Key Ideas
[Unit 6]

There are two key ideas in this unit:

  1. Files are represented by java.io.File objects. One common way to get a path for these is using a java.io.FileDialog:

    FileDialog fd = new FileDialog(new Frame(), "Save", FileDialog.SAVE);
    fd.setVisible(true);
    File f = null;

    if((fd.getDirectory() != null)||( fd.getFile() != null)) {
       f = new File(fd.getDirectory() + fd.getFile());
    }

  2. The second is that if we are reading or writing text files, we do this using streams, but wrapped in buffers. If you use a stream, it is always polite to close it after use:

    BufferedReader br = null;

    try {
       br = new BufferedReader(new FileReader(new File("a:/data.txt")));
    } catch (FileNotFoundException fnfe) {
       return null;
    }

    try {
       return br.readLine();
    } catch(IOException ioe) {
       throw ioe;
    } finally {
       try {
          br.close();
       } catch (IOException ioe) {
          return null;
       }
    }

Here's some code which uses these ideas. Note that it sticks with printStackTrace just for clarity.

double[][] data = null;

BufferedReader br = null;
File f = new File("c:/in.txt");

// Count the number of lines.

try {
   br = new BufferedReader(new FileReader(f));
} catch (FileNotFoundException e) {
   e.printStackTrace();
}

int lines = -1;
String textIn = " ";
String[] file = null;

try {
   while (textIn != null) {
      textIn = br.readLine();
      lines++;
   }
} catch (IOException e) {
   e.printStackTrace();
} finally {
   try {
      br.close();
   } catch (IOException e2) {
      e2.printStackTrace();
   }
}

// Reopen the file at the top and read the
// file into a String array of the right size.

try {
   br = new BufferedReader(new FileReader(f));
} catch (FileNotFoundException e) {
   e.printStackTrace();
}

file = new String[lines];

try {
   for (int i = 0; i < lines; i++) {
      file[i] = br.readLine();
   }
} catch (IOException e) {
   e2.printStackTrace();
} finally {
   try {
      br.close();
   } catch (IOException e2) {
      e2.printStackTrace();
   }
}

// Convert each line into an array of double data.

data = new double [lines][];

for (int i = 0; i < lines; i++) {
   StringTokenizer st = new StringTokenizer(file[i],", ");
   data[i] = new double[st.countTokens()];
   int j = 0;
   while (st.hasMoreTokens()) {
      data[i][j] = Double.parseDouble(st.nextToken());
      j++;
   }
}

// Check data.

for (int i = 0; i < data.length; i++) {
   for (int j = 0; j < data[i].length; j++) {
      System.out.print (data[i][j] + ", ");
      data[i][j]++;
   }
   System.out.println("");
}

// Write data. Note we're going to change the file from
// comma separated variables (CSV) to space separated.

BufferedWriter bw = null;

try {
   bw = new BufferedWriter (new FileWriter(new File("c:/out.txt")));
} catch (IOException e) {
   e.printStackTrace();
}

String tempStr = "";

try{
   for (int i = 0; i < data.length; i++) {
      for (int j = 0; j < data[i].length; j++) {

         tempStr = String.valueOf(data[i][j]);
         System.out.print(tempStr + ", ");
         bw.write(tempStr + ", ");

      }
      System.out.println("");
      bw.newLine();
   }
   bw.flush();
} catch (IOException e) {
   e.printStackTrace();
} finally {
   try {
      br.close();
   } catch (IOException e2) {
      e2.printStackTrace();
   }
}