Using Scanner class in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading an integer and a string using Scanner

What is the output of this Java program when the user inputs:
42
Hello

Java

import java.util.Scanner;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int num = sc.nextInt();
    sc.nextLine();
    String text = sc.nextLine();
    System.out.println("Number: " + num);
    System.out.println("Text: " + text);
  }
}

A

Number: 42
Text: Hello

B

Number: 42
Text: 

C

Number: 42
Text: \nHello

D

Number: 42
Text: null

💡 Hint

Remember that nextInt() does not consume the newline character.

The nextInt() method reads the integer but leaves the newline character in the input buffer. The nextLine() then reads that leftover newline as an empty string. So the text variable is empty.

❓ Predict Output

intermediate

Reading multiple tokens with Scanner

What will be the output of this program if the user inputs:
John 25 true

Java

import java.util.Scanner;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String name = sc.next();
    int age = sc.nextInt();
    boolean isStudent = sc.nextBoolean();
    System.out.println(name + ", " + age + ", " + isStudent);
  }
}

A John 25 true

B Compilation error

C John, 25, false

D John, 25, true

💡 Hint

next() reads one word, nextInt() reads an integer, nextBoolean() reads true/false.

The Scanner reads tokens separated by whitespace. next() reads "John", nextInt() reads 25, nextBoolean() reads true. The output concatenates them with commas.

🔧 Debug

advanced

Why does this Scanner code skip input?

This code is supposed to read an integer and then a full line of text. But it skips reading the text input. Why?
Identify the problem and select the correct explanation.

Java

import java.util.Scanner;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter number: ");
    int n = sc.nextInt();
    sc.nextLine();
    System.out.print("Enter text: ");
    String line = sc.nextLine();
    System.out.println("Number: " + n);
    System.out.println("Text: " + line);
  }
}

A nextLine() cannot be used after nextInt() because it causes an infinite loop.

B nextInt() leaves a newline in the buffer, so nextLine() reads it as empty.

C The Scanner object must be closed before reading the next line.

D The program needs to call sc.next() before nextLine() to clear the buffer.

💡 Hint

Think about what happens to the newline character after nextInt().

nextInt() reads only the integer but leaves the newline character. nextLine() then reads that leftover newline as an empty string, causing it to skip the actual input.

📝 Syntax

advanced

Identify the syntax error in Scanner usage

Which option contains a syntax error when trying to read a double value using Scanner?

A double d = sc.nextDouble();

B double d = sc.nextDouble("");

C double d = sc.nextDouble;

D double d = sc.nextdouble();

💡 Hint

Method names in Java are case-sensitive.

Option B is invalid because nextDouble() does not take any arguments. Option C is invalid because nextDouble is a method and must be called with parentheses. Option D is invalid because method names are case-sensitive; nextdouble() is not defined.

🚀 Application

expert

How many tokens are read by Scanner?

Given this input line:
apple, banana; cherry orange
and this code:
Scanner sc = new Scanner(System.in); sc.useDelimiter(",|;| "); int count = 0; while(sc.hasNext()) { sc.next(); count++; } System.out.println(count);
What is the output?

Java

import java.util.Scanner;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    sc.useDelimiter(",|;| ");
    int count = 0;
    while(sc.hasNext()) {
      sc.next();
      count++;
    }
    System.out.println(count);
  }
}

A 4

B 3

C 5

D 6

💡 Hint

Count the words separated by comma, semicolon, or space.

The delimiters split the input on ",", ";", or space. Scanner skips consecutive delimiters (like ", " or "; "), so the tokens are "apple", "banana", "cherry", "orange" = 4 tokens.

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