Reading and Writing Files in Java

Below is a good example of reading and writing files in Java
http://www.caveofprogramming.com/frontpage/articles/java/java-file-reading-and-writing-files-in-java/

About File Handling in Java

You can read files using these classes:

• FileReader for text files in your system’s default encoding (for example, files containing Western European characters on a Western European computer).
• FileInputStream for binary files and text files that contain ‘weird’ characters.

FileReader (for text files) should usually be wrapped in aBufferedFileReader. This saves up data so you can deal with it a line at a time or whatever instead of character by character (which usually isn’t much use).

If you want to write files, basically all the same stuff applies, except you’ll deal with classes named FileWriter with BufferedFileWriter for text files, orFileOutputStream for binary files.

import java.io.*;

public class Test {
    public static void main(String [] args) {

        // The name of the file to open.
        String fileName = "temp.txt";

        // This will reference one line at a time
        String line = null;

        try {
            // FileReader reads text files in the default encoding.
            FileReader fileReader = 
                new FileReader(fileName);

            // Always wrap FileReader in BufferedReader.
            BufferedReader bufferedReader = 
                new BufferedReader(fileReader);

            while((line = bufferedReader.readLine()) != null) {
                System.out.println(line);
            } 

            // Always close files.
            bufferedReader.close();   
        }
        catch(FileNotFoundException ex) {
            System.out.println(
                "Unable to open file '" + 
                fileName + "'");    
        }
        catch(IOException ex) {
            System.out.println(
                "Error reading file '" 
                + fileName + "'");     
            // Or we could just do this: 
            // ex.printStackTrace();
        }
    }
}

If “temp.txt” contains this:

I returned from the City about three o'clock on that 
May afternoon pretty well disgusted with life. 
I had been three months in the Old Country, and was 
fed up with it. 
If anyone had told me a year ago that I would have 
been feeling like that I should have laughed at him; 
but there was the fact. 
The weather made me liverish, 
the talk of the ordinary Englishman made me sick, 
I couldn't get enough exercise, and the amusements 
of London seemed as flat as soda-water that 
has been standing in the sun. 
'Richard Hannay,' I kept telling myself, 'you 
have got into the wrong ditch, my friend, and 
you had better climb out.'

The program outputs this:

I returned from the City about three o'clock on that
May afternoon pretty well disgusted with life.
I had been three months in the Old Country, and was
fed up with it.
If anyone had told me a year ago that I would have
been feeling like that I should have laughed at him;
but there was the fact.
The weather made me liverish,
the talk of the ordinary Englishman made me sick,
I couldn't get enough exercise, and the amusements
of London seemed as flat as soda-water that
has been standing in the sun.
'Richard Hannay,' I kept telling myself, 'you
have got into the wrong ditch, my friend, and
you had better climb out.'

Reading Binary Files in Java

If you want to read a binary file, or a text file containing ‘weird’ characters (ones that your system doesn’t deal with by default), you need to useFileInputStream instead of FileReader. Instead of wrapping FileInputStream in a buffer, FileInputStream defines a method called read() that lets you fill a buffer with data, automatically reading just enough bytes to fill the buffer (or less if there aren’t that many bytes left to read).

Here’s a complete example.

import java.io.*;

public class Test {
    public static void main(String [] args) {

        // The name of the file to open.
        String fileName = "temp.txt";

        try {
            // Use this for reading the data.
            byte[] buffer = new byte[1000];

            FileInputStream inputStream = 
                new FileInputStream(fileName);

            // read fills buffer with data and returns
            // the number of bytes read (which of course
            // may be less than the buffer size, but
            // it will never be more).
            int total = 0;
            int nRead = 0;
            while((nRead = inputStream.read(buffer)) != -1) {
                // Convert to String so we can display it.
                // Of course you wouldn't want to do this with
                // a 'real' binary file.
                System.out.println(new String(buffer));
                total += nRead;
            } 

            // Always close files.
            inputStream.close();  

            System.out.println("Read " + total + " bytes");
        }
        catch(FileNotFoundException ex) {
            System.out.println(
                "Unable to open file '" + 
                fileName + "'");    
        }
        catch(IOException ex) {
            System.out.println(
                "Error reading file '" 
                + fileName + "'");     
            // Or we could just do this: 
            // ex.printStackTrace();
        }
    }
}

I returned from the City about three o'clock on that
May afternoon pretty well disgusted with life.
I had been three months in the Old Country, and was
fed up with it.
If anyone had told me a year ago that I would have
been feeling like that I should have laughed at him;
but there was the fact.
The weather made me liverish,
the talk of the ordinary Englishman made me sick,
I couldn't get enough exercise, and the amusements
of London seemed as flat as soda-water that
has been standing in the sun.
'Richard Hannay,' I kept telling myself, 'you
have got into the wrong ditch, my friend, and
you had better climb out.'

Read 649 bytes

Of course, if you had a ‘real’ binary file — an image for instance — you wouldn’t want to convert it to a string and print it on the console as above.

Writing Text Files in Java

To write a text file in Java, use FileWriter instead of FileReader, andBufferedOutputWriter instead of BufferedOutputReader. Simple eh?

Here’s an example program that creates a file called ‘temp.txt’ and writes some lines of text to it.

import java.io.*;

public class Test {
    public static void main(String [] args) {

        // The name of the file to open.
        String fileName = "temp.txt";

        try {
            // Assume default encoding.
            FileWriter fileWriter =
                new FileWriter(fileName);

            // Always wrap FileWriter in BufferedWriter.
            BufferedWriter bufferedWriter =
                new BufferedWriter(fileWriter);

            // Note that write() does not automatically
            // append a newline character.
            bufferedWriter.write("Hello there,");
            bufferedWriter.write(" here is some text.");
            bufferedWriter.newLine();
            bufferedWriter.write("We are writing");
            bufferedWriter.write(" the text to the file.");

            // Always close files.
            bufferedWriter.close();
        }
        catch(IOException ex) {
            System.out.println(
                "Error writing to file '"
                + fileName + "'");
            // Or we could just do this:
            // ex.printStackTrace();
        }
    }
}

The output file now looks like this (after running the program):

Hello there, here is some text.
We are writing the text to the file.