Java Scanner

Introduction

The Scanner class in Java, part of the java.util package, is a utility class used to parse primitive types and strings using regular expressions. 

It can read input from various sources, including strings, files, and input streams. 

The Scanner class is widely used for reading user input in console applications.

Table of Contents

1. What is the Scanner Class?
2. Common Methods
3. Examples of Using the Scanner Class
4. Conclusion

1. What is the Scanner Class?

The Scanner class provides methods for reading and parsing primitive types (like int, double, etc.) and strings from different sources such as the console, files, and other input streams. It uses delimiters (default is whitespace) to separate the tokens in the input.

2. Common Methods

• next(): Finds and returns the next complete token from this scanner.
• nextLine(): Advances the scanner past the current line and returns the input that was skipped.
• nextInt(): Scans the next token of the input as an int.
• nextDouble(): Scans the next token of the input as a double.
• hasNext(): Returns true if this scanner has another token in its input.
• hasNextInt(): Returns true if the next token in this scanner's input can be interpreted as an int value.
• hasNextDouble(): Returns true if the next token in this scanner's input can be interpreted as a double value.
• close(): Closes the scanner.
• useDelimiter(String pattern): Sets this scanner's delimiting pattern.

3. Examples of Using the Scanner Class

Example 1: Reading User Input from the Console

This example demonstrates how to read different types of user input from the console.

import java.util.Scanner;

public class ScannerConsoleExample {
    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter your name: ");
            String name = scanner.nextLine();
            System.out.println("Hello, " + name + "!");

            System.out.print("Enter your age: ");
            int age = scanner.nextInt();
            System.out.println("You are " + age + " years old.");

            System.out.print("Enter your height (in meters): ");
            double height = scanner.nextDouble();
            System.out.println("Your height is " + height + " meters.");
        }
    }
}

Output:

Enter your name: Rajesh
Hello, Rajesh!
Enter your age: 28
You are 28 years old.
Enter your height (in meters): 1.75
Your height is 1.75 meters.

Example 2: Reading from a File

This example shows how to use the Scanner class to read data from a file.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ScannerFileExample {
    public static void main(String[] args) {
        File file = new File("example.txt");
        try (Scanner scanner = new Scanner(file)) {
            while (scanner.hasNextLine()) {
                String line = scanner.nextLine();
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            System.out.println("File not found.");
            e.printStackTrace();
        }
    }
}

Output (assuming example.txt contains the following content):

This is the first line.
This is the second line.
This is the third line.

Example 3: Using Different Delimiters

This example demonstrates how to use different delimiters with the Scanner class.

import java.util.Scanner;

public class ScannerDelimiterExample {
    public static void main(String[] args) {
        String input = "apple,orange,banana";
        try (Scanner scanner = new Scanner(input)) {
            scanner.useDelimiter(",");

            while (scanner.hasNext()) {
                System.out.println(scanner.next());
            }
        }
    }
}

Output:

apple
orange
banana

Example 4: Reading Multiple Data Types

This example shows how to read different data types from a single input string.

import java.util.Scanner;

public class ScannerMultipleTypesExample {
    public static void main(String[] args) {
        String input = "Anjali 25 5.4 true";
        try (Scanner scanner = new Scanner(input)) {
            String name = scanner.next();
            int age = scanner.nextInt();
            double height = scanner.nextDouble();
            boolean isStudent = scanner.nextBoolean();

            System.out.println("Name: " + name);
            System.out.println("Age: " + age);
            System.out.println("Height: " + height);
            System.out.println("Is Student: " + isStudent);
        }
    }
}

Output:

Name: Anjali
Age: 25
Height: 5.4
Is Student: true

Example 5: Handling Exceptions

This example demonstrates how to handle exceptions when using the Scanner class.

import java.util.InputMismatchException;
import java.util.Scanner;

public class ScannerExceptionExample {
    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter an integer: ");
            int number = scanner.nextInt();
            System.out.println("You entered: " + number);
        } catch (InputMismatchException e) {
            System.out.println("Invalid input. Please enter an integer.");
        }
    }
}

Output (when entering a non-integer value):

Enter an integer: hello
Invalid input. Please enter an integer.

4. Conclusion

The Scanner class in Java provides a convenient way to read and parse various types of input from different sources. By using the methods provided by the Scanner class, developers can easily handle user input, read from files, and process strings with custom delimiters. 

Using the try-with-resources statement ensures that the scanner is closed properly, following best practices for resource management.