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Java BufferedWriter

Jakob Jenkov
Last update: 2019-08-31

The Java BufferedWriter class, java.io.BufferedWriter, provides buffering to Writer instances. Buffering can speed up IO quite a bit. Rather than writing one character at a time to the network or disk, the BufferedWriter writes a larger block at a time. This is typically much faster, especially for disk access and larger data amounts.

The Java BufferedWriter is a Java Writer subclass, so it can be used anywhere a Writer is required.

Java BufferedWriter Example

To add buffering to a Writer simply wrap it in a Java BufferedWriter. Here is how that looks:

BufferedWriter bufferedWriter = 
    new BufferedWriter(new FileWriter("c:\\data\\output-file.txt"));

This example creates a BufferedWriter which writes characters to a FileWriter. Simple, isn't it?

BufferedWriter Buffer Size

You can set the buffer size to use internally by the Java BufferedWriter. You provide the size as a constructor parameter, like this:

int bufferSize = 8 * 1024;
    
BufferedWriter bufferedWriter = 
    new BufferedWriter(
        new FileWriter("c:\\data\\output-file.txt"),
            bufferSize);

This example sets the internal buffer of the BufferedWriter to 8 KB. It is best to use buffer sizes that are multiples of 1024 bytes. That works best with most built-in buffering in hard disks etc.

Except for adding buffering to your input streams, BufferedWriter behaves pretty much like a Writer. The BufferedWriter adds one extra method though: The newLine() method which can write a new-line character to the underlying Writer. In addition, you may need to call flush() if you need to be absolutely sure that the characters written until now is flushed out of the buffer and onto the network or disk.

write(int)

The Java BufferedWriter write(int) method writes the lower 16 bit of the int to its internal buffer, as a single character. Here is an example of writing a single character to a Java BufferedWriter:

bufferedWriter.write('A');

write(char[])

The Java BufferedWriter also has a write(char[]) method which can write an array of characters to its internal buffer. The write(char[]) method returns the number of characters actually written to the Writer. Here is an example of writing an array of chars to a Java Writer:

char[] chars = new char[]{'A','B','C','D','E'};
bufferedWriter.write(chars);

Write Performance

It is faster to write an array of characters to a Java BufferedWriter than writing one character at a time. Since the BufferedWriter collects the characters written internally in a buffer before writing them to the underlying Writer, the speedup is not as noticeable as with other Writer classes (that do not use buffering). However, there is still a small speedup.

flush()

The Java BufferedWriter's flush() method flushes all data written to the BufferedWriter to the underlying data destination. By calling flush() you can assure that any buffered data will be flushed (written) to disk (or network, or whatever else the destination of your BufferedWriter has). Here is an example of flushing data written to a Java BufferedWriter by calling its flush() method:

bufferedWriter.flush();

Closing a BufferedWriter

When you are finished writing characters to the Java BufferedWriter you should remember to close it. Closing a BufferedWriter will also close the Writer instance to which the BufferedWriter is writing.

Closing a BufferedWriter is done by calling its close() method. Here is how closing a BufferedWriter looks:

bufferedWriter.close();

You can also use the try-with-resources construct introduced in Java 7. Here is how to use and close a BufferedWriter looks with the try-with-resources construct:

FileWriter output = new FileWriter("data/data.bin");

try(BufferedWriter bufferedWriter =
    new BufferedWriter(output)){

    bufferedWriter.write("Hello World");

}

Notice how there is no longer any explicit close() method call. The try-with-resources construct takes care of that.

Notice also that the first FileWriter instance is not created inside the try-with-resources block. That means that the try-with-resources block will not automatically close this FileWriter instance. However, when the BufferedWriter is closed it will also close the OutputStream instance it writes to, so the FileWriter instance will get closed when the BufferedWriter is closed.

Jakob Jenkov

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