I'm not understanding the following code

I have to understand this code to create my own app(almost based on this function):

public static String[][] ReadFilePerLine(Context context, String nom) {
        int i = 0;
        try {
            FileInputStream fIn = context.openFileInput(nom);
            InputStreamReader ipsr = new InputStreamReader(fIn);
            BufferedReader b = new BufferedReader(ipsr);
            i = getLineNumber(context, nom);
            String[][] s = new String[2][i/2];
            i = 0;
            String ligne;
            int j = 0;
            while ((ligne = b.readLine()) != null) {
                if (i % 2 == 0)
                    s[0][j] = ligne;
                else {
                    s[1][j] = ligne;
                    j++;
                }
                i++;
            }
            fIn.close();
            ipsr.close();
            return s;

        } 
        catch (Exception e) 
        {}

I'm not understanding why the using of a 2D array? and with two rows ?(String[][] s = new String[2][i/2];) here is the data that it will be stored in the file:

data = date + " : " + y + "L/100KM"+ " " + value1 + "L "+ value2 + "KM\n";

Necessary functions:

 public void updatelv(Activity activity) {
        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(context);
        String fileName = getResources().getString(R.string.fileName);
        fileDir = "" + preferences.getString("login", "") + "."+ preferences.getString("marque", "") + ".";
        s = myIO.ReadFilePerLine(getApplicationContext(), fileDir+fileName);
        ListView L = (ListView) findViewById(R.id.lv);
        L.setAdapter(new ArrayAdapter<String>(this, R.layout.list_item, s[0]));
        for (int i = 0; i< s[0].length; i++) { 
            Log.d("Saves",s[0][i]); 
        } 
        }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.histo);
        context = getApplicationContext();
        activity = this;
        final SharedPreferences preferences = PreferenceManager
                .getDefaultSharedPreferences(context);
        String fileName = getResources().getString(R.string.fileName);
        fileDir = "" + preferences.getString("login", "") + "."+ preferences.getString("marque", "") + ".";
        s = myIO.ReadFilePerLine(getApplicationContext(), fileDir + fileName);

        updatelv(this);
        ListView L = (ListView) findViewById(R.id.lv);
        L.setTextFilterEnabled(true);

        L.setOnItemClickListener(new OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view,
                    i开发者_开发技巧nt position, long id) {
                // When clicked, show a toast with the TextView text
                String tmp = s[1][position];
                if (tmp == null)
                    tmp = "Aucun fichier trouvé!";
                Toast.makeText(getApplicationContext(), tmp, Toast.LENGTH_SHORT)
                        .show();
            }
        });

ReadFilePerLine function:

public static String[][] ReadFilePerLine(Context context, String nom) {
        int i = 0;
        try {
            FileInputStream fIn = context.openFileInput(nom);
            InputStreamReader ipsr = new InputStreamReader(fIn);
            BufferedReader b = new BufferedReader(ipsr);
            i = getLineNumber(context, nom);
            String[][] s = new String[2][i/2];
            i = 0;
            String ligne;
            int j = 0;
            while ((ligne = b.readLine()) != null) {
                if (i % 2 == 0)
                    s[0][j] = ligne;
                else {
                    s[1][j] = ligne;
                    j++;
                }
                i++;
            }
            fIn.close();
            ipsr.close();
            return s;

        } 
        catch (Exception e) 
        {

        }

Thank you for you help.

The code is clearly reading from a file whose format consists of pairs of lines; it puts the first line of each pair in s[0][...] and the second line of each pair in s[1][...]. If your format doesn't have that peculiarity -- which it doesn't sound as if it does -- then you don't need to do that. Just make an ordinary 1-dimensional array of Strings.

It appears that what they are doing is breaking the file down into two lists (or String arrays, in this case), one which contains all the even-numbered lines, and one which contains all the odd-numbered lines. I'll comment up the code for you:

public static String[][] ReadFilePerLine(Context context, String nom) {
    int i = 0;
    try {
        //open the specified input file and create a reader
        FileInputStream fIn = context.openFileInput(nom);
        InputStreamReader ipsr = new InputStreamReader(fIn);
        BufferedReader b = new BufferedReader(ipsr);

        //get the total number of lines in the file, and allocate 
        //a buffer large enough to hold them all
        i = getLineNumber(context, nom);
        String[][] s = new String[2][i/2];

        i = 0;      //set the current line to 0
        String ligne;
        int j = 0;  //set the section index to 0

        //now read through the lines in the file, and place every
        //even-numbered line in the first section ('s[0]'), and every 
        //odd-numbered line in the second section ('s[1]')
        while ((ligne = b.readLine()) != null) {
            if (i % 2 == 0)
                //even-numbered line, it goes into the first section
                s[0][j] = ligne;
            else {
                //odd-numbered line, it goes into the second section
                s[1][j] = ligne;
                j++;  //increment the section index
            }
            i++;  //increment the line count
        }

        //done, cleanup and return
        fIn.close();
        ipsr.close();
        return s;
    } 
    catch (Exception e) {
        //should at least log an error here...
    }
}

As to why they chose to use a String[][], I cannot say. Probably for convenience, since they want a single object that they can return from this function that contains both lists. Personally I would use a Map that has two List instances in it, but the String[][] works just as well and is probably marginally more efficient.

Judging from your example data it does not appear that you need to use this format. But if you want to use it, you need to structure your data so that the key is on one line, and its associated value is on the next, like:

date
2011-03-19
userName
someGuy

it seems to read from a file, split it into the two dimensional array (based on row count).

Why it does it? I have no idea why you'd want that. Check out the function that it returns s to and find out!