Java Scanner Class

Scanner lets your program read what a person types, like a teacher asking a question and writing down your answer. You can read words, numbers and more.

In this page:

What is Scanner?
Reading Strings
Reading Numbers
Checking for Inputs
Closing the Scanner

Syntax

java

import java.util.Scanner;

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

What is Scanner?

Scanner, from java.util, wraps an input source -- typically System.in for keyboard input, but it can just as easily read from a file -- and gives you convenient methods to pull out words, lines, or numbers.

Example: What is Scanner?

java

import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner("25 apples"); // wraps a source; System.in is typical, this is a hardcoded stand-in
		System.out.println(sc.nextInt() + " " + sc.next());
	}
}

Reading Strings

next() reads input up to the next whitespace, so it stops at the first space in a name like 'John Smith'; nextLine() instead reads everything up to the newline, capturing the full line including spaces.

Example: Reading Strings

java

import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner("John Smith");
		System.out.println(sc.next());     // stops at the first space: "John"
		Scanner sc2 = new Scanner("John Smith");
		System.out.println(sc2.nextLine()); // reads the whole line: "John Smith"
	}
}

Reading Numbers

Methods like nextInt() and nextDouble() parse the next token as that specific numeric type and throw an InputMismatchException if what's typed doesn't actually look like a number.

Example: Reading Numbers

java

// Import java.util.Scanner so it can be used by its short name
import java.util.Scanner;
// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Create a new Scanner object and store it in sc
		Scanner sc = new Scanner("42 3.14");
		// Declare whole and set it to sc.nextInt()
		int whole = sc.nextInt();
		// Declare decimal and set it to sc.nextDouble()
		double decimal = sc.nextDouble();
		// Print a line to the console
		System.out.println(whole + " " + decimal);
	}
}

Checking for Inputs

Calling hasNextInt() before nextInt() lets you check whether the next token is actually a valid integer first, which is the standard way to validate user input without crashing on bad data.

Example: Checking for Inputs

java

// Import java.util.Scanner so it can be used by its short name
import java.util.Scanner;
// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Create a new Scanner object and store it in sc
		Scanner sc = new Scanner("42");
		// Check whether sc.hasNextInt()
		if (sc.hasNextInt()) {
			// Print a line to the console
			System.out.println("Valid int: " + sc.nextInt());
		}
	}
}

Closing the Scanner

Scanner holds an open connection to its input source, so calling close() when you're done frees that underlying resource -- skipping this rarely causes visible problems for console input, but it's a resource leak for file-backed Scanners.

Example: Closing the Scanner

java

import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner("done");
		System.out.println(sc.next());
		sc.close(); // frees the underlying resource
	}
}

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Java Scanner Class

What is Scanner?

Reading Strings

Reading Numbers

Checking for Inputs

Closing the Scanner

Chapter Quiz — Complete all 6 topics to unlock

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