Reading and writing text files in Java

File I/O

Java offers two main APIs for text file I/O: the classic java.io streams and the modern java.nio.file (NIO.2) API from Java 7. Prefer NIO.2 for new code — it is more expressive and handles character encoding explicitly.

NIO.2 — Preferred

import java.nio.file.*;
import java.util.List;

// Write entire string
Path path = Path.of("output.txt");
Files.writeString(path, "Hello\nWorld");

// Read all lines into a List
List lines = Files.readAllLines(path);
lines.forEach(System.out::println);

// Append to existing file
Files.writeString(path, "\nNew line", StandardOpenOption.APPEND);

Classic Buffered I/O — Large Files

try (BufferedWriter bw = new BufferedWriter(new FileWriter("log.txt", true))) {
    bw.write("Entry 1");
    bw.newLine();
    bw.write("Entry 2");
}

try (BufferedReader br = new BufferedReader(new FileReader("log.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
}

Always buffer unbuffered streams. FileReader/FileWriter alone make one system call per character — wrapping in Buffered* batches I/O into chunks and is orders of magnitude faster for large files.

Specify StandardCharsets.UTF_8 explicitly when charset matters. On some platforms the default system charset is not UTF-8, causing silent encoding bugs when reading files written on a different OS.

For very large files that exceed available memory, use Files.lines(path) which returns a lazy Stream<String> and reads one line at a time. Always wrap it in a try-with-resources since streams from files hold an open file handle.