How to append text to a file

Short answer

Pass the CREATE and APPEND options — the file is created if missing, and new text goes to the end:

void main() throws IOException {
    Path log = Path.of("app.log");
    Files.writeString(log, "first\n", StandardOpenOption.CREATE, StandardOpenOption.APPEND);
    Files.writeString(log, "second\n", StandardOpenOption.CREATE, StandardOpenOption.APPEND);
 
    IO.print(Files.readString(log));
}

Output:

first
second

A log with timestamps

It is convenient to move the writing into a method:

void log(String message) throws IOException {
    String line = LocalTime.of(12, 0, 5) + " " + message + System.lineSeparator();
    Files.writeString(Path.of("events.log"), line, StandardOpenOption.CREATE, StandardOpenOption.APPEND);
}
 
void main() throws IOException {
    log("user logged in");
    log("order created");
    IO.print(Files.readString(Path.of("events.log")));
}

Output:

12:00:05 user logged in
12:00:05 order created

The time is fixed here for the example — in real code use LocalTime.now().

With a BufferedWriter

newBufferedWriter() takes the same options:

void main() throws IOException {
    Path file = Path.of("list.txt");
    for (String item : List.of("tea", "cake")) {
        try (var writer = Files.newBufferedWriter(file, StandardOpenOption.CREATE, StandardOpenOption.APPEND)) {
            writer.write(item);
            writer.newLine();
        }
    }
    IO.print(Files.readString(file));
}

Output:

tea
cake

Common mistake

Passing only APPEND. If the file does not exist yet, the write fails:

void main() throws IOException {
    Files.writeString(Path.of("new.log"), "text", StandardOpenOption.APPEND);
}

Output:

Exception in thread "main" java.nio.file.NoSuchFileException: new.log

Thanks for reading!

Short answer

A log with timestamps

With a BufferedWriter

Common mistake

Functions used in this recipe

Thanks for reading!