Reading integer input in Java

Introduction

We use reading integer input to get numbers from the user while the program is running. This helps the program work with data the user provides.

When you want the user to enter their age.

When you need a number to calculate something, like adding two numbers.

When you ask the user to choose an option from a menu by typing a number.

When you want to get a score or count from the user.

When you need to read any whole number from the keyboard.

Syntax

Java

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
int number = scanner.nextInt();

You must import java.util.Scanner to read input.

nextInt() reads the next whole number typed by the user.

Examples

This reads an integer and stores it in the variable age.

Java

Scanner scanner = new Scanner(System.in);
int age = scanner.nextInt();

This asks the user to enter a number and reads it into num.

Java

Scanner scanner = new Scanner(System.in);
System.out.print("Enter a number: ");
int num = scanner.nextInt();

Sample Program

This program asks the user to type an integer, reads it, and then prints it back.

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Please enter an integer: ");
        int userNumber = scanner.nextInt();
        System.out.println("You entered: " + userNumber);
        scanner.close();
    }
}

Important Notes

Always close the Scanner after use with scanner.close() to free resources.

If the user types something that is not an integer, the program will throw an error.

Summary

Use Scanner and nextInt() to read integer input from the user.

Remember to import java.util.Scanner and close the scanner when done.

Reading input lets your program interact with the user by using their numbers.

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