Learning Java - Chapter 9 : Java

The following program, FormatWriteFileApp, follows closely to that of FormatWriteApp discussed in the Formatter to Console section. There we used the java.util.Formatter class to send formatted output to the console. However, FormatWriteFileApp program instead sends output to a file.

The Formattter class does much of the work. We don't even have to create a FileWriter stream as we did in the previous section. We simply pass the File object to the Formatter constructor and it takes care of all the output work. If the file doesn't exist yet, then the Formatter will create it, though you will need to catch an exception that is thrown in such a case.

FormatWriteFileApp.java

import java.io.*;
import java.util.Formatter;

/**
  * Demonstrate the java.util.Formatter capabilities for
  * formatting primitive types and sending them to a file.
**/
public class FormatWriteFileApp
{
  public static void main (String arg[]) {
    Formatter formatter = null;
    File file = null;

    // Get the file from the argument line.
    if (arg.length > 0) file = new File (arg[0]);
    // Else use a default file name.
    if (file == null) {
      System.out.println ("Default: textOutput.txt");
      file = new File ("textOutput.txt");
    }

    // Send formatted output to the file.
    try {
      formatter = new Formatter (file);
    }
    catch  (FileNotFoundException e) {
      // File not found exception thrown since this is a new
      // file name. However, Formatter will create the new file.
    }

    formatter.format ("Text output with Formatter. %n");
    formatter.format ("Primitives converted to strings: %n");

    boolean a_boolean =  false;
    byte    a_byte    =  114;
    short   a_short   =  1211;
    int     an_int    =  1234567;
    long    a_long    =  987654321;
    float   a_float   =  983.6f;
    double  a_double  = -4.297e-15;

    formatter.format ("boolean = %9b %n",   a_boolean);
    formatter.format ("byte    = %9d %n",   a_byte);
    formatter.format ("short   = %9d %n",   a_short);
    formatter.format ("int     = %9d %n",   an_int);
    formatter.format ("long    = %9d %n",   a_long);
    formatter.format ("float   = %9.3f %n", a_float);
    formatter.format ("double  = %9.2e %n", a_double);

    // Need to flush the data out of the buffer.
    formatter.flush ();
    formatter.close ();

  } // main

} // class FormatWriteFileApp

This program creates a file that contains the same output as we obtained on the console with FormatWriteApp.

References and Web Resources

Chapter 5: Tech : System.out.printf() - J2SE5.0
java.util.Formatter - class specifications for J2SE5.0
- Format String Syntax
- Details

Latest update: Nov. 12, 2004