Lesson 7 of 20

Input and Output

Display information and read keyboard input using Scanner, while avoiding common input-buffer problems.

Learning objectives

• Create a Scanner connected to keyboard input.
• Read text and numeric values safely.
• Format console output with printf.
• Explain the difference between nextLine() and numeric Scanner methods.
• Build a small interactive program.

1. Reading keyboard input

Scanner is a standard Java class for reading input. For console programs, it is commonly connected to System.in.

Example: read a name

import java.util.Scanner;

public class NameInput {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter your name: ");
        String name = input.nextLine();

        System.out.println("Hello, " + name + "!");
        input.close();
    }
}

nextLine() reads a complete line, including spaces between words.

2. Reading numbers

Example: read price and quantity

import java.util.Scanner;

public class OrderInput {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Unit price: ");
        double price = input.nextDouble();

        System.out.print("Quantity: ");
        int quantity = input.nextInt();

        double total = price * quantity;
        System.out.printf("Total: RM %.2f%n", total);

        input.close();
    }
}

3. The nextInt/nextLine trap

After nextInt() or nextDouble(), the newline entered by the user remains in the input buffer. A following nextLine() may read that leftover newline immediately.

Example: consume the leftover newline

System.out.print("Age: ");
int age = input.nextInt();
input.nextLine(); // consume the pending newline

System.out.print("Full name: ");
String name = input.nextLine();

4. A robust beginner alternative

Another approach is to read everything with nextLine() and convert text when you need numbers.

System.out.print("Enter quantity: ");
int quantity = Integer.parseInt(input.nextLine());

System.out.print("Enter price: ");
double price = Double.parseDouble(input.nextLine());

Mini project: simple receipt

import java.util.Scanner;

public class SimpleReceipt {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Product name: ");
        String product = input.nextLine();

        System.out.print("Unit price: RM ");
        double price = Double.parseDouble(input.nextLine());

        System.out.print("Quantity: ");
        int quantity = Integer.parseInt(input.nextLine());

        double total = price * quantity;

        System.out.println("
--- Receipt ---");
        System.out.println("Product : " + product);
        System.out.println("Quantity: " + quantity);
        System.out.printf("Total   : RM %.2f%n", total);

        input.close();
    }
}

Common mistakes

Summary

You can now build interactive console programs that accept user input and display formatted output. Lesson 8 introduces operators so you can perform richer calculations and comparisons with those values.