Reading integer input in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What Java class is commonly used to read integer input from the user?

The <strong>Scanner</strong> class is commonly used to read integer input from the user in Java.

beginner

How do you create a Scanner object to read input from the keyboard?

You create it with: Scanner scanner = new Scanner(System.in);

beginner

Which Scanner method reads an integer value from the user?

The method nextInt() reads an integer value from the user.

intermediate

What happens if the user enters a non-integer value when using nextInt()?

An InputMismatchException is thrown because the input is not an integer.

intermediate

Why should you close the Scanner object after use?

Closing the Scanner frees system resources associated with it. Use scanner.close(); when done.

Which import statement is needed to use Scanner in Java?

A import java.io.Scanner;

B import java.input.Scanner;

C import java.lang.Scanner;

D import java.util.Scanner;

Scanner is part of the java.util package, so you import it with import java.util.Scanner;.

What does nextInt() do in Scanner?

A Reads the next integer from input

B Reads the next string from input

C Reads the next line from input

D Reads the next double from input

nextInt() reads the next integer value from the input.

How do you create a Scanner to read from the keyboard?

A Scanner scanner = new Scanner(System.out);

B Scanner scanner = new Scanner(System.in);

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

D Scanner scanner = new Scanner(System.input);

System.in represents keyboard input, so you create Scanner with new Scanner(System.in);.

What exception occurs if input is not an integer when calling nextInt()?

A InputMismatchException

B NullPointerException

C NumberFormatException

D IOException

If input is not an integer, Scanner throws InputMismatchException.

Why is it good practice to close a Scanner after use?

A To prevent syntax errors

B To clear the input buffer

C To free system resources

D To reset the Scanner object

Closing Scanner frees system resources like input streams.

Explain how to read an integer input from the user in Java using Scanner.

Think about the steps from importing Scanner to reading and closing it.

What should you do if the user enters invalid input when expecting an integer?

Consider how to handle errors gracefully when reading input.

Practice

1.

Which Java class is commonly used to read integer input from the user?

easy

A. FileReader

B. BufferedReader

C. Scanner

D. PrintWriter

Solution

1. Step 1: Identify the class for input reading

   The Scanner class is designed to read input from various sources including user input.

2. Step 2: Confirm the class for integer input

   Scanner provides the nextInt() method to read integers specifically.

3. Final Answer:

   Scanner -> Option C

4. Quick Check:

   Input class = Scanner [OK]

Hint: Use Scanner class to read integers easily [OK]

Common Mistakes:

• Confusing Scanner with FileReader which reads files
• Using PrintWriter which is for output, not input
• Thinking BufferedReader reads integers directly without parsing

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.console());

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

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

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

Solution

1. Step 1: Understand input source for Scanner

   To read user input from the keyboard, System.in is used as the input stream.

2. Step 2: Match correct Scanner constructor

   The correct syntax is new Scanner(System.in). Other options use output or error streams, which are incorrect.

3. Final Answer:

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

4. Quick Check:

   Keyboard input = System.in [OK]

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

Common Mistakes:

• Using System.out or System.err instead of System.in
• Trying to use System.console() which returns Console, not InputStream
• Forgetting to import java.util.Scanner

3.

What will be the output of the following Java code if the user enters 42?

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num = sc.nextInt();
        System.out.println(num + 10);
        sc.close();
    }
}

medium

A. 52

B. 42

C. 10

D. Error at runtime

Solution

1. Step 1: Read integer input

   The program reads an integer from the user using nextInt(). User inputs 42, so num = 42.

2. Step 2: Add 10 and print result

   The program prints num + 10, which is 42 + 10 = 52.

3. Final Answer:

   52 -> Option A

4. Quick Check:

   42 + 10 = 52 [OK]

Hint: Add 10 to input number and print [OK]

Common Mistakes:

• Printing input without adding 10
• Forgetting to close Scanner (not an error but good practice)
• Assuming input is string, causing error

4.

Identify the error in the following code snippet that reads an integer from the user:

import java.util.Scanner;

public class Test {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int number = sc.nextLine();
        System.out.println(number);
        sc.close();
    }
}

medium

A. Scanner object is not closed properly.

B. Using nextLine() to read an integer causes a type mismatch error.

C. Missing import statement for Scanner.

D. System.out.println() cannot print integers.

Solution

1. Step 1: Check method used to read input

   The code uses nextLine() which returns a String, but tries to assign it to an int variable.

2. Step 2: Understand type mismatch

   Assigning a String to an int variable causes a compile-time error. The correct method to read an int is nextInt().

3. Final Answer:

   Using nextLine() to read an integer causes a type mismatch error. -> Option B

4. Quick Check:

   nextLine() returns String, not int [OK]

Hint: Use nextInt() to read integers, not nextLine() [OK]

Common Mistakes:

• Using nextLine() instead of nextInt() for integers
• Ignoring type mismatch errors
• Not closing Scanner (though not the main error here)

5.

You want to read two integers from the user and print their sum. Which code snippet correctly does this?

?

hard

A. Scanner sc = new Scanner(System.in);
int a = sc.nextInt();
int b = sc.nextInt();
System.out.println(a + b);
sc.close();

B. Scanner sc = new Scanner(System.in);
int a = Integer.parseInt(sc.nextLine());
int b = sc.nextLine();
System.out.println(a + b);

C. Scanner sc = new Scanner(System.in);
String a = sc.nextLine();
String b = sc.nextLine();
System.out.println(a + b);
sc.close();

D. Scanner sc = new Scanner(System.in);
int a = sc.nextInt();
String b = sc.nextLine();
System.out.println(a + b);
sc.close();

Solution

1. Step 1: Identify the correct code for reading two integers

   Scanner sc = new Scanner(System.in); int a = sc.nextInt(); int b = sc.nextInt(); System.out.println(a + b); sc.close(); uses sc.nextInt() twice to read integers directly, handling whitespace or newlines, then prints their sum.

2. Step 2: Eliminate incorrect options

   A: Compile error - int b = sc.nextLine(); (String to int).
   C: Concatenates strings ("1" + "2" = "12").
   D: nextLine() after nextInt() reads leftover newline (b = "", e.g., 5 + "" = "5").

3. Final Answer:

   Scanner sc = new Scanner(System.in); int a = sc.nextInt(); int b = sc.nextInt(); System.out.println(a + b); sc.close(); -> Option A

4. Quick Check:

   Two nextInt() reads ints correctly [OK]

Hint: Use nextInt() for integer input reliably [OK]

Common Mistakes:

• Mixing nextInt() and nextLine() causing input skipping
• Adding strings instead of integers
• Not closing Scanner after use