Module 3: Elementary Java, Part II

import java.io.*;

public class file_io2 {

  public static void main (String[] argv)
  {
    try {
      // Need to associate a file with a PrintWriter.
      // The last parameter is set to "true" to indicate
      // auto-flush should be activated.
      FileWriter fr = new FileWriter ("testdata2");
      PrintWriter pw = new PrintWriter (fr, true);

      // Now we're ready for writing.
      pw.println ("Hello");
      pw.println ("Hello again");

      // Done.
      pw.close();
    }
    catch (IOException e) {
      System.out.println (e);
    }
    
  }
}

Note:

• The stream is closed after writing is complete.
• PrintWriter is used for text files.

File I/O: reading and writing to the same file

Reading and writing to the same file can be done in many ways. We will consider the following simple task:

• Read the file into memory.
• Change its contents.
• Write the new contents back to the same file.

Example: ( source file )

import java.io.*;

public class file_io3 {

  public static void main (String[] argv)
  {
    String filename = "testdata3";
    int n_lines = 0;
    String[] line_buffer = null;

    // First, we read in the file to get the size.
    try {
      FileReader fr = new FileReader (filename);
      LineNumberReader lr = new LineNumberReader (fr);

      boolean over = false;         
      n_lines = 0;
      do {
        String input_line = lr.readLine ();
        if (input_line != null)
          n_lines ++;
        else
          over = true;
      } while (! over);

      lr.close();
    }
    catch (IOException e) {
      System.out.println (e);
    }

    // We have now counted the number of lines and
    // we will read the file into a buffer.
    try {
      FileReader fr = new FileReader (filename);
      LineNumberReader lr = new LineNumberReader (fr);

      // Allocate necessary space.
      line_buffer = new String[n_lines];

      int i = -1;
      boolean over = false;
      do {
        String s = lr.readLine ();
        if (s == null) 
          over = true;
        else 
           line_buffer [++i] = s;
      } while (! over);

      lr.close();
    }
    catch (IOException e) {
      System.out.println (e);
    }
    
    // Next, write the buffer out to the same file
    // with line numbers added.
    try {
      // Open the file for writing
      FileWriter fr = new FileWriter (filename);
      PrintWriter pw = new PrintWriter (fr, true);

      // Write from buffer with line numbers:
      for (int i=0; i < n_lines; i++) 
        pw.println ("Line " + (i+1) + ": " + line_buffer[i]);

      pw.close();
    }
    catch (IOException e) {
      System.out.println (e);
    }

  }
}

File I/O: Appending to a file

To append to a file, simply use the appropriate arguments to FileWriter: include "true" to indicate appending.

Example: ( source file )

import java.io.*;
import java.io.*;

public class file_io_append {

  public static void main (String[] argv)
  {
    try {
      // Using "true" as a second argument indicates "append"
      // in FileWriter.
      FileWriter fr = new FileWriter ("testdata_append", true);
      PrintWriter pw = new PrintWriter (fr, true);

      // Now we're ready for writing.
      pw.println ("Hello");
      pw.println ("Hello again");

      // Done.
      pw.close();
    }
    catch (IOException e) {
      System.out.println (e);
    }
    
  }
}

Simple input parsing

Consider an input file that looks like this:

Circle:
  center.x=50
  center.y=60
  radius=20
Circle:
  center.x=320
  center.y=180
  radius=40
Rectangle: 
  topleft.x=50
  topleft.y=400
  bottomright.x=500
  bottomright.y=40

What we would like to do is to parse the input to extract the numbers.

• Reading multiple numbers (or even one number following a text string) is difficult in Java.
• Accordingly, let us develop a useful method to read a file in the above format -- a type of `properties' file.
• The library provides a StringTokenizer object that performs rudimentary lexical analysis.
• We will write a method to parse a single `property' string like "radius=9.5".

Here is how it can be done ( source file ):

// We need to import the StringTokenizer object in
// the java.util package:

import java.util.*;

public class file_io4 {

  // Parameters:
  //   in_string: the property string (e.g. "radius=0.9")
  //   property:  the property name (e.g., "radius")
  // The value is assumed to be a real number.

  public static double read_property (String in_string, String property)
  {
    // Obtain a copy of the StringTokenizer object.
    StringTokenizer st = new StringTokenizer (in_string);

    // Get the first token, specifying the delimiter.
    String first_part = st.nextToken ("=");

    // Trim whitespace on either side - a String method.
    first_part = first_part.trim ();

    System.out.println ("First part: " + first_part);

    if (!first_part.equals (property)){
      System.out.println ("Improper string: " + in_string);
      System.exit (0);
    }

    // Get the next token, now allowing for end-of-line.
    String second_part = st.nextToken (" =\t\n\r");

    // Trim whitespace.
    second_part = second_part.trim();

    System.out.println ("Second part: " + second_part);

    // Read the number in the second part using the library's
    // Double object.
    try {
      double d = Double.parseDouble (second_part);
      return d;
    }
    catch (NumberFormatException e) {
      System.out.println ("Second part not a number: " + second_part);
      System.exit (0);
    }
    return 0;
  }

  public static void main (String[] argv)
  {
    // Test
    double d = read_property ("x=5", "x");
    System.out.println (d);

    d = read_property ("center.x=5", "center.x");
    System.out.println (d);

    d = read_property ("center.x=5   ", "center.x");
    System.out.println (d);

    d = read_property (" center.x = 5   ", "center.x");
    System.out.println (d);
  
  }
}

Note:

• We have used that strange new operator again to create a StringTokenizer.
• The string to be parsed is fed into this object in the new statement.
• The object so created has a nextToken() method that returns tokens, one at a time.
• A String object has a trim() method that trims off whitespace, returning the result as a String.
• Once we have a real number in a String, use the parseDouble in Double to extract the value.
• Finally, we have import'ed a library object (StringTokenizer by importing the package (java.util) containing it:

 import java.util.*;
 

In-class exercise 3.2: Use the above method, and the methods for reading to a file for the following task. Suppose we want to extract information from a file containing a description of a circle. For example (source file):

Circle:
  center.x=50
  center.y=60
  radius=20

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

class useful_io {

  // Read a string from the screen.

  static String read_string (String prompt)
  {
     // Code not shown
  }


  // Read an int from the screen.

  public static int read_int (String prompt)
  {
     // Code not shown    
  }


  // Read a double from the screen.

  public static double read_double (String prompt)
  {
     // Code not shown
  }


  // Parse a string for a property.

  public static double read_property (String in_string, String property)
  {
     // Code not shown
  }

} // End of class useful_io

public class read_geodata2 {

  public static void main (String[] argv)
  {
    // Get file name from user
    file name = useful_io.read_string ("Enter filename: ");

    // ... rest of code not shown.
  }

}

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

public class useful_io {

  // Read a string from the screen.

  public static String read_string (String prompt)
  {
     // Code not shown
  }


  // Read an int from the screen.

  public static int read_int (String prompt)
  {
     // Code not shown    
  }


  // Read a double from the screen.

  public static double read_double (String prompt)
  {
     // Code not shown
  }


  // Parse a string for a property.

  public static double read_property (String in_string, String property)
  {
     // Code not shown
  }

} // End of class useful_io

import java.io.*;

public class read_geodata3 {

  public static void main (String[] argv)
  {
    // Get file name
    String filename = useful_io.read_string ("Enter filename: ");

    // Rest of code not shown

  }
} // End of read_geodata3.java