Java Input and Output Explained — Scanner, System.out and Real Examples

public class PrintingExamples {
    public static void main(String[] args) {

        // println adds a newline after printing — cursor moves to the next line
        System.out.println("Welcome to TheCodeForge!");

        // print does NOT add a newline — the next output continues on the same line
        System.out.print("First name: ");
        System.out.print("Ada ");
        System.out.print("Lovelace");

        // We need a manual newline here to move to the next line
        System.out.println(); // prints an empty line

        // printf lets you format values inline using format specifiers
        // %s = String placeholder, %d = integer placeholder, %n = newline
        String productName = "Java Book";
        int quantityInStock = 42;
        double priceInDollars = 29.99;

        System.out.printf("Product : %s%n", productName);
        System.out.printf("In Stock : %d units%n", quantityInStock);

        // %.2f means: print a floating-point number with exactly 2 decimal places
        System.out.printf("Price    : $%.2f%n", priceInDollars);
    }
}

import java.util.Scanner; // We MUST import Scanner before using it

public class UserProfileInput {
    public static void main(String[] args) {

        // System.in is the keyboard input stream — we wrap it in Scanner to read it easily
        Scanner keyboardInput = new Scanner(System.in);

        // Prompt the user — always print a message BEFORE reading input
        System.out.print("Enter your full name: ");

        // nextLine() reads everything the user types until they press Enter
        String fullName = keyboardInput.nextLine();

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

        // nextInt() reads a whole number — it will crash if the user types letters!
        int userAge = keyboardInput.nextInt();

        System.out.print("Enter your account balance: ");

        // nextDouble() reads a decimal number like 1250.75
        double accountBalance = keyboardInput.nextDouble();

        // Print a formatted summary back to the user
        System.out.println("\n--- Profile Summary ---");
        System.out.printf("Name    : %s%n", fullName);
        System.out.printf("Age     : %d years old%n", userAge);
        System.out.printf("Balance : $%.2f%n", accountBalance);

        // Close the Scanner to release the keyboard resource
        keyboardInput.close();
    }
}

import java.util.Scanner;

public class SafeInputReading {
    public static void main(String[] args) {

        Scanner keyboardInput = new Scanner(System.in);

        // APPROACH 1 — Flush the buffer manually after nextInt()
        System.out.print("Enter your birth year: ");
        int birthYear = keyboardInput.nextInt();

        // This nextLine() drains the leftover '\n' from pressing Enter
        // Without this line, the next nextLine() would return "" (empty string)
        keyboardInput.nextLine(); // <--- the buffer flush

        System.out.print("Enter your home city: ");
        String homeCity = keyboardInput.nextLine(); // now works correctly

        System.out.printf("Born in %d, living in %s.%n", birthYear, homeCity);

        System.out.println();

        // APPROACH 2 — Read everything as String, then parse (safer & more flexible)
        System.out.print("Enter your salary: ");
        String salaryInput = keyboardInput.nextLine(); // read raw text

        double salary;
        try {
            // Parse the string into a number — throws NumberFormatException if invalid
            salary = Double.parseDouble(salaryInput);
            System.out.printf("Annual salary: $%.2f%n", salary);
        } catch (NumberFormatException conversionError) {
            // Friendly message instead of a crash
            System.out.println("That doesn't look like a valid number. Please enter digits only.");
        }

        keyboardInput.close();
    }
}

import java.io.BufferedReader;    // for fast, buffered reading
import java.io.InputStreamReader; // converts raw bytes (System.in) into characters
import java.io.IOException;       // must handle this checked exception

public class BufferedReaderExample {

    // 'throws IOException' tells Java: this method might throw an IO error
    // Java requires us to either handle it or declare it — we're declaring it here
    public static void main(String[] args) throws IOException {

        // Step 1: Wrap System.in in InputStreamReader to handle character encoding
        // Step 2: Wrap that in BufferedReader for efficient line reading
        BufferedReader consoleReader = new BufferedReader(new InputStreamReader(System.in));

        System.out.print("Enter the item name: ");
        String itemName = consoleReader.readLine(); // readLine() always returns a String

        System.out.print("Enter the quantity: ");
        // readLine() gives us "10" as a String — we parse it to int manually
        int quantity = Integer.parseInt(consoleReader.readLine());

        System.out.print("Enter the unit price: ");
        // Similarly, parse the string to a double
        double unitPrice = Double.parseDouble(consoleReader.readLine());

        double totalCost = quantity * unitPrice;

        System.out.println("\n--- Order Receipt ---");
        System.out.printf("Item     : %s%n", itemName);
        System.out.printf("Quantity : %d%n", quantity);
        System.out.printf("Unit Price: $%.2f%n", unitPrice);
        System.out.printf("Total    : $%.2f%n", totalCost);

        consoleReader.close(); // always close your reader
    }
}