Write and read binary files in Android

Question

I created a custom object of type Task and I want to save it in a binary file in internal storage. Here is the class I created:

public class Task {

    private String title;
    private int year;
    private int month;
    private int day;
    private int hour;
    private int minute;

    public Task(String inputTitle, int inputYear, int inputMonth, int inputDay, int inputHour, int inputMinute) {

        this.title = inputTitle;
        this.year = inputYear;
        this.month = inputMonth;
        this.day = inputDay;
        this.hour = inputHour;
        this.minute = inputMinute;

    }

    public String getTitle() {

        return this.title;

    }

    public int getYear() {

        return this.year;

    }

    public int getMonth() {

        return this.month;

    }

    public int getDay() {

        return this.day;

    }

    public int getHour() {

        return this.hour;

    }

    public int getMinute() {

        return this.minute;

    }
}

In an activity, I created a method that will save my object to a file. This is the code I used:

public void writeData(Task newTask) {
    try {
        FileOutputStream fOut = openFileOutput("data", MODE_WORLD_READABLE);
        fOut.write(newTask.getTitle().getBytes());
        fOut.write(newTask.getYear());
        fOut.write(newTask.getMonth());
        fOut.write(newTask.getDay());
        fOut.write(newTask.getHour());
        fOut.write(newTask.getMinute());
        fOut.close();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Now I would like to create a method that will extract the data from the file. By reading on the internet, a lot of people use FileInputStream but I have trouble with extracting the bytes from it and knowing how long a String can be. Furthermore, I used a simple method found online but I get permission denied. As I said, I am very new to Android development.

public void readData(){
    FileInputStream fis = null;
    try {
        fis = new FileInputStream("data");
        System.out.println("Total file size to read (in bytes) : "
                + fis.available());
        int content;
        while ((content = fis.read()) != -1) {
            // convert to char and display it
            System.out.print((char) content);
        }

    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (fis != null)
                fis.close();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

Any help will be appreciated.

Answer 1

For permission issues, I encourage you to use an external storage such as an SD card.

Environment.getExternalStorageDirectory()

you can create a folder there and save your files. You can also use "/data/local/" if your system permits user files to be saved there. You can refer to this page regarding the various ways you can save files to internal and external storage,

For the second problem I suggest you to use DataInputStream,

File file = new File("myFile");
byte[] fileData = new byte[(int) file.length()];
DataInputStream dis = new DataInputStream(new FileInputStream(file));
dis.readFully(fileData);
dis.close();

You can code something like this,

import java.io.*;
public class Sequence {
    public static void main(String[] args) throws IOException {
    DataInputStream dis = new DataInputStream(System.in);
    String str="Enter your Age :";
    System.out.print(str);
    int i=dis.readInt();
    System.out.println((int)i);
    }
}

You can also Use Serializable interface for reading and writing serializable objects. In fact, I used this once when I tried to write data values directly to files instead of any traditional databases (In my very first undergraduate years, I was not familiar with databases). A good example is here,

import java.io.*;
import java.util.*;
import java.util.logging.*;

/** JDK before version 7. */
public class ExerciseSerializable {

  public static void main(String... aArguments) {
    //create a Serializable List
    List<String> quarks = Arrays.asList(
      "up", "down", "strange", "charm", "top", "bottom"
    );

    //serialize the List
    //note the use of abstract base class references

    try{
      //use buffering
      OutputStream file = new FileOutputStream("quarks.ser");
      OutputStream buffer = new BufferedOutputStream(file);
      ObjectOutput output = new ObjectOutputStream(buffer);
      try{
        output.writeObject(quarks);
      }
      finally{
        output.close();
      }
    }  
    catch(IOException ex){
      fLogger.log(Level.SEVERE, "Cannot perform output.", ex);
    }

    //deserialize the quarks.ser file
    //note the use of abstract base class references

    try{
      //use buffering
      InputStream file = new FileInputStream("quarks.ser");
      InputStream buffer = new BufferedInputStream(file);
      ObjectInput input = new ObjectInputStream (buffer);
      try{
        //deserialize the List
        List<String> recoveredQuarks = (List<String>)input.readObject();
        //display its data
        for(String quark: recoveredQuarks){
          System.out.println("Recovered Quark: " + quark);
        }
      }
      finally{
        input.close();
      }
    }
    catch(ClassNotFoundException ex){
      fLogger.log(Level.SEVERE, "Cannot perform input. Class not found.", ex);
    }
    catch(IOException ex){
      fLogger.log(Level.SEVERE, "Cannot perform input.", ex);
    }
  }

  // PRIVATE 

  //Use Java's logging facilities to record exceptions.
  //The behavior of the logger can be configured through a
  //text file, or programmatically through the logging API.
  private static final Logger fLogger =
    Logger.getLogger(ExerciseSerializable.class.getPackage().getName())
  ;
}