Scanner

About

The Scanner class in Java is part of the java.util package and is widely used for parsing and reading input. It allows developers to read and parse text or primitive data types (like int, double, etc.) from various sources, including keyboard input, files, and strings.

• The Scanner class is designed to break input into tokens using a delimiter (default is whitespace).

• It provides methods to read and parse different data types, such as integers, floats, strings, etc.

• Commonly used in applications that require user input or processing text-based data.

• It is simple to use, making it a popular choice for basic input handling in beginner Java programs.

Features

1. Input Sources: Can read input from a wide variety of sources such as the console, files, strings, and streams.

2. Tokenization: Automatically splits input into tokens based on delimiters.

3. Parsing Data: Provides methods to parse input into primitive types (int, double, boolean, etc.) and strings.

4. Custom Delimiters: Supports the use of custom delimiters for splitting input.

5. Locale Support: Can use different Locale settings for parsing numbers and text.

6. Built-in Validation: Offers methods to check the presence of a specific type of token (hasNextInt, hasNextDouble, etc.), ensuring safe input parsing.

Declaration

To use the Scanner class, it needs to be imported from the java.util package:

import java.util.Scanner;

Methods Available

1. Reading Data

• next(): Reads the next token as a string.

• nextLine(): Reads the entire line of input.

• nextInt(), nextDouble(), nextFloat(), nextLong(), nextShort(), nextByte(): Reads and parses a token as a specific primitive type.

• nextBoolean(): Reads a token and parses it as a boolean.

2. Checking for Input

• hasNext(): Checks if there is another token available.

• hasNextInt(), hasNextDouble(), etc.: Checks if the next token can be parsed as the specified type.

3. Token Delimiters

• useDelimiter(String pattern): Sets a custom delimiter pattern.

• reset(): Resets the scanner to the default delimiter and locale.

4. Locale and Patterns

• useLocale(Locale locale): Sets the scanner's locale for parsing numbers.

• findInLine(String pattern): Finds a pattern in the current line.

5. Closing the Scanner

• close(): Closes the scanner and releases any underlying resources.

Usage

Basic Input from Console

import java.util.Scanner;

public class ScannerExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter your name: ");
        String name = scanner.nextLine();

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

        System.out.println("Hello, " + name + ". You are " + age + " years old.");
        scanner.close(); // Always close the scanner
    }
}

Reading from a File

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

public class FileScannerExample {
    public static void main(String[] args) throws FileNotFoundException {
        File file = new File("example.txt");
        Scanner scanner = new Scanner(file);

        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            System.out.println(line);
        }

        scanner.close();
    }
}

Using a Custom Delimiter

import java.util.Scanner;

public class CustomDelimiterExample {
    public static void main(String[] args) {
        String input = "John,Doe,25,Engineer";
        Scanner scanner = new Scanner(input);
        scanner.useDelimiter(",");

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

        scanner.close();
    }
}

Validating Input

import java.util.Scanner;

public class InputValidationExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter an integer: ");
        while (!scanner.hasNextInt()) {
            System.out.println("That's not a valid integer. Try again:");
            scanner.next(); // Consume the invalid token
        }
        int number = scanner.nextInt();
        System.out.println("You entered: " + number);

        scanner.close();
    }
}

Applications and Real-World Usage

1. Command-Line Applications: Used to handle user input for interactive console-based applications.

2. File Parsing: Reading and processing text files for data analysis or configuration.

3. Data Validation: Ensuring user input meets specific criteria before processing.

4. Custom Data Formats: Parsing delimited data (e.g., CSV, TSV) using custom delimiters.

5. Educational Use: Commonly used in programming exercises to teach input handling in Java.