Scanner Class

1. Introduction

The Scanner class in Java (from java.util) is used to read input from different sources like:

• keyboard (System.in)
• files
• strings

It is most commonly used to take user input from the console in beginner and intermediate Java programs.

Scanner makes input handling easier because it can directly read different data types like:

• int
• long
• double
• boolean
• String (word or full line)

2. Why Scanner Class Is Needed

Before Scanner, input was commonly taken using BufferedReader and manual parsing.
That approach is faster but requires more code.

Scanner is popular because:

• simple syntax
• supports many data types directly
• beginner friendly
• useful for quick programs and competitive coding basics

3. Import and Setup

Scanner is in java.util, so you must import it:

import java.util.Scanner;

Create Scanner object for keyboard input:

Scanner sc = new Scanner(System.in);

4. Reading Integers

Use nextInt() to read integer:

Scanner sc = new Scanner(System.in);

System.out.print("Enter age: ");
int age = sc.nextInt();

System.out.println("Age: " + age);

If user types 25, output becomes:

Age: 25

5. Reading Other Primitive Types

5.1 nextLong()

long salary = sc.nextLong();

5.2 nextDouble()

double price = sc.nextDouble();

5.3 nextBoolean()

boolean isActive = sc.nextBoolean();

Scanner automatically converts input to the requested type.

6. Reading String Input (Word vs Line)

6.1 next() for single word

next() reads input until space.

String name = sc.next();

If input is:

Shiva Srivastava

Then next() reads only:

Shiva

6.2 nextLine() for full line

nextLine() reads the entire line including spaces.

String fullName = sc.nextLine();

If input is:

Shiva Srivastava

Then it reads full line.

7. Very Common Issue: nextInt() + nextLine() Problem

This is the most common Scanner confusion.

Example:

int age = sc.nextInt();
String name = sc.nextLine();

Problem:

• nextInt() reads only number
• newline remains in buffer
• nextLine() immediately consumes that newline and returns empty string

Fix:

int age = sc.nextInt();
sc.nextLine(); // consume leftover newline
String name = sc.nextLine();

This is mandatory when mixing nextInt() and nextLine().

8. Input Validation and Exceptions

If user enters invalid input type, Scanner throws:

• InputMismatchException

Example: expecting int but user enters "abc".

Best practice for safer input:

if (sc.hasNextInt()) {
    int n = sc.nextInt();
} else {
    System.out.println("Please enter a valid number");
}

This avoids runtime exception.

9. Using Scanner with Loops

Example: Keep reading numbers until user enters 0

Scanner sc = new Scanner(System.in);

while (true) {
    int n = sc.nextInt();
    if (n == 0) break;
    System.out.println("You entered: " + n);
}

Used in many interactive programs.

10. Closing Scanner (Important)

Always close scanner when done:

sc.close();

But note: If you close a Scanner on System.in, you cannot read input again from System.in in the same program.

So in small programs, closing is fine. In long-running apps, handle input streams carefully.

11. Summary

• Scanner is in java.util and used for reading input.
• It supports many data types using methods like nextInt(), nextDouble(), nextBoolean().
• next() reads one word, nextLine() reads full line.
• Mixing nextInt() and nextLine() requires consuming newline using sc.nextLine().
• Use hasNextInt() and similar methods for validation.
• Close Scanner carefully when using System.in.

Written By: Shiva Srivastava