Java: File I/O - Text Files

Java can read several types of information from files: binary, Java objects, text, zipped files, etc. One of the most common problems is reading lines of text.

Example: Copy one file to another

This example reads text files using the classes FileReader, BufferedReader, FileWriter, and BufferedWriter. It's a main program without a graphical user interface, taking parameters from the command line.

To change this to a graphical user interface, you can use JFileChooser to get a File object instead of new File(args[...]).

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// File:  io/readwrite/CopyTextFile.java
// Description: Example to show text file reading and writing.
// Author: Fred Swartz
// Date:   Februar 2005

import java.io.*;

public class CopyTextFile {

    public static void main(String args[]) {
        //... Get two file names from parameters on command line.
        if (args.length != 2) {
            System.err.println("Usage: java CopyTextFile sourcefile targetfile");
            System.exit(1);
        }

        //... Create File objects.
        File inFile  = new File(args[0]);  // read from first file specified
        File outFile = new File(args[1]);  // write into second file

        //... Enclose in try..catch because of possible io exceptions.
        try {
            //... Create reader and writer for text files.
            BufferedReader reader = new BufferedReader(new FileReader(inFile));
            BufferedWriter writer = new BufferedWriter(new FileWriter(outFile));

            //... Loop as long as there are input lines.
            String line = null;
            while ((line=reader.readLine()) != null) {
                writer.write(line);
                writer.newLine();   // Write system dependent end of line.
            }

            //... Close reader and writer.
            reader.close();  // Close to unlock.
            writer.close();  // Close to unlock and flush to disk.

        } catch (IOException e) {
            System.err.println(e);
            System.exit(1);
        }
    }
}

GUI interface

javax.swing.JFileChooser creates a standard file dialog and allows you to easily get a File object for reading or writing. See JFileChooser.

Working with the text lines

The above program does nothing with the lines of text except write them. Typically, you need to get values from the lines. There are several useful ways to extract data from the lines.

1. java.util.Scanner (as of Java 1.5) is very useful for parsing fields from a string / file.
2. Regular expresssions, as of Java 1.4, provide the best way to breaking strings into tokens like words, numbers, punctuation, etc. Regular expressions are a good replacement for the deprecated java.util.StringTokenizer class. The simplest way to break the input apart is to use the String split() method.
3. java.util.StringTokenizer is an older class for breaking strings apart into tokens. Usually Scanner or regular expressions will be a better solution, altho they require newer versions of Java.