Java Scanner Class
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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?
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());
}
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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
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"
}
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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
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner("42 3.14");
int whole = sc.nextInt();
double decimal = sc.nextDouble();
System.out.println(whole + " " + decimal);
}
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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
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner("42");
if (sc.hasNextInt()) {
System.out.println("Valid int: " + sc.nextInt());
}
}
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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
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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