User Input and Output

# CHAPTER 6

User Input and Output #

1. Introduction #

2. Learning Objectives #

• Import and use the Scanner class.

• Read different data types from the keyboard.

• Handle common Scanner pitfalls.

• Format output with printf().

• Build an interactive application.

3. The Scanner Class #

import java.util.Scanner;

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

4. Reading Different Data Types #

import java.util.Scanner;

public class AllInputTypes {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        System.out.print("Enter your age: ");
        int age = sc.nextInt();
        
        System.out.print("Enter your GPA: ");
        double gpa = sc.nextDouble();
        
        System.out.print("Are you enrolled? (true/false): ");
        boolean enrolled = sc.nextBoolean();
        
        System.out.println("Age: " + age);
        System.out.println("GPA: " + gpa);
        System.out.println("Enrolled: " + enrolled);
        
        sc.close();
    }
}

5. The nextLine() Trap (Critical!) #

Scanner sc = new Scanner(System.in);

System.out.print("Enter age: ");
int age = sc.nextInt();        // Reads the number, leaves \n in buffer

System.out.print("Enter name: ");
String name = sc.nextLine();   // Reads the leftover \n — SKIPS INPUT!

System.out.println("Name: " + name); // Prints empty string!

Fix: Add a dummy nextLine() after nextInt() or nextDouble():

int age = sc.nextInt();
sc.nextLine();  // Consume the leftover newline
String name = sc.nextLine();  // Now reads correctly

6. Formatted Output with printf() #

String name = "Alice";
int age = 25;
double gpa = 3.856;

System.out.printf("Name: %s%n", name);
System.out.printf("Age: %d years%n", age);
System.out.printf("GPA: %.2f%n", gpa);  // 3.86 (rounded!)

7. String Concatenation with + #

String first = "John";
String last = "Doe";
int age = 30;

System.out.println(first + " " + last + " is " + age + " years old.");

8. Mini Project: User Information Form #

import java.util.Scanner;

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

        System.out.println("============================");
        System.out.println("   USER REGISTRATION FORM   ");
        System.out.println("============================");

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

        System.out.print("Age: ");
        int age = sc.nextInt();
        sc.nextLine(); // Consume newline

        System.out.print("Email: ");
        String email = sc.nextLine();

        System.out.print("GPA: ");
        double gpa = sc.nextDouble();

        System.out.println("\n============================");
        System.out.println("   REGISTRATION SUMMARY     ");
        System.out.println("============================");
        System.out.printf("Name:  %s%n", name);
        System.out.printf("Age:   %d%n", age);
        System.out.printf("Email: %s%n", email);
        System.out.printf("GPA:   %.2f%n", gpa);
        System.out.println("============================");

        sc.close();
    }
}

9. Common Mistakes #

• Forgetting to import Scanner: import java.util.Scanner; is required.

• The nextLine() trap: Always consume the dangling newline after nextInt() or nextDouble().

• Not closing the Scanner: Always call scanner.close() to prevent resource leaks.

• InputMismatchException: Entering text when nextInt() expects a number crashes the program.

10. Best Practices #

• Always close your Scanner when done.

• Use sc.nextLine() after numeric reads to prevent input skipping.

• Validate input when possible (we'll learn exception handling later).

• Use printf() for clean, aligned output formatting.

11. Exercises #

1. 1. Write a program that asks for two numbers and prints their sum, difference, product, and quotient.

1. 2. Build a "Mad Libs" style game that asks for a noun, adjective, and verb, then constructs a funny sentence.

1. 3. Create a temperature converter that asks for Fahrenheit and converts to Celsius.

12. MCQ Quiz with Answers #

13. Interview Questions #

• Q: What is the difference between next() and nextLine()?

• Q: Explain the newline buffer issue with Scanner and how to fix it.

• Q: What is InputMismatchException and when does it occur?

14. Summary #

15. Next Chapter Recommendation #