Chapter 93: BufferedReader in Java

BufferedReader sits on top of another Reader—often Files.newBufferedReader or InputStreamReader wrapping bytes. It fills an internal char array so readLine does not hammer the OS for one character at a time.

Reading files (Chapter 91) showed readLine; here we stress behavior and streams.

1. Topic title

BufferedReader: cheap line reads by batching characters behind the scenes

2. What it means

readLine() returns null only at end of stream—not for blank lines (those become empty strings).

lines() (Java 8+) exposes a Stream<String>—handy with filter/map, but remember the stream’s close ties to the reader—use try-with-resources.

mark/reset exist but mark has a readAheadLimit—overshooting invalidates the mark.

3. Why it is used

Log tailing, CSV row loops, protocol parsers that are naturally line-based.

4. Mental sketch

Without buffering, reading a line is like walking to the store for every single ingredient. BufferedReader is a full pantry fetch—fewer trips.

5. Java code example

import java.io.BufferedReader;
import java.io.IOException;
import java.io.StringReader;
import java.nio.charset.StandardCharsets;
import java.util.stream.Collectors;
 
public class BufferedReaderDemo {
    public static void main(String[] args) throws IOException {
        String data = "alpha\n\nbeta\n";
        try (BufferedReader br = new BufferedReader(new StringReader(data))) {
            long nonBlank =
                    br.lines().filter(line -> !line.isBlank()).count();
            System.out.println("nonBlank lines=" + nonBlank);
        }
 
        try (BufferedReader br2 = new BufferedReader(new StringReader(data))) {
            String joined = br2.lines().collect(Collectors.joining("|"));
            System.out.println(joined);
        }
    }
}

Two separate try blocks because lines() consumes the stream—do not call lines() twice on the same exhausted reader without reopening.

6. Explanation of code

StringReader turns String into a Reader—great for unit tests without temp files.

7. Common mistakes

Mixing readLine and lines() on one reader—easy to lose elements; pick one style per scope.

Huge lines—readLine still allocates a whole line string; protocol abuse can blow memory.

8. Best practices

Wrap InputStreamReader(stream, UTF_8) with BufferedReader for socket text—same batching win.

Use Files.readAllLines only when memory budget is known safe.

9. Small practice task

Implement long countLines(Path path) using try-with-resources and readLine loop—no lines()—and compare results to Files.readAllLines on a small test file.

Beginner tip

If readLine blocks forever on a socket, you are waiting on bytes that never arrive—timeouts live at Socket or HttpClient layers, not inside BufferedReader.