How to read a file and store in an arraylist of double?

Question

How can I read a textfile of numbers and store it in arraylist of double? The file has multiple lines of numbers. Also how can I retrieve a random number from the arraylist?

I can store it in a file of string. But when I read the file and display it, it shows the full line of numbers instead of just one number.

public static void main(String[] args) throws IOException
{
    ArrayList<String> list = new ArrayList<>();

    // New BufferedReader.
    BufferedReader reader = new BufferedReader(new FileReader(
            "filename.txt"));

    // Add all lines from file to ArrayList.
    while (true) {
        String line = reader.readLine();
        if (line == null) {
            break;
        }
        list.add(line);
    }

    // Close it.
    reader.close();

    // Print size of ArrayList.
    System.out.println("Lines: " + list.size());

    // Print each line.
    for (String line : list) {
        System.out.println(line);
    }


    Random rand = new Random();
    String randomElement = list.get(rand.nextInt(list.size()));
    System.out.println(randomElement);


    }

I expect the output of 0.48. But the actual output is 0.48 0.66 0.32 0.55 0.21

Answer 1

Break the line by spaces as follow: line.split("\\s+"), so you will have array of String which contains number (0.48)

List<Double> list = new ArrayList<>();
while (true) {
    String line = reader.readLine();
    if (line == null) {
        break;
    }
    String[] stringDoubles = line.split("\\s+");
    for(int i=0;i<stringDoubles.length;i++){
            list .add(Double.parseDouble(stringDoubles[i]))
    }
}

Answer 2

Updated the program as per your needs.

public class Main {

public static void main(String[] args) throws IOException
{
    try {
        List<Double> list = new ArrayList<>();

        // New BufferedReader.
        BufferedReader reader = new BufferedReader(new FileReader(
                "filename.txt"));

        // Add all lines from file to ArrayList.
        String line = null;
        while ((line = reader.readLine()) != null) {
                list.addAll(Arrays.asList(line.split("\\s+")).stream().map(p -> Double.parseDouble(p.trim())).collect(Collectors.toList()));
        }

        // Close it.
        reader.close();

        Random rand = new Random();
        Double randomElement = list.get(rand.nextInt(list.size()));
        System.out.println(randomElement);

    }catch (IOException e){
        System.out.println(e);
    }


}

}

Answer 3

I would try something like this. Assuming you don't need the strings at all and you have a working file without letters ore somthing else equaly distribing.

List<Double> list = new ArrayList<>();

String line = rader.readLine();
while (line != null) {
    list.add(Double.valueOf(line));
    line = reader.readLine();
};

If you want to retrive an random member i would do it whith:

list.get((int)(Math.random(list.size()));