Converting int to byte[] in android

Question

I am using the write() method in order to write in a file of the external storage. This method only accepts byte[] as an input. I have tried passing a String and I get an error message ("The method write(int) in the type FileOutputStream is not applicable for the arguments String"). If I pass an int, I don't get error but in the file nothing is written. The value I get from calling getNumSentPackets() is an int and I need to convert it to byte[]. I have been looking at other questions already answered here and I have tried the ByteBuffer option but the result I get in the file is not what I want, this means, I don't get the number of sent packets. Can anybody help me, please?

This is my code:

     public void createFile(String name) {

    try {
      String filename = name;
      File myFile = new File(Environment.getExternalStorageDirectory(), filename);
      if (!myFile.exists())
        myFile.createNewFile();
      String title = "FLOODING RESULTS FILE\n\n";
      String sent = "Number of sent packets\n";
      FileOutputStream fos;
      byte[] data = title.getBytes();
      byte[] intSent = sent.getBytes();
      int numSent = mSender.getNumSentPackets();
      byte[] numSentBytes = ByteBuffer.allocate(10).putInt(numSent).array();
      try{
      fos = new FileOutputStream(myFile);
      fos.write(data);
      fos.write(intSent);
      fos.write(numSentBytes);
      fos.flush();
      fos.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
  } catch (IOException e) {
    e.printStackTrace();
  }
}

public static int getNumSentPackets() {
  return nSentPackets;
}

The expected output file would be as follows:

FLOODING RESULTS FILE

Number of sent packets 200

200 is only an example, meaning with this that I would like to see there a number which would correspond to the total number of sent packets.

Thank you in advance.

Could you please add the complete expected output in your question ? — SirDarius, Oct 31 '14 at 09:38
This is what I am expecting FLOODING RESULTS FILE Number of sent packets 200 200 is an example but I would like to have the value of numSentPackets — Jairo Bustos, Oct 31 '14 at 09:41
I'm asking you to update your question, because using a comment for this discards all newlines ;) — SirDarius, Oct 31 '14 at 09:44
Can I not simply print out in the file an int and forget about all this converting process? — Jairo Bustos, Oct 31 '14 at 09:49

SirDarius · Answer 1 · 2014-10-31T10:04:29.980

As I am a lazy developer, I like to use the existing facilities in my languages of choice, for example, for java, a PrintWriter.

public void createFile(String filename) {
    try {
      File myFile = new File(Environment.getExternalStorageDirectory(), filename);

      PrintWriter out = new PrintWriter(myFile); // this will create the file if necessary

      out.println("FLOODING RESULTS FILE");
      out.println();
      out.print("Number of sent packets ");
      out.println(mSender.getNumSentPackets());
      out.close();
  } catch (IOException e) {
    e.printStackTrace();
  }
}

This is much easier to read and maintain than your current approach, and looks more idiomatic.

Thank you very much! This works out! I am completely new to android and java and I am not really aware of the facilities provided by java. Thanks!! — Jairo Bustos, Oct 31 '14 at 10:01

score 0 · Answer 2 · edited Oct 31 '14 at 09:39

0

ByteBuffer.allocate(capacity).putInt(yourInt).array();

edited Oct 31 '14 at 09:39

Piyush

18,895
5
32
63

answered Oct 31 '14 at 09:36

Kevin Tan

1,038
8
19

Thanks for the answer but maybe you misread. If you read my post and have a look at my code you will see that I am using that and I don't get what I expect. Instead I get a whole bunch of numbers which I suppose are hexadecimal. – Jairo Bustos Oct 31 '14 at 09:39

score 0 · Answer 3 · edited May 23 '17 at 11:57

The text representation of "200" requires you to write 3 characters. All files are just a bunch of bytes in the end so there needs to be a mapping from character to some byte value. Assuming ASCII(*) the data to write into the file would be

//                    '2','0','0'
byte[] textVersion = { 50, 48, 48 }

int on the other hand is a 32bit numeric value, i.e. has 4 bytes and 200 is equivalent to

byte[] intVersion = { 0, 0, 0, 200 }

When using a ByteBuffer, you'll get this. If you write that into a file and a text viewer tries to display that it would display something like ◻◻◻Č if you're lucky. A 0 is actually a non printable control character and therefore often either skipped when printing or replaced with strange looking character like boxes. The 200 would be equivalent to Č in Windows-CP1250. It has no meaning on it's own when interpreted as UTF8 - it's the start of a 2 byte sequence and so the next 2 byte are required to determine which character to display.

You could have used

String.valueOf(200).getBytes( /* you should specify which encoding to use here */ );

which will create the "200" string first, then return you the required bytes for those 3 characters.

You should however use Java's character based IO facility: The numerous (and confusing) Reader & Writer implementations. They all(*^2) wrap an InputStream or OutputStream in the end and do the text to byte conversion for you.

PrintWriter is probably the most convenient to use but not without flaw: https://stackoverflow.com/a/15803472/995891 FileWriter should be avoided because you can't specify the encoding

The longer alternative route would be

BufferedWriter writer = new BufferedWriter(
        new OutputStreamWriter(
             new FileOutputStream(file), encoding));
writer.write("Hello ");
writer.write(String.valueOf(200));
writer.newLine();

(*) most encodings are ASCII compatible for the first 127 characters which basically covers normal english text.

(*^2) nothing forces a Writer to output the characters into a stream, e.g. StringWriter. But they are used mostly that way.

Converting int to byte[] in android

3 Answers3