Using Scanner class in Java - Mini Project: Build & Apply

Using Scanner class

📖 Scenario: You are creating a simple Java program that asks a user for their name and age, then prints a greeting message.

🎯 Goal: Build a Java program that uses the Scanner class to read user input from the keyboard and then prints a message using that input.

📋 What You'll Learn

Use the Scanner class to read input

Read a string input for the user's name

Read an integer input for the user's age

Print a greeting message including the name and age

💡 Why This Matters

🌍 Real World

Many Java programs need to get input from users, like command-line tools or simple interactive apps.

💼 Career

Knowing how to use Scanner is essential for beginner Java developers to build programs that interact with users.

1

Create a Scanner object

Write a line to create a Scanner object called scanner that reads from System.in.

Java

// Your code here

Hint

Use new Scanner(System.in) to read from the keyboard.

2

Read user's name

Add a line to read a string input from scanner into a variable called name using scanner.nextLine().

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        // Your code here
    }
}

Hint

Use scanner.nextLine() to read a full line of text.

3

Read user's age

Add a line to read an integer input from scanner into a variable called age using scanner.nextInt().

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String name = scanner.nextLine();
        // Your code here
    }
}

Hint

Use scanner.nextInt() to read an integer number.

4

Print greeting message

Write a line to print the message Hello, {name}! You are {age} years old. using System.out.println and the variables name and age.

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String name = scanner.nextLine();
        int age = scanner.nextInt();
        // Your code here
    }
}

Hint

Use System.out.println and string concatenation with name and age.

Practice

1. What is the primary purpose of the Scanner class in Java?

easy

A. To write output to the console

B. To handle file operations

C. To create graphical user interfaces

D. To read input from the user

Solution

1. Step 1: Understand Scanner's role

   The Scanner class is designed to read input from various sources, commonly from the user via the keyboard.

2. Step 2: Differentiate from other classes

   Writing output is done by System.out, GUI creation uses other libraries, and file handling uses classes like FileReader.

3. Final Answer:

   To read input from the user -> Option D

4. Quick Check:

   Scanner reads input = B [OK]

Hint: Scanner reads user input from keyboard easily [OK]

Common Mistakes:

• Confusing Scanner with output classes
• Thinking Scanner creates GUI
• Assuming Scanner handles files only

2. Which of the following is the correct way to create a Scanner object to read from the keyboard?

easy

A. Scanner input = new Scanner(System.in);

B. Scanner input = new Scanner(System.out);

C. Scanner input = new Scanner(System.err);

D. Scanner input = new Scanner(System.in.read());

Solution

1. Step 1: Identify correct Scanner constructor

   The Scanner constructor requires an InputStream to read input. System.in is the standard input stream (keyboard).

2. Step 2: Check other options

   System.out and System.err are output streams, not input. System.in.read() returns an int, not an InputStream.

3. Final Answer:

   Scanner input = new Scanner(System.in); -> Option A

4. Quick Check:

   Use System.in for input Scanner [OK]

Hint: Use System.in to read keyboard input with Scanner [OK]

Common Mistakes:

• Using System.out or System.err instead of System.in
• Passing System.in.read() which is an int, not InputStream
• Missing 'new' keyword

3. What will be the output of the following code if the user inputs the number 25?

import java.util.Scanner;
public class Test {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int num = sc.nextInt();
    System.out.println(num * 2);
    sc.close();
  }
}

medium

A. Error at runtime

B. 25

C. 50

D. 0

Solution

1. Step 1: Understand input reading

   The program reads an integer from the user using nextInt(). User inputs 25, so num = 25.

2. Step 2: Calculate output

   The program prints num * 2, which is 25 * 2 = 50.

3. Final Answer:

   50 -> Option C

4. Quick Check:

   Input 25 * 2 = 50 [OK]

Hint: Multiply input number by 2 to get output [OK]

Common Mistakes:

• Confusing input with output
• Forgetting to multiply by 2
• Not closing Scanner (though not causing output error)

4. Identify the error in the following code snippet:

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

medium

A. Not closing the Scanner

B. Using nextInt() to read a String

C. Missing import statement for Scanner

D. Incorrect print statement syntax

Solution

1. Step 1: Check input method used

   The code uses nextInt() to read the user's name, which is a String, not an integer.

2. Step 2: Identify correct method

   To read a String, nextLine() or next() should be used instead of nextInt().

3. Final Answer:

   Using nextInt() to read a String -> Option B

4. Quick Check:

   Use nextLine() for String input, not nextInt() [OK]

Hint: Use nextLine() for strings, nextInt() for numbers [OK]

Common Mistakes:

• Using nextInt() for String input
• Ignoring Scanner import
• Not closing Scanner (not error here)
• Misunderstanding print syntax

5. You want to read a full line of text including spaces from the user after reading an integer using Scanner. Which sequence of method calls correctly handles this without skipping input?

Scanner sc = new Scanner(System.in);
int age = sc.nextInt();
// What next?

hard

A. sc.nextLine(); String name = sc.nextLine();

B. String name = sc.nextLine();

C. sc.next(); String name = sc.nextLine();

D. sc.nextInt(); String name = sc.nextLine();

Solution

1. Step 1: Understand Scanner behavior after nextInt()

   After nextInt(), the newline character remains in the input buffer, so a direct nextLine() reads that leftover newline.

2. Step 2: Consume leftover newline before reading full line

   Calling sc.nextLine() once consumes the leftover newline, then the next nextLine() reads the actual full line input.

3. Final Answer:

   sc.nextLine(); String name = sc.nextLine(); -> Option A

4. Quick Check:

   Consume newline with nextLine() before reading full line [OK]

Hint: Call nextLine() twice after nextInt() to read full line [OK]

Common Mistakes:

• Skipping the first nextLine() to consume newline
• Using next() which reads only one word
• Calling nextInt() again unnecessarily