Question

How can I read from, and write to, files in Java?

Answer

Stand-alone applications written in Java can access the local file-system, but applets executing under the control of a browser cannot. With this in mind, lets take a look at two simple applications that write a line of text to a file, and read it back.

import java.io.*;

public class MyFirstFileWritingApp
{
	// Main method
	public static void main (String args[])
	{
		// Stream to write file
		FileOutputStream fout;		

		try
		{
		    // Open an output stream
		    fout = new FileOutputStream ("myfile.txt");

		    // Print a line of text
		    new PrintStream(fout).println ("hello world!");

		    // Close our output stream
		    fout.close();		
		}
		// Catches any error conditions
		catch (IOException e)
		{
			System.err.println ("Unable to write to file");
			System.exit(-1);
		}
	}	
}

This simple application creates a FileOutputStream, and writes a line of text to the file. You'll notice that the file writing code is enclosed within a try { .... } catch block. If you're unfamiliar with exception handling in Java, this requires a little explanation.

Certain methods in Java have the potential to throw an error condition, known as an exception. We have to trap these error conditions, so we 'catch' any exceptions that may be thrown.

The task of reading from a file is also just as easy. We create a FileInputStream object, read the text, and display it to the user.

import java.io.*;

public class MyFirstFileReadingApp
{
	// Main method
	public static void main (String args[])
	{
		// Stream to read file
		FileInputStream fin;		

		try
		{
		    // Open an input stream
		    fin = new FileInputStream ("myfile.txt");

		    // Read a line of text
		    System.out.println( new DataInputStream(fin).readLine() );

		    // Close our input stream
		    fin.close();		
		}
		// Catches any error conditions
		catch (IOException e)
		{
			System.err.println ("Unable to read from file");
			System.exit(-1);
		}
	}	
}

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