Java BufferedReader, BufferedWriter & Scanner: High-Performance I/O

Reading disk files byte-by-byte or character-by-character is notoriously slow because every single read triggers a physical disk sector read or OS kernel context switch. Buffered Streams solve this bottleneck by wrapping existing streams with an internal memory buffer (default 8 KB), drastically reducing OS-level system calls.

BufferedReader: High-Speed Line-by-Line Reading

BufferedReader provides the invaluable readLine() method, which reads an entire line of text up to a newline character (\n, \r, or \r\n) and returns it as a String, or null when the end of file is reached.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

public class FastFileRead {
    public static void main(String[] args) {
        String filePath = "server-logs.txt";
        
        try (BufferedReader reader = new BufferedReader(new FileReader(filePath, StandardCharsets.UTF_8))) {
            String line;
            int lineNumber = 1;
            
            while ((line = reader.readLine()) != null) {
                System.out.printf("[%04d] %s%n", lineNumber++, line);
            }
        } catch (IOException e) {
            System.err.println("Read error: " + e.getMessage());
        }
    }
}

BufferedWriter & PrintWriter: Efficient Writing

BufferedWriter caches outgoing characters in an 8 KB buffer and flushes them to disk when full. PrintWriter wraps writers to provide familiar methods like println() and printf().

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

public class FastFileWrite {
    public static void main(String[] args) {
        try (FileWriter fw = new FileWriter("students.csv", StandardCharsets.UTF_8, false);
             BufferedWriter bw = new BufferedWriter(fw);
             PrintWriter out = new PrintWriter(bw)) {
            
            out.println("RollNo,Name,Course,Grade");
            out.printf("%d,%s,%s,%.1f%n", 101, "Aarav Sharma", "BCA", 92.5);
            out.printf("%d,%s,%s,%.1f%n", 102, "Neha Patel", "MCA", 88.0);
            out.printf("%d,%s,%s,%.1f%n", 103, "Rohan Verma", "B.Tech", 95.2);
            
            System.out.println("CSV export completed successfully.");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Scanner vs BufferedReader: Detailed Benchmark

Dry-Run & Memory Execution Trace