Write to file line by line in Java

We can use java.io.BufferedWriter class for efficiently writing text to the file line by line.The java.io.BufferedReader and java.io.BufferedWriter classes
provide internal character buffers. Text that's written to a buffered writer is stored in the internal buffer and only written to the underlying writer when the buffer fills up or is flushed.Likewise, reading text from a buffered reader may cause more characters to be read than were requested; the extra characters are stored in an internal buffer. Future reads first access characters from the internal buffer and only access the underlying reader when the buffer is
emptied.

public class BufferedWriter 

extends Writer

The java.io.BufferedWriter class is a subclass of java.io.Writer that is chained to another Writer class to buffer output.This allows more efficient writing of characters and lines.The buffer size may be specified, or the default size may be used. The default is large enough for most purposes.

There are two constructors. One has a default buffer size (8192 characters); the other requires the programmer to specify the buffer size:

public BufferedWriter(Writer in, int buffer_size)

public BufferedWriter(Writer in)

In general, each write request made of a Writer causes a corresponding write request to be made of the underlying character or byte stream. It is therefore advisable to wrap a BufferedWriter around any Writer whose write() operations may be costly, such as FileWriters and OutputStreamWriters. For example,

 BufferedWriter out = new BufferedWriter(new FileWriter("t1.txt"));
 

Write to File Line by Line using BufferedWriter and FileWriter

The following code shows how to write text to a file line by line using FileWriter and BufferedWriter.

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

public class WriteToFileLineByLine {

    /**
     *
     * @param Pass
     *
     *            Filename as command-line argument
     *
     * @throws IOException
     **/

    public static void main(String[] args) {

        BufferedWriter bw = null;

        try {

            bw = new BufferedWriter(new FileWriter(args[0]));

            bw.write("This is the first line in the file");
            bw.newLine();
            bw.write("This is the second line in the file");
            bw.newLine();

        } catch (IOException e) {

            System.out.println(e);

        } finally {

            try {

                bw.close();

            } catch (IOException e) {

                e.printStackTrace();

            }

        }

    }

}

The one new method in BufferedWriter class is newLine(). This method writes a platform-dependent line terminator string: \n on Unix, \r on the Mac, \r\n on Windows. The value of this string is taken from the system property line.separator.

One drawback of the above code is that FileReader and FileWriter, which implicitly use the system's default character encoding.In some scenarios this default is not appropriate(for example, read an XML file with specified decoding). Here you can make use of java.io.InputStreamReader and java.io.OutputStreamWriter classes.

public class InputStreamReader

extends Reader

An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted.

public class OutputStreamWriter
extends Writer

An OutputStreamWriter is a bridge from character streams to byte streams: Characters written to it are encoded into bytes using a specified charset. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted.Its constructor connects a character writer to an underlying output stream:

The following code shows how to write a text to a file line by line, using an explicit encoding.

import java.io.BufferedWriter;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;

public class WriteToUTF8FileLineByLine {

    /**
     *
     * @param Pass
     *
     *            Filename as command-line argument
     *
     * @throws IOException
     **/

    public static void main(String[] args) {

        BufferedWriter bw = null;

        try {
            FileOutputStream fout = new FileOutputStream(args[0]);

            bw = new BufferedWriter(new OutputStreamWriter(fout, "UTF-8"));

            bw.write("This is the first line in the file");
            bw.newLine();
            bw.write("This is the second line in the file");
            bw.newLine();

        } catch (IOException e) {

            System.out.println(e);

        } finally {

            try {

                bw.close();

            } catch (IOException e) {

                e.printStackTrace();

            }

        }

    }

}

If you remove reference to encoding from OutputStreamWriter class, it will still work -- the system's default encoding will simply be used instead.