Module 3: Input and Output

Before a program can interact meaningfully with users, it must communicate through standard input and output streams. In Java, console output is managed through System.out, which represents a standard output byte stream wrapped in a java.io.PrintStream instance.

The Three Output Methods

• print(): Writes text to the console without terminating the line. The cursor remains on the same line, allowing subsequent outputs to append immediately adjacent to it.
• println(): Writes text and automatically terminates the current line with an OS-dependent line separator ( on Linux/macOS, on Windows). This is the standard choice for structured terminal logging.
• printf(): Borrowed from C, this method produces formatted strings using formatting placeholders and argument lists:

  String name = "Alice";
  int score = 95;
  double gpa = 3.876;
  System.out.printf("%s scored %d points with GPA: %.2f%n", name, score, gpa);

The Scanner class (residing in java.util) is the standard, beginner-friendly tool for parsing keyboard input. It breaks console input into tokens using whitespace delimiters and provides typed utility methods to parse primitives directly:

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
System.out.print("Enter your age: ");
int age = sc.nextInt();
System.out.println("You are " + age + " years old");

The Classic Scanner Newline Bug

A notorious pitfall encountered by every Java developer occurs when combining token-based input methods (like nextInt() or nextDouble()) with line-based input (nextLine()):

Scanner sc = new Scanner(System.in);
System.out.print("Enter Roll No: ");
int roll = sc.nextInt(); // Reads integer, but leaves '\n' in the input stream buffer!

System.out.print("Enter Name: ");
String name = sc.nextLine(); // Immediately consumes the leftover '\n' and returns an empty string!

While Scanner is convenient, it is noticeably slow because it utilizes complex regular expression parsing and does little buffering. For competitive programming or data-intensive applications parsing hundreds of thousands of lines, BufferedReader wrapped around an InputStreamReader is significantly faster.

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class FastIO {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.print("Enter number: ");
        String line = br.readLine();
        int val = Integer.parseInt(line);
        System.out.println("Square: " + (val * val));
    }
}

Why BufferedReader Outperforms Scanner

• 8KB Memory Buffer: BufferedReader reads large character blocks (default 8,192 characters) from the operating system stream into memory at once, minimizing costly OS system calls.
• No Regex Overhead: Scanner runs regular expressions over tokens, consuming CPU cycles. BufferedReader simply reads raw text lines.
• Synchronized: BufferedReader methods are synchronized, making it thread-safe.