Read lines from a TXT to Arraylist

Question

I need to read a log saved in a TXT file that contains geographical coordinates in the following form:

-54.123440,-21.123456
-54.123425,-21.123467
-54.123435,-21.123480
-54.123444,-21.123444
-54.123452,-21.123100

Each line has the longitude and latitude of each coordinate (point of the graph), separated by a comma (,).

I’m going to use these coordinates to generate a graph where each line is a point, so I need to have access to the latitude and longitude of each point. I would like to know how to store the latitude and longitude of each point in one ArrayList, and if that’s really the best way to do that for me to have access to that data to generate the graph afterwards.

I can read the file and print line by line, through the following class:

public class Leitura {

    public void leTXT() {
        Scanner ler = new Scanner(System.in);
        try {
            FileReader arq = new FileReader("c:/dados/log.txt");
            BufferedReader lerArq = new BufferedReader(arq);
            String str = lerArq.readLine();
            while (str != null) {
                System.out.printf("%s\n", str);
                str = lerArq.readLine();
            }
            arq.close();
        } catch (IOException e) {
            System.err.printf("Erro na abertura do arquivo!");
        }

        System.out.println();
    }
}

Answer 1

In your file, each line corresponds to a coordinated and each coordinated has a longitude (between -180 and +180) and a latitude (between -90 and +90). The contents of the archive are coordinate list.

Note that the central concept we have here is a coordinate. Since Java is an object-oriented language, these concepts map to objects that are modeled in classes. So, Coordenada is a class:

public final class Coordenada {
    private final double longitude;
    private final double latitude;

    public Coordenada(double longitude, double latitude) {
        if (longitude < -180.0 || longitude > 180.0) {
            throw new IllegalArgumentException("Longitude inválida.");
        }
        if (latitude < -90.0 || latitude > 90.0) {
            throw new IllegalArgumentException("Latitude inválida.");
        }
        this.longitude = longitude;
        this.latitude = latitude;
    }

    public double getLongitude() {
        return longitude;
    }

    public double getLatitude() {
        return longitude;
    }

    public static Coordenada parse(String linha) {
        String[] partes = linha.split(",");
        if (partes.length != 2) throw new IllegalArgumentException("Linha malformada.");

        double a, b;
        try {
            a = Double.parseDouble(partes[0]);
            b = Double.parseDouble(partes[1]);
        } catch (NumberFormatException e) {
            throw new IllegalArgumentException("Linha malformada.");
        }
        return new Coordenada(a, b);
    }
}

Note this method parse(String). He is responsible for interpreting a line and converting into a coordinate.

You can use the method Files.readAllLines(Path, Charset) to get all the file lines easily.

With that, we can do so:

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Leitura {

    public List<Coordenada> lerCoordenadas() throws IOException {
        return lerCoordenadas(Paths.get("c:/dados/log.txt"));
    }

    public List<Coordenada> lerCoordenadas(Path arquivo) throws IOException {
        return Files.readAllLines(arquivo, StandardCharsets.UTF_8)
               .stream()
               .map(Coordenada::parse)
               .collect(Collectors.toList());
    }
}

In such case, you don’t have to worry about opening, reading and closing the file manually because the Files.readAllLines already does it. But if you do it manually, remember to use the Try-with-Resources.

Answer 2

Without knowing which graphic library you will use, here is a generic solution, which uses a Map to store the latitude set / longitude.

public HashMap<Double, Double> getCoordenadas() throws IOException{     

    HashMap<Double, Double> retorno = new HashMap<>();

    List<String> linhas = Files.readAllLines(Paths.get("C:\\dados\\log.txt"));

    for (String linha : linhas) {
        String[] coordenadas = linha.split(",");
        retorno.put(Double.valueOf(coordenadas[0]), Double.valueOf(coordenadas[1]));
    }

    return retorno;
}

Note that I used a different method to read the file, the class Files.

If your project uses Java 8, I recommend using it, because as you can see, it greatly decreases the complexity of the code.