Reading and writing files

File utility class

To work with files, it is a good idea to make a utility class.

import java.nio.file.*;
import java.util.List;

public class FileIO {
  private Path filePath;

  public FileIO(String fileName) {
    this.filePath = Paths.get("src", "data", fileName);
  }
}

nio is a library of tools for working with "input and output".

The filePath property saves the location of the file we are working with. Paths.get is from the nio library and helps build a correct file path on different operating systems.

Reading a file

To read a file, we can use the Files.readAllLines() method provided by nio.

public List<String> readLines() throws IOException {
  return Files.readAllLines(this.filePath);
}

Since Files.readAllLines() returns a List<String>, we can use list methods to work with the lines in the file:

public String readLine(int i) throws IOException {
  List<String> lines = this.readLines();
  return lines.get(i);
}

Writing a file

We can overwrite the entire contents of a file with the Files.write() method.

public void writeLine(String line) throws IOException {
  Files.write(this.filePath, List.of(line));
}

DANGER

This will delete and overwrite the contents of the file. It cannot be restored once it is deleted.

Note that Files.write() expects a list of lines to write, and so we must put the string to write into a list.

Appending a file

We can pass StandardOpenOption.APPEND an additional argument, which ensures that the data is appended to the file rather that overwriting.

public void writeLine(String line) throws IOException {
  Files.write(this.filePath, List.of(line), StandardOpenOption.APPEND);
}

CSV

We can use split and join to work with CSV files. We add some utility methods to our Meal class.

public static Meal fromCSV(String row) {
  String[] parts = row.split(",");
  return new Meal(parts[0], Integer.parseInt(parts[1]));
}

public String toCSV() {
  String[] parts = { this.mealName, Integer.toString(calories) };
  return String.join(",", parts);
}

Then, in the AddMeal command we can save the user information instead of just printing it back to them.

try {
  Exercise exercise = new Exercise(activity, duration, unit);
  public static FileIO file = new FileIO("exercise.csv");
  file.writeLine(exercise.toCSV());
} catch (Exception e) {
  System.err.println("Unable to create exercise. Error:");
  System.err.println(e.getMessage());
}