In this quick article, I'll show you how to append text to an existing file using Java legacy I/O API as well as non-blocking new I/O API (NIO).

Using Files.write() Method

The simplest and most straightforward way of appending text to an existing file is to use the Files.write() static method. This method is a part of Java's new I/O API (classes in java.nio.* package) and requires Java 7 or higher.

Here is an example that uses Files.write() to append data to a file:

try {
    // append data to a file
    Files.write(Paths.get("output.txt"), "Hey, there!".getBytes(),
            StandardOpenOption.APPEND);

} catch (IOException ex) {
    ex.printStackTrace();
}

The above code will append Hey, there! to a file called output.txt. If the file doesn't exist, it will throw a NoSuchFileException exception. It also doesn't append a new line automatically which is often required when appending to a text file.

If you want to create a new file if it doesn't already exist and also append new line automatically, use another variant of Files.write() as shown below:

try {
    // data to append
    List<String> contents = Arrays.asList("Hey, there!", "What's up?");

    // append data to a file
    Files.write(Paths.get("output.txt"), contents,
            StandardOpenOption.CREATE,
            StandardOpenOption.APPEND);

} catch (IOException ex) {
    ex.printStackTrace();
}

If the file has encoding other than the default character encoding of the operating system, you can specify it like below:

Files.write(Paths.get("output.txt"), contents,
        StandardCharsets.UTF_8,
        StandardOpenOption.CREATE,
        StandardOpenOption.APPEND);

Note: Files.write() is good if you want to append to a file once or a few times only. Because it opens and writes the file every time to the disk, which is a slow operation. For frequent append requests, you should rather BufferedWriter (explained below).

Using BufferedWriter Class

The BufferedWriter class is a part of Java legacy I/O API that can also be used to append text to a file. Here is an example that uses the Files.newBufferedWriter() static method to create a new writer (require Java 8+):

try {
    // create a writer
    BufferedWriter bw = Files.newBufferedWriter(Paths.get("output.txt"),
            StandardOpenOption.APPEND);

    // append text to file
    bw.write("Hey, there!");
    bw.newLine();
    bw.write("What's up?");

    // close the writer
    bw.close();

} catch (IOException ex) {
    ex.printStackTrace();
}

The above code will append text to file. If the file doesn't already exist, it will throw a NoSuchFileException exception. However, you can change it to create a new file if not available with the following:

BufferedWriter bw = Files.newBufferedWriter(Paths.get("output.txt"),
        StandardOpenOption.CREATE,
        StandardOpenOption.APPEND);

To specify a different character encoding, you can do the following:

BufferedWriter bw = Files.newBufferedWriter(Paths.get("output.txt"),
        StandardCharsets.UTF_8,
        StandardOpenOption.CREATE,
        StandardOpenOption.APPEND);

Using FileWriter Class

If you are using Java 7 or below, you can use FileWriter wrapped in a BufferedWriter object to append data to a file as shown below:

try {
    // create a writer
    BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt", true));

    // append text to file
    bw.write("Hey, there!");
    bw.newLine();
    bw.write("What's up?");

    // close the writer
    bw.close();

} catch (IOException ex) {
    ex.printStackTrace();
}

The second argument to the FileWriter constructor will tell it to append data to the file, rather than writing a new file. If the file does not already exist, it will be created.

Further Reading

You may be interested in other Java I/O articles:

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