throws IOException
{
    Path path = Paths.get(input);
    return Files.lines(path)
                .mapToDouble(Double::parseDouble)
                .toArray();
}

BTW, the first method can also be simplified. There's no reason to create a Stream of a single element only to convert it to an array of a single double. You can write:

public static double[][] Double2DMatrixFromFile(String input) throws IOException 
{

    Path path = Paths.get(input);
    return Files.lines(path)
                .map(x -> new double[]{Double.parseDouble(x)})
                .toArray(double[][]::new);

}

Inference variable has incompatible bounds when trying to build 1D array

1 Answers