Scanner

The Scanner class in Java is used to get user input from various sources including the console (keyboard), files, and strings.

Basic Usage

To use the Scanner class, you first need to import it:

import java.util.Scanner;

Then create a Scanner object:

// Create a Scanner object for console input
Scanner scanner = new Scanner(System.in);

Reading Different Types of Input

Reading Strings

System.out.print("Enter your name: ");
String name = scanner.nextLine();  // Reads a line of text

Reading Integers

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

Reading Floating Point Numbers

System.out.print("Enter your height in meters: ");
double height = scanner.nextDouble();  // Reads a double

Reading a Single Word

System.out.print("Enter a word: ");
String word = scanner.next();  // Reads until whitespace

Important Methods

Method	Description
nextLine()	Reads a line of text (until newline)
next()	Reads the next token (word)
nextInt()	Reads the next token as an int
nextDouble()	Reads the next token as a double
nextBoolean()	Reads the next token as a boolean
hasNext()	Returns true if more tokens exist
hasNextLine()	Returns true if more lines exist

Common Issues

Mixing nextLine() with other next methods

When you use nextInt() or other non-line methods followed by nextLine(), you may encounter issues because nextInt() doesn’t consume the newline character:

// Issue example
int number = scanner.nextInt();  // User enters "42" and presses Enter
String text = scanner.nextLine();  // This immediately returns empty string!

Solution:

// Solution
int number = scanner.nextInt();  // User enters "42" and presses Enter
scanner.nextLine();  // Consume the leftover newline
String text = scanner.nextLine();  // Now this waits for user input

Closing the Scanner

It’s good practice to close the Scanner when you’re done with it:

scanner.close();

Reading from Files

You can also use Scanner to read from files:

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

public class ReadFile {
  public static void main(String[] args) {
    try {
      File file = new File("example.txt");
      Scanner fileScanner = new Scanner(file);

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

      fileScanner.close();
    } catch (FileNotFoundException e) {
      System.out.println("File not found");
      e.printStackTrace();
    }
  }
}