Question

I want to write first a sequence of strings and then a sequence of bytes into a file, using Java. I started by using FileOutputStream because of the array of bytes. After searching the API, I realised that FileOutputStream cannot write Strings, only ints and bytes, so I switched to DataOutputStream. When I run the program, I get an exception. Why?

Here’s a portion of my code:

 try {
            // Create the file
            FileOutputStream fos;
            DataOutputStream dos; // = new DataOutputStream("compressedfile.ecs_h");
            File file= new File("C:\\MyFile.txt");
            fos = new FileOutputStream(file);
            dos=new DataOutputStream(fos);

            /* saves the characters as a dictionary into the file before the binary seq*/
            for (int i = 0; i < al.size(); i++) {
                String name= al.get(i).name; //gets the string from a global arraylist, don't pay attention to this!
                dos.writeChars(name); //saving the name in the file
            }

            System.out.println("\nIS SUCCESFULLY WRITTEN INTO FILE! ");

            dos.writeChars("><");
            String strseq;

            /*write all elements from the arraylist into a string variable*/
            strseq= seq.toString();
            System.out.println("sTringSeq: " + strseq);

            /*transpose the sequence string into a byte array*/
            byte[] data = new byte[strseq.length() / 8];

            for (int i = 0; i < data.length; i++) {
                data[i] = (byte) Integer.parseInt(strseq.substring(i * 8, (i + 1) * 8), 2);
                dos.write(data[i]);
            }


            dos.flush();
            //Close the output stream
            dos.close(); 
} catch(Exception e){}

Answer 1

The problem with your code is that the last for loop was counting over the wrong number of bytes. The code below fixes your problem writing your test data to a file. This works on my machine.

public static void main(String[] args) {

    ArrayList<String> al = new ArrayList<String>();
    al.add("String1");
    al.add("String2");

    try {
        // Create the file
        FileOutputStream fos = new FileOutputStream("MyFile.txt");
        DataOutputStream dos = new DataOutputStream(fos);

        /* saves the characters as a dictionary into the file before the binary seq */
        for (String str : al) {
            dos.writeChars(str);
        }

        System.out.println("\nIS SUCCESFULLY WRITTEN INTO FILE! ");

        dos.writeChars("><");
        String strseq = "001100111100101000101010111010100100111000000000";

        // Ensure that you have a string of the correct size
        if (strseq.length() % 8 != 0) {
            throw new IllegalStateException(
                    "Input String is cannot be converted to bytes - wrong size: "
                            + strseq.length());
        }

        int numBytes = strseq.length() / 8;
        for (int i = 0; i < numBytes; i++) {
            int start = i * 8;
            int end = (i + 1) * 8;
            byte output = (byte) Integer.parseInt(strseq.substring(start, end), 2);
            dos.write(output);
        }

        dos.writeChars("> Enf of File");
        dos.flush();
        // Close the output stream
        dos.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

The approach of writing bytes directly to a test file does have a few problems (I assume that it’s a text file in that your test file name ends with .txt), the most obvious one being that some text editors don’t handle/display null characters very well (your last test byte was: 00000000 or null). If you want to see the bytes as readable bytes then you could investigate encoding them using Base64 encoding.