Java append to file

last modified July 4, 2024

In this article we show how to append to a file in Java. We use FileWriter, FileOutputStream, Files, RandomAccessFile, and Google Guava.

Appending to a file is often used in logging.

In the examples, we append text to the files.

Bratislava
Moldava
Košice
Trenčín
Prešov

We use this text file. It is located in src/main/resources directory.

Append to file with FileWriter

FileWriter class is used for writing streams of characters. FileWriter takes an optional second parameter: append. If set to true, then the data will be written to the end of the file.

import java.io.FileWriter;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

void main() throws IOException {

    String fileName = "src/main/resources/towns.txt";

    try (var fw = new FileWriter(fileName, StandardCharsets.UTF_8, true)) {

        fw.append("Žilina\n");
    }
}

This example appends data to a file with FileWriter.

try (var fw = new FileWriter(fileName, StandardCharsets.UTF_8, true)) {

    fw.append("Žilina\n");
}

We define the FileWriter and use its append method.

Append to file with FileOutputStream

FileOutputStream is an output stream for writing data to a File or to a FileDescriptor. It takes an optional second parameter, which determines whether the data is appended to the file.

import java.io.FileOutputStream;
import java.io.IOException;

void main() throws IOException {

    String fileName = "src/main/resources/towns.txt";

    byte[] tb = "Žilina\n".getBytes();

    try (var fos = new FileOutputStream(fileName, true)) {
        fos.write(tb);
    }
}

This example appends data to a file with FileOutputStream.

try (var fos = new FileOutputStream(fileName, true)) {
    fos.write(tb);
}

We create an instance of the FileOutputStream with append parameter set to true and use its write method.

Append to file with Files

The java.nio.file.Files class is a convenient class to easily append data to a file.

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

void main() throws IOException {

    String fileName = "src/main/resources/towns.txt";

    String town = "Žilina\n";
    Files.writeString(Paths.get(fileName), town, StandardCharsets.UTF_8,
        StandardOpenOption.APPEND);
}

This example appends data with Files.

Files.writeString(Paths.get(fileName), town, StandardCharsets.UTF_8,
    StandardOpenOption.APPEND);

The fourth parameter of Files.writeString tells how the file was opened for writing. With the StandardOpenOption.APPEND, the file was opened for appending.

Append to file with RandomAccessFile

RandomAccessFile is used for reading and writing to a random access file.

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.charset.StandardCharsets;

void main() throws IOException {

    String fileName = "src/main/resources/towns.txt";

    var file = new File(fileName);
    var town = "Žilina\n";

    try (var raf = new RandomAccessFile(file, "rw")) {
        
        raf.seek(raf.length());
        raf.write(town.getBytes(StandardCharsets.UTF_8));
    }
}

In this example we append data with RandomAccessFile.

raf.seek(raf.length());
raf.write(town.getBytes(StandardCharsets.UTF_8));

We determine the length of the file, locate to the end of the file, and write the bytes.

Append to file with Guava

We can use Guava library for appending to a file.

implementation 'com.google.guava:guava:33.2.1-jre'

We need a Guava dependency.

import com.google.common.base.Charsets;
import com.google.common.io.CharSink;
import com.google.common.io.FileWriteMode;
import com.google.common.io.Files;
import java.io.File;
import java.io.IOException;

void main() throws IOException {

    String fileName = "src/main/resources/towns.txt";

    File file = new File(fileName);

    CharSink chs = Files.asCharSink(file, Charsets.UTF_8, FileWriteMode.APPEND);
    chs.write("Žilina\n");
}

In the example, we append to a file with with Guava's CharSink class.

CharSink chs = Files.asCharSink(file, Charsets.UTF_8, FileWriteMode.APPEND);

The third parameter of Files.asCharSink specifies the file writing mode; with FileWriteMode.APPEND option the file is opened for writing.

Source

Java basic I/O - tutorial

In this article we have shown how to append to a file in Java with built-in tools and Guava library.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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