BufferedWriter

This Java class helps us write Strings and characters to a file. We can write a String with write() and with newLine() we add a newline.

With BufferedWriter we are not limited to Strings. We can write chars, ints, arrays and even substrings. We often pass a FileWriter to the BufferedWriter constructor.

First example

This program introduces the syntax for BufferedWriter. We create a FileWriter and pass an example path to it. This is the target text file.

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

public class Program {
    public static void main(String[] args) throws IOException {

        // Create a BufferedWriter around a FileWriter.
        // ... Write to an output text file.
        BufferedWriter writer = new BufferedWriter(new FileWriter(
                "C:\\programs\\output.txt"));
        // Write these lines to the file.
        // ... We call newLine to insert a newline character.
        writer.write("CAT");
        writer.newLine();
        writer.write("DOG");
        writer.newLine();
        writer.write("BIRD");
        writer.close();
    }
}
CAT
DOG
BIRD

Write string array

This example writes an entire String array to a file. It specifies a newline after each String. So we write an array to lines in a file.

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

public class Program {
    public static void main(String[] args) throws IOException {

        String[] array = new String[3];
        array[0] = "100A";
        array[1] = "200B";
        array[2] = "300C";

        // Create our BufferedWriter.
        BufferedWriter writer = new BufferedWriter(new FileWriter(
                "C:\\programs\\output.txt"));

        // Loop over the elements in the string array and write each line.
        for (String line : array) {
            writer.write(line);
            writer.newLine();
        }
        writer.close();
    }
}
100A
200B
300C

Substrings, chars, ints

BufferedWriter handles more than strings. It can write just a part of a string—a substring (specified with a start index and length).

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

public class Program {
    public static void main(String[] args) throws IOException {

        BufferedWriter writer = new BufferedWriter(new FileWriter(
                "C:\\programs\\output.txt"));

        // Write the second two characters of this string.
        String value = "abcdefghi";
        writer.write(value, 2, 2);
        writer.newLine();

        // Append a character to the file.
        writer.append(value.charAt(1));
        writer.newLine();

        // Write an integer to the file.
        writer.write(97);
        writer.newLine();

        // Write a character array.
        // ... A range of characters can be instead written.
        char[] array = value.toCharArray();
        writer.write(array);

        // Close our writer.
        writer.close();
    }
}
cd
b
a
abcdefghi

IOException, stream closed

Often the BufferedWriter will throw exceptions. In this example, we get an IOException because we write to a file we had already closed.

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

public class Program {
    public static void main(String[] args) throws IOException {

        BufferedWriter writer = new BufferedWriter(new FileWriter(
                "C:\\programs\\output.txt"));

        // Close the writer.
        writer.close();

        // This will cause an exception.
        writer.write("X");
    }
}
Exception in thread "main" java.io.IOException: Stream closed
    at java.io.BufferedWriter.ensureOpen(Unknown Source)
    at java.io.BufferedWriter.write(Unknown Source)
    at java.io.Writer.write(Unknown Source)
    at Program.main(Program.java:15)

Throws IOException

In the examples, we use the "throws IOException" notation on our main methods. This is required—BufferedReader can cause an error for many reasons.

With BufferedReader, and the FileWriter type as help, we write Strings to file with ease. We can form lines with the newLine method.