make a list out of a text file in java?

Question

I have a text file full of numbers and I want to read the numbers into Java and then make a list that I can sort. it has been a while since I have used java and I am forgetting how to do this.

the text file looks something like this

4.5234  
9.3564
1.2342
4.4674
9.6545
6.7856

Answer 1

Short instructions without code:

1. Create a BufferedReader to read from your file.
2. Iterate all over the file reading line by line with reader.readLine(), until readLine() returns null (== end of file)
3. Parse each line as a Float or Double (e.g. Float.valueOf(line) or Double.valueOf(line)
4. Add your Float or Double objects to an ArrayList.

Answer 2

Not accounting for the fact that you need to setup exception handling and such based on your sourrounding code, it'll look something like this:

BufferedReader br = new BufferedReader(new FileReader(new File("filename.txt")));
ArrayList<Double> ald = new ArrayList<Double>();
String line;
while(true)
{
    line = br.readLine();
    if(line == null) break;
    ald.add(new Double(line));
}

Answer 3

You do something like this.

EDIT: I've tried to implement the changes dbkk mentioned in his comments so the code actually will be correct. Tell me if I got something wrong.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class ReadList {

    public static void main(String[] args) throws IOException {

        BufferedReader in = null;
        FileReader fr = null;
        List<Double> list = new ArrayList<Double>();

        try {
            fr = new FileReader("list.txt");
            in = new BufferedReader(fr);
            String str;
            while ((str = in.readLine()) != null) {
                list.add(Double.parseDouble(str));
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            in.close();
            fr.close();
        }

        for (double d : list) System.out.println(d);
    }

}

Answer 4

You can use a Scanner on a File and use the nextDouble() or nextFloat() method.

Scanner scanner = new Scanner(new File("pathToYourFile"));
List<Double> doubles = new ArrayList<Double>();
while(scanner.hasNextDouble()){
    doubles.add(scanner.nextDouble());
}
Collections.sort(doubles);

---

Resources :

• Javadoc - Scanner.nextDouble()

Answer 5

This is really fun and simple if you use Guava:

final File f = new File("your/file.txt");
final List<Float> listOfFloats =
    Lists.transform(Files.readLines(f, Charset.defaultCharset()),
        new Function<String, Float>(){

            @Override
            public Float apply(final String from){
                return Float.valueOf(from);
            }
        });

And here's a similar version using Apache Commons / IO:

final File f = new File("your/file.txt");
final List<String> lines = FileUtils.readLines(f);
final List<Float> listOfFloats = new ArrayList<Float>(lines.size());
for(final String line : lines){
    listOfFloats.add(Float.valueOf(line));
}

Answer 6

Use java.util.Scanner:

public List<Doubles> getNumbers(String fileName) {
  List<Double> numbers = new ArrayList<Double>();
  Scanner sc = new Scanner(new File(fileName));

  while (sc.hasNextDouble()) {
      numbers.add(sc.nextDouble());
  }

  return numbers;
}