ENGR 131: TEXT FILE INPUT/OUTPUT IN JAVA

Overview: TextFileIO.java

The class is located in the file TextFileIO.java in the ENGR 131 Code Repository. To use its methods in a Java program, it should be copied to the same directory (DrJava and javac/java) or same project (BlueJ) as the program you call its methods from.

Reading from a text file with openRead

This particular line sets up a Scanner (which we have named input) to read from the file named infile.txt.

Notes:

• In this case, infile.txt must be a file that is located in the same directory as the Java program you are running. If it does not exist in that directory, Java will report an error.
• openRead is preceded by the name of the class, "TextFileIO", and the '.' because the class is contained in a different file. Compare this to calling Math.random()
• Since we are using Scanner, your program would require you to import it using import java.util.Scanner;

The Scanner input can now be used to read from this file in the same way that you used a Scanner to read from the keyboard:

• Use input.nextInt() to read the next int
• Use input.nextDouble() to read the next double
• Use input.next() to read the next String (that contains no whitespace)

Technical Note: You don't have to use "input" as the name of the Scanner. However, the name of the Scanner must match the name that precedes the '.' in the method calls.

Example 1

// This file demonstrates reading data from a text file using
// the Scanner class and the TextFileIO.openRead method
//
//  -- M. Branicky, 10/09/06

import java.util.Scanner; // needed for input processing using Scanner

public class ReadDataTrial
{
  public static void main(String args[])
  {
    // Open the file [versus creating a new Scanner for the keyboard]
    Scanner input = TextFileIO.openRead("MadlibDataTrial.txt");
    // KB: Scanner input = new Scanner( System.in );

    // Variables to be used in the story
    double number = 0.0;
    String pluralNoun = "", adjective = "";

    // Get input from file [no need to ask user, else no changes]
    // KB: System.out.print("Enter a plural noun and press : ");
    pluralNoun = input.next();
    // KB: System.out.print("Enter an adjective and press : ");
    adjective = input.next();
    // KB: System.out.print("Enter a number and press : ");
    number = input.nextDouble();

    // Display story [No changes]
    System.out.printf("After the man ate %f %s, he felt very %s.\n",
       number, pluralNoun, adjective);

    // Close the file [NEW]
    input.close();
  }
}

eggs
full
50

After the man ate 50.000000 eggs, he felt very full.

Example 2

eggs full 50
worms icky 3.5
hotdogs sick 11

After the man ate 50.000000 eggs, he felt very full.
After the man ate 3.500000 worms, he felt very icky.
After the man ate 11.000000 hotdogs, he felt very sick.

// This file demonstrates reading data from a text file using
// the Scanner class and the TextFileIO.openRead method
//
//  -- M. Branicky, 10/09/06

import java.util.Scanner; // needed for input processing using Scanner

public class ReadData
{
  public static void main(String args[])
  {
    // Open the file
    Scanner input = TextFileIO.openRead("MadlibData.txt");

    // Variables to be used in the story
    double number = 0.0;
    String pluralNoun = "", adjective = "";

    // Get input from file
    while (input.hasNext()) {
      pluralNoun = input.next();
      adjective = input.next();
      number = input.nextDouble();

      // Display story
      System.out.printf("After the man ate %f %s, he felt very %s.\n",
         number, pluralNoun, adjective);
    }

    // Close the file
    input.close();
  }
}

Writing to a text file with openWrite

This particular line sets up a PrintWriter (which we have named output) in order to write to the file named outfile.txt.

Notes:

• In this case, outfile.txt will be located in the same directory as the Java program you are running. If a file by that name already exists in that directory, Java will overwrite it.
• Again, openWrite is preceded by the name of the class, TextFileIO and the '.' because the class is contained in a different file.
• Since we are using PrintWriter, your program would require you to import it using import java.io.PrintWriter;

The PrintWriter output can now be used to write to the file in the same way that you used "System.out.*" to write to the screen:

• Use output.print() to print without a newline
• Use output.println() to print with a newline
• Use output.printf() for formatted printing

Technical Note: You don't have to use "output" as the name of the PrintWriter. However, the name of the PrintWriter must match the name that precedes the '.' in the method calls.

Example 3

// This file demonstrates writing data to a text file using
// the PrintWriter class and the TextFileIO.openWrite method
//
//  -- M. Branicky, 10/09/06

import java.io.PrintWriter; // needed for output processing using PrintWriter

public class WriteData
{
  public static void main(String args[])
  {
    // Open the file
    PrintWriter output = TextFileIO.openWrite("out.txt");

    // Write output to the file
    output.println("Prints an entire line, plus a carriage return.");
    output.print("Prints a string without ... ");
    output.print("a new line ... ");
    output.println("."); // end the line
    output.println();
    output.println("The square root of 2 is " + Math.sqrt(2.0));
    output.printf("The square root of 2 is %f%n", Math.sqrt(2.0));
    output.printf("%d + %d = %d%n", 2, 2, 2+2);
    output.println(2 + " + " + 2 + " = " + (2+2));
    output.println();

    // Close the file
    output.close();
  }
}

Important Note: If you look closely at the program above, you will see one thing different than we've done before. Namely, in the printf statements, "%n" is used instead of "\n". The reason for this is that Windows uses a different format to end lines in files than MacOSX/Linux/Unix. The "%n" will (tell the Java Virtual Machine to) do the right thing on the different platforms.

The code for TextFileIO.java

Still, it is not that hard to understand. The major new item is Java's try-catch block to catch exceptions (aka errors) that could be raised, viz. that the file could not be accessed. Briefly, the program tries to execute the code in the try block; if an exception is raised (an error occurs) relating to the file not being found, it is caught by the catch block, which executes some contingency code (in this case, printing an error message and exiting the method and the whole program). To read more about try and catch, see page 645 of Deitel and Deitel.

// This class encapsulates the details of text file IO in Java
// 
// It has two methods:
//    openRead --- takes a filename (String) as a parameter and
//                 returns a Scanner for reading from that file
//    openWrite -- takes a filename (String) as a parameter and
//                 returns a PrintWriter for writing to that file
//
// M. Branicky, 10/09/06

import java.io.File;        // needed for opening/accessing/closing files
import java.util.Scanner;   // needed for input processing using Scanner
import java.io.PrintWriter; // needed for output processing using PrintWriter
import java.io.FileNotFoundException;  // needed for exception handling

public class TextFileIO 
{
  public static Scanner openRead (String filename) {
    Scanner input = null;
    File file = new File(filename);
    try {
      input = new Scanner(file);
    }
    catch (FileNotFoundException f)
    {
      System.out.println(filename + " not found.");
      System.exit(1);
    }
    return input;
  }

  public static PrintWriter openWrite (String filename) {
    PrintWriter output = null;
    File file = new File(filename);
    try {
      output = new PrintWriter(file);
    }
    catch (FileNotFoundException f)
    {
      System.out.println("Could not open "+filename);
      System.exit(1);
    }
    return output;
  }
}

Example Files