In programming, Input/Output (I/O) refers to the communication between your program and the outside world.

• Input: Data that your program receives from an external source. This could be user input from the keyboard, data from a file, or information from a network connection.

• Output: Data that your program sends to an external destination. This could be text displayed on the console, data written to a file, or information sent over a network.

For introductory programming, we'll focus on the most common forms of basic I/O:

1. Standard Output: Displaying text and data to the console (the screen where your program runs).

2. Standard Input: Reading data entered by the user from the keyboard.

Basic I/O is fundamental because it allows your programs to interact with users, receive data to process, and present results.

1. Standard Output: System.out.print() and System.out.println()

In Java, the System.out object is your primary tool for sending output to the standard output stream, which is typically your console.

You'll primarily use two methods of System.out:

• System.out.print(value): Prints the specified value to the console. The cursor remains on the same line after printing.

• System.out.println(value): Prints the specified value to the console, and then moves the cursor to the next line (prints a "new line" character). This is equivalent to System.out.print(value + "\n");.

Syntax:

System.out.print("Your message here");
System.out.println("Another message");
System.out.println(variableName); // Prints the value of a variable

Example: Displaying Output

// BasicOutput.java

public class BasicOutput {

public static void main(String[] args) {

// Using System.out.println() - each statement starts on a new line

System.out.println("Hello, Java!");

System.out.println("This is my first output program.");

// Using System.out.print() - stays on the same line

System.out.print("This text will be ");

System.out.print("on the same line.");

System.out.println(); // Prints a new line character to move to the next line

// Combining text and variables

String name = "Alice";

int age = 30;

System.out.println("My name is " + name + " and I am " + age + " years old.");

// Printing numbers directly

System.out.println(123);

System.out.println(3.14);

}

}

2. Standard Input: The Scanner Class

To get input from the keyboard in Java, you use the Scanner class, which is part of the java.util package. The Scanner class provides convenient methods for reading various types of input from the console.

Steps to use Scanner:

1. Import the Scanner class: You need to tell Java where to find the Scanner class. This is done at the top of your Java file, outside of any class definition:

   import java.util.Scanner;
   
   

2. Create a Scanner object: You need to create an instance of the Scanner class, typically linked to the standard input stream (System.in).

   Scanner scanner = new Scanner(System.in);
   
   

3. Read input using Scanner methods: The Scanner object has various methods to read different data types:

   • nextInt(): Reads the next integer.

   • nextDouble(): Reads the next double (decimal number).

   • nextBoolean(): Reads the next boolean (true or false).

   • next(): Reads the next word (stops at whitespace).

   • nextLine(): Reads the entire line of input, including spaces, until a new line character is encountered.

4. Close the Scanner: It's good practice to close the Scanner object when you are finished using it to release system resources.

   scanner.close();
   
   

Example: Getting Input from the User

// BasicInput.java

import java.util.Scanner; // Step 1: Import the Scanner class

public class BasicInput {

public static void main(String[] args) {

// Step 2: Create a Scanner object, linked to System.in (keyboard input)

Scanner keyboardInput = new Scanner(System.in);

System.out.println("--- Getting String Input ---");

System.out.print("Enter your full name: ");

// Step 3: Read a full line of text

String fullName = keyboardInput.nextLine();

System.out.println("Hello, " + fullName + "!");

System.out.println("\n--- Getting Integer Input ---");

System.out.print("Enter your age: ");

// Step 3: Read an integer

int age = keyboardInput.nextInt();

System.out.println("You are " + age + " years old.");

// IMPORTANT NOTE about nextLine() after nextInt()/nextDouble()/next():

// When you use nextInt(), nextDouble(), or next(), they only read the value,

// but leave the "new line" character (pressed after typing input) in the input buffer.

// If you immediately call nextLine() after these, it will consume that leftover new line,

// resulting in an empty string. To fix this, add an extra nextLine() call.

keyboardInput.nextLine(); // Consume the leftover new line character

System.out.println("\n--- Getting a Single Word Input ---");

System.out.print("Enter your favorite color (single word): ");

String color = keyboardInput.next(); // Reads only one word

System.out.println("Your favorite color is: " + color);

keyboardInput.nextLine(); // Consume the leftover new line character

System.out.println("\n--- Getting Double Input ---");

System.out.print("Enter a decimal number (e.g., 3.14): ");

double decimalNumber = keyboardInput.nextDouble();

System.out.println("You entered: " + decimalNumber);

// Step 4: Close the Scanner object to release resources

keyboardInput.close();

System.out.println("\nScanner closed. Program finished.");

}

}

Combining Input and Output: A Simple Interactive Program

Let's put both input and output together to create a simple interactive program that asks the user for information and then uses it to display a personalized message.

Example: Simple Interactive Program

// SimpleInteractiveProgram.java

import java.util.Scanner; // Import the Scanner class

public class SimpleInteractiveProgram {

public static void main(String[] args) {

Scanner userInput = new Scanner(System.in); // Create a Scanner object

System.out.println("Welcome to the Interactive Greeter!"); // Output

// Get user's name

System.out.print("Please enter your name: "); // Output prompt

String userName = userInput.nextLine(); // Input

// Get user's favorite programming language

System.out.print("What is your favorite programming language? "); // Output prompt

String favLanguage = userInput.nextLine(); // Input

// Get user's experience level (integer)

System.out.print("How many years have you been programming? "); // Output prompt

int yearsExperience = userInput.nextInt(); // Input

userInput.nextLine(); // Consume the leftover newline character after nextInt()

// Display a personalized summary

System.out.println("\n--- Your Profile ---"); // Output

System.out.println("Name: " + userName); // Output

System.out.println("Favorite Language: " + favLanguage); // Output

System.out.println("Years of Experience: " + yearsExperience); // Output

// Give a little message based on experience

if (yearsExperience < 2) {

System.out.println("Keep learning and happy coding!");

} else {

System.out.println("Great to see your dedication to programming!");

}

userInput.close(); // Close the Scanner

System.out.println("Thank you for using the program!");

}

}

Conclusion

Basic Input/Output is your program's window to the world. System.out.print() and System.out.println() allow you to display information, while the Scanner class empowers your program to gather information from the user, making your applications interactive and dynamic. Mastering these fundamental I/O techniques is a crucial step in building engaging and functional Java programs.