<head>
<link rel="stylesheet" type="text/css" href="../../notes/cs365_notes.css" />
</head>
<center>
<h2>Java File I/O</h2>
</center>
<hr>
This example shows you a simple way to read and write from files
using the following classes:
<p>
<ol>
<li> FileReader/BufferedReader: FileReader opens a file for input
     and BufferedReader wraps it so that buffered reading is performed
<li> Scanner: The BufferedReader's readLine method reads a line at a
     time. This line is input to a Scanner object, which tokenizes it
<li> FileWriter/BufferedWriter/PrintWriter: 
     <ul>
     <li> FileWriter opens a file for output,
     <li> BufferedWriter wraps it so that buffered writing is performed
     <li> PrintWriter wraps a BufferWriter so that you can use formatting
       commands to write your output
     </ul>
</ol>
<p>
The example reads lines from a file, formats them, and writes them to
an output file. The lines have the form:
<pre>
age true/false
</pre>
where true/false is a person's guess as to whether or not the age is correct.
For example:
<pre>
40 true
35 false
7 true
</pre>
The corresponding output would be:
<pre>
age =   40 correct = true
age =   35 correct = false
age =    7 correct = true
</pre>
The program takes the input file as a command line argument and hard codes
the output file.
<p>
<hr>
<h2> The Example Program </h2>
<pre>
import java.util.*;
import java.io.*;

public class fileio {

    public static void main(String[] args) throws IOException {
        new fileio(args);
    }

    fileio(String[] args) throws IOException {
	BufferedReader input_reader;
	Scanner line_scanner;
	PrintWriter fw;
	String nextLine;
        int age;
        boolean correct;
          input_reader = new BufferedReader(new FileReader(args[0]));
  	  fw = new PrintWriter(
			new BufferedWriter(
				new FileWriter("test.txt")));
	  // readLine() returns null when it reaches EOF
	  while ((nextLine = input_reader.readLine()) != null) {
	    // if the user hit return after the last line of the file,
	    // then there is an empty last line and the while condition
	    // will appear to succeed. The next if statement
	    // caters to that possibility and gets us out of the
	    // loop if there is an empty last line
	    if (nextLine.equals("")) break;
	    line_scanner = new Scanner(nextLine);
	    age = line_scanner.nextInt();
	    correct = line_scanner.nextBoolean();
	    line_scanner.close();
            fw.format("age = %4d correct = %b%n", age, correct);
	  }
          input_reader.close();
	  fw.close();
    }
}
</pre>