Java Process Text Through Buffered Readers and Writers

Understand the idea

BufferedReader.readLine returns a line without its line terminator, or null at end of input. An empty string is therefore a real empty line, not EOF. BufferedWriter.newLine writes the platform line separator, while explicit is useful when a format specifies LF. Creating readers and writers through Files lets this example name UTF-8 instead of relying on a default encoding.

Run it locally

Use a local JDK 21 or newer. Save the complete example as Main.java in a separate lab folder, then compile and run it with the commands below. Java examples do not run in Bhylu’s browser playground. The -ea option enables assertion checks used by some lessons.

JDK and terminal setup

javac --release 21 -encoding UTF-8 -d out Main.java
java -ea -cp out Main

A worked example

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.nio.file.*;
import java.nio.charset.StandardCharsets;

public class Main {
    public static void main(String[] args) throws Exception {
        Path folder = Files.createTempDirectory("bhylu-buffer-");
        Path file = folder.resolve("labels.txt");
        try {
            try (BufferedWriter writer = Files.newBufferedWriter(file, StandardCharsets.UTF_8)) {
                writer.write("Clay"); writer.newLine();
                writer.write("Wood"); writer.newLine();
            }
            try (BufferedReader reader = Files.newBufferedReader(file, StandardCharsets.UTF_8)) {
                String line;
                while ((line = reader.readLine()) != null) System.out.println(line.toUpperCase(java.util.Locale.ROOT));
            }
        } finally {
            Files.deleteIfExists(file);
            Files.delete(folder);
        }
    }
}

Expected output

CLAY
WOOD

The writer is closed before the reader is opened. Each loop iteration receives one complete line and prints a transformed result. The transformation does not rewrite the original file. For formats where line endings carry special meaning, do not assume readLine preserves the exact original separators.