Store text file into array and vice versa

Question

I am trying to read a text file into an array, modify the array, and then store it back into the text file for future use.

The array is just one column wide, so I would like each line of the text file to be stored in each array element.

I am doing this in the middle of a big program, so the related answers which I've found before don't seem to fit in.

Here is my code:

checkReadHeader = parts[0];

if (checkReadHeader.equals("LETSDOIT"))
{
    readMsg = parts[1];
    readj = 0;

    if(readMsg.equals(logging1)){

        //---------------------------------------
        // READ readlist1.txt AND STORE IT INTO STRING ARRAY readlist
        //---------------------------------------
        try
        {
            fIn = context.openFileInput("readList1.txt");
            isr = new InputStreamReader(fIn);
            while ((charRead = isr.read(inputBuffer)) > 0)
            {
                String readString = String.copyValueOf(inputBuffer, 0, charRead);
                if(!readString.equals("\n"))
                {
                    readList[readj][0] += readString;
                }
                else
                {
                    readj += 1;
                }         
                inputBuffer = new char[100];
            }
        }
        catch (IOException ioe)
        {
            ioe.printStackTrace();
        }
    }

    //---------------------------------------
    // MODIFY readlist
    //---------------------------------------

    readList[j][0] = testdate;

    //---------------------------------------
    // STORE readlist BACK INTO TEXT FILE readlist1.txt
    //---------------------------------------

    try
    {
        fOut = context.openFileOutput("readList1.txt", context.MODE_WORLD_READABLE);
        osw = new OutputStreamWriter(fOut);
        osw.write(readList.toString());
        osw.flush();
        osw.close();
    }
    catch (IOException ioe)
    {
        ioe.printStackTrace();
    }
}

My declaration of variables are all ok, as I am only coming across a run-time error now. Please advise me of any errors in my code - thanks in advance:-)

Answer 1

First of all, it doesn't make sense to use an array as the internal data structure for your file. Because you don't know how many lines you will read beforehand. A List<String> is more sufficient with ArrayList or LinkedList as an implementation.

Second: Don't use a raw Reader but a BufferedReader. With this BufferedReader you can read the file line by line with the method readLine(). Similarly you can use PrintWriter to write line by line to a file.

Third: You should use explicit character encoding. Don't rely on the standard encoding, because the standard encoding can be different for different operating systems (e.g. Windows-ANSI aka Cp1252 for Windows and UTF-8 for Linux).

Fourth: Use the try-with-resources statement to open the input and output streams. So it is easier for you to be sure they are closed in every case.

I assume the return type of context.openFileInput("readList1.txt") is 'InputStream` and the character encoding is UTF-8:

List<String> readList = new ArrayList<String>();
// Read the file line by line into readList
try(
  BufferedReader reader = new BufferedReader(new InputStreamReader(
      context.openFileInput("readList1.txt"), "UTF-8"));
) {
  String line;
  while((line = reader.readLine()) != null) {
    readList.add(line);
  } 
} catch(IOException ioe) {
  ioe.printStackTrace();
}

// Modify readList
// ...

// Write readList line by line to the file
try(
  PrintWriter writer = new PrintWriter(new OutputStreamWriter(
    context.openFileOutput("readList1.txt", context.MODE_WORLD_READABLE), "UTF-8")));
) {
  for(String line: readList) {
    writer.println(line); 
  }
  writer.flush();
} catch (IOException ioe) {
  ioe.printStackTrace();
}