Java - Reading numbers from text file, and sorting them makes negative numbers turn to zero

Question

I have a double array

double[] numbers = new double[1000];
int size = 0;

I then read the numbers from the text file numbers.txt

input = new Scanner(new FileReader("numbers.txt"));
output= new PrintWriter(new FileWriter("sorted.txt"));

while (input.hasNext()) {
    numbers[size] = input.nextDouble();
    size = size + 1;
    if (size == 1000) break;

And then sort them using following:

numbers = Arrays.stream(numbers)
                .boxed()
                .sorted(Comparator.reverseOrder())
                .mapToDouble(Double::doubleValue)
                .toArray();

And write them to file sorted.txt

for (int i = 0; i < size; i++)
    output.println(numbers[i]);

When I try to sort an array that includes negative numbers, for example if numbers.txt contains 5 -3 1 -4 , then sorted.txt will contain 5 1 0 0 .

I'm relatively a beginner in Java and I'm not sure why this happens. When I use Array.sort(numbers,0,size) it works well, but I'm trying to sort them in descending order, as this method sorts them in ascending order.

Answer 1

The problem is that Arrays.stream(numbers) streams the entire array ... including the parts of the array that you didn't put values into. Those parts will contain 0.0 .

So what is happening is:

1. You read 4 numbers from the file into the array. This gives you:

    [5.0, -3.0, 1.0, -4.0, 0.0, 0.0, ..... 0.0]

2. You sort the array in reverse order:

    [5.0, 1.0, 0.0, 0.0, ..... 0.0, -1.0, -3.0]

3. Print out the first 4 elements of the array.

    [5.0, 1.0, 0.0, 0.0]

There are various approaches to solving this, including:

• Use an ArrayList<Double> instead of a double[] .
• Pre-initialize all array elements with (say) Double.POSITIVE_INFINITY
• Use Arrays.copy to copy the subarray you need before sorting.

• Rewrite the stream code to select elements 0 to size - 1 before sorting; eg

   numbers = Arrays.stream(numbers).limit(size). ....