Read special line from txt file in j2me

Question

I have a text file, for example : file.txt, I want to read a line, for example line 7, have any way to read directly line 7 without reading other lines? I want to save memory from this work.

Answer 1

Because of the way JME been cut down you cannot do this. You would have to read whole file. Only other way, but may not be well suitable is to read the file , store it in RecordStore new entry per line, but is it really worth...

Answer 2

Common Code

private String readLine(InputStream _inStream, int lineNum) 
            throws IOException {
    if (null == _inStream) {
        throw new IOException("Inputstream null.");
    }
    if (lineNum < 0) {
        return ("Cannot read line a number " + lineNum);
    }

    final StringBuffer buf = new StringBuffer();
    byte c;

    int curLine = 1;
    while (((c = (byte) _inStream.read()) != -1)) {
        //System.out.println((char)c);
        if (c == '\n') {
            ++curLine;
            if (curLine > lineNum) {
                break;
            } else if (curLine < lineNum) {
                continue;
            }
        } else if (curLine != lineNum) {
            continue;
        }
        buf.append((char) c);
    }

    if (0 == buf.length()) {
        return null;
    } else {
        return buf.toString().trim();
    }
}

.

private String readLineWithSkip(InputStream _inStream, long skipCharacters) 
            throws IOException {
    if (null == _inStream) {
        throw new IOException("Inputstream null.");
    }
    if (skipCharacters < 1) {
        return ("Cannot skip stream of " + skipCharacters + " characters");
    }

    final StringBuffer buf = new StringBuffer();
    byte c;

    _inStream.skip(skipCharacters);
    while ((c = (byte) _inStream.read()) != '\n') {
        //System.out.println((char)c);
        buf.append((char) c);
    }

    if (0 == buf.length()) {
        return null;
    } else {
        return buf.toString().trim();
    }
}

.

InputStream inStream = null;
int fileLength = 39;
int skipCharacters = 10;
int lineNumber = 3;
String myLine = "No line read.";
try {
    inStream = Class.class.getResourceAsStream("/test.txt");
    if (null != inStream) {
        inStream.mark(fileLength);

    //For Approach II
        myLine = readLine(inStream, lineNumber);

        inStream.reset();
    //For Approach I
        myLine = readLineWithSkip(inStream, skipCharacters);
    }
} catch (SecurityException se) {
    se.printStackTrace();

} catch (IOException ioe) {
    ioe.printStackTrace();

} finally {
    try {
        inStream.close();
    } catch (IOException ex) {
        ex.printStackTrace();
    } catch (NullPointerException e) {
        e.printStackTrace();
    }
    inStream = null;

    System.out.println(myLine);
}

.

Approach I: Map the line number with no of cumulative characters

1. Run the file through a code that maps the line number with the last of charater in that line from the 0th position of the file (to be used as skip() value) all +2 ('\r\n\') for each line. You can store this Mapping table either at the start or at the end of the same file.
2. The run the above common code with method readLineWithSkip(inStream, skipCharacters); ONLY and judiciously commenting other method calls.

Points to Consider:

1. Skips to the desired position in the inputstream
2. Has an overhead of parsing the file and storing the mapping table.

.

Approach II: Read each line till the Nth line is read

1. The run the above common code with method readLine(inStream, lineNumber); ONLY and judiciously commenting other method calls.

Points to Consider:

1. Slow since it has to read each character till it reaches the desired line
2. No overhead of parsing the file and no storing of the mapping table.

Answer 3

I think it is possible however you need to use a hashtable which might result to more heap usage.

Anyway, first, the contents of the text file should be stored in a char array. Then, second, the contents of the char array must be moved to the hashtable.

Each line in the text file is separated by a new line. In the char array, the new line (maybe) is translated to '\n'. Concatenate the characters in the array until the new line character is reached. The concatenated characters (minus '\n') will form the string in the first line. There should also be a counter here which should have been initialized to 0 (or 1, whatever you prefer). Save the text to the hashtable; The value will be the string that has been created and the key will be the counter. Increment the counter afterwards. Repeat this process for the remainder of the array until the end of file is reached.

With the hashtable, you can now get the string at line 7 without going through the other lines. Well, basically, each line has been read once. But, at least, you don't have to traverse each line once they have been stored in the hashtable.

Like what I have said earlier, doing this might increase heap usage especially if the text file is very large.

[And, by the way, sorry for the very late response. This is my first time here (I mean I just registered and answered this question) :D ]

Answer 4

I would like to further simplify the problem of reading the chars without requiring any mapping of characters with line numbers.

...
                  Form form=new Form("filename");
                  InputStream fin=fconn.openDataInputStream();
                  StringBuffer buf =new StringBuffer();
                  int c;
                  int counter=-1;
                  while((c=fin.read())!=-1)
                  {   
                      counter++;
                      if(counter==23)
                      {
                          form.append(buf.toString());
                          buf=null;counter=0;
                          continue;
                      }

                      buf.append((char)c);
                  }
                  if(counter<23 && counter>=0) // write the skipped chars between the last number read and the end of file
                  {
                    form.append(buf.toString());
                    buf=null;counter=0;
                  }
                  fin.close();
                  ...

Hope this would help others.