How to store bytes and read them back to an array

Question

I am trying to store a list of numbers(bytes) into a file so that I can retrieve them into a byte[].

59 20 60 21 61 22 62 23 63 24 64 25 65 26 66 27 67 28 68 29 
67 30 66 31 65 32 64 33 63 34 62 35 61 36 60 37 59 38
66 29 65 30 64 31 63 32 62 33 61 34 60 35 59 36 58 37
65 28 64 29 63 30 62 31 61 32 60 33 59 34 58 35 57 36...

I have tried saving them into a text file but the relevant code doesn't seem to read it properly.

    try {
        File f = new File("cube_mapping2.txt");
        array = new byte[file.size()]
        FileInputStream stream = new FileInputStream(f);
        stream.read(array);
    } catch (Exception e) {
        e.printStackTrace();
    }

Is there a proper way to save the file so that FileInputReader.read(byte[] buffer) will populate the array with my bytes?

Answer 1

I'd be using Scanner. Something like this:

public static void main(String[] args) throws IOException {
    InputStream stream = new FileInputStream("cube_mapping2.txt");
    Scanner s = new Scanner(stream);
    List<Byte> bytes = new ArrayList<Byte>();
    while (s.hasNextByte()) {
        bytes.add(s.nextByte());
    }
    System.out.println(bytes);
}

I tested this on a file containing your exact input and it worked. Output was:

[59, 20, 60, 21, 61, 22, 62, 23, 63, 24, 64, 25, 65, 26, 66, 27, 67, 28, 68, 29, 67, 30, 66, 31, 65, 32, 64, 33, 63, 34, 62, 35, 61, 36, 60, 37, 59, 38, 66, 29, 65, 30, 64, 31, 63, 32, 62, 33, 61, 34, 60, 35, 59, 36, 58, 37, 65, 28, 64, 29, 63, 30, 62, 31, 61, 32, 60, 33, 59, 34, 58, 35, 57, 36]

Answer 2

FileInputStream works on binary files. The code you posted would read from a binary file, but isn't quite right because stream.read(array) reads up to the length of the array; it doesn't promise to read the whole array. The return value from read(array) is the number of bytes actually read. To be sure of getting all the data you want you need the read() call to be in a loop.

To answer your actual question: to write to a file in such a way that stream.read(array) will be able to read it back it, use FileOutputStream.write(array).

If you're happy with a text file instead of a binary file, go with @Bohemian's answer.

Answer 3

array = new byte[file.size()]

Is that means, there is no space left to store the separate mark for each two numbers? According to your byte array, if each one of them is only two spaces, then you can use a two spaces temp byte array to read each byte you store in your file. Something like

byte[] temp = new byte[2];
stream.read(temp);

Which can make sure to read the byte number one by one.