Java Programming Input/Output Study Guide

Java Input/Output (I/O) Tutorial

Master Java input/output operations: Learn to read user input, write to console, handle files, and format output with practical examples using Scanner, BufferedReader, and System.out methods.

1. Introduction to Java I/O

Input/Output (I/O) lets programs communicate with users and external systems. Java provides classes in java.io and java.util for console and file operations.

Output: System.out for console display
Input: Scanner and BufferedReader for keyboard
Streams abstract byte and character data flow
Always close resources or use try-with-resources

Basic I/O overview

import java.util.Scanner;

public class IOIntro {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = sc.nextLine();
        System.out.println("Hello, " + name);
        sc.close();
    }
}

2. Console Output Methods

System.out is a PrintStream used for standard output. println adds a newline; print stays on the same line; printf formats values like C-style printf.

System.out.println() — print with newline
System.out.print() — print without newline
System.out.printf() — formatted output
System.err — separate stream for error messages

Output methods

public class OutputDemo {
    public static void main(String[] args) {
        System.out.print("Name: ");
        System.out.println("Alice");
        System.out.printf("Score: %.1f%n", 92.5);
    }
}

3. Reading Input with Scanner

Scanner parses primitive types and strings from readable sources. It is beginner-friendly but can leave newline characters in the buffer when mixing nextInt with nextLine.

nextInt(), nextDouble(), nextLine() for common types
Use hasNext() to check before reading
Call nextLine() after nextInt() to consume leftover newline
Close Scanner when done

Scanner example

import java.util.Scanner;

public class ScannerDemo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter age: ");
        int age = sc.nextInt();
        sc.nextLine();
        System.out.print("Enter city: ");
        String city = sc.nextLine();
        System.out.println(age + " from " + city);
        sc.close();
    }
}

4. Reading with BufferedReader

BufferedReader reads text efficiently through a character buffer. It is preferred for reading large text input and integrates with try-with-resources.

Wrap InputStreamReader around System.in
readLine() returns null at end of stream
More efficient than Scanner for line-based input
Use with try-with-resources for auto-close

BufferedReader example

import java.io.*;

public class ReaderDemo {
    public static void main(String[] args) throws IOException {
        try (BufferedReader br = new BufferedReader(
                new InputStreamReader(System.in))) {
            System.out.print("Enter text: ");
            String line = br.readLine();
            System.out.println("You typed: " + line);
        }
    }
}

5. File I/O Basics

Files store persistent data on disk. Java offers character streams (Reader/Writer) for text and byte streams (InputStream/OutputStream) for binary data.

File class checks existence and paths
FileReader/FileWriter for text files
Always handle IOException
Prefer NIO Files API in modern Java

Write to file

import java.io.*;

public class FileWriteDemo {
    public static void main(String[] args) throws IOException {
        try (FileWriter fw = new FileWriter("notes.txt")) {
            fw.write("Java file I/O works!");
        }
    }
}

6. Practical I/O Examples

Combining input validation with formatted output is a common pattern in console applications. Always validate user input before processing.

Menu-driven programs use Scanner in a loop
Parse numeric input with exception handling
Format currency with printf
Log errors to System.err

Input validation

import java.util.Scanner;

public class ValidInput {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        if (sc.hasNextInt()) {
            int n = sc.nextInt();
            System.out.println("Double: " + (n * 2));
        } else {
            System.out.println("Invalid input");
        }
        sc.close();
    }
}