Reading Input with Scanner

Asking the user for input

So far our programs always print the same thing. To make them react to the user, we read input with a tool called Scanner.

Think of it like this: Scanner is like a waiter taking your order. You tell it what kind of thing to listen for (a number? a word?) and it brings that back to your program.

Using Scanner takes three steps:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("What is your name?");
        String name = sc.nextLine();
        System.out.println("Hello, " + name + "!");
    }
}

Input (what the user types)

Priya

Output

What is your name?
Hello, Priya!

• import java.util.Scanner; – at the very top: “I want to use Scanner”.
• Scanner sc = new Scanner(System.in); – make a scanner that reads the keyboard (System.in).
• sc.nextLine() – wait for the user to type a line, and give it back as a String.

Reading different types

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        System.out.println(a + " + " + b + " = " + (a + b));
    }
}

Input (what the user types)

7
5

Output

7 + 5 = 12

nextInt() skips spaces and new lines, so the two numbers can be on one line (7 5) or on two lines.

The nextInt + nextLine trap

nextInt() reads the number but leaves the Enter key behind. A nextLine() right after it reads that empty leftover instead of the next line:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int age = sc.nextInt();
        sc.nextLine();               // throw away the rest of the age line
        String city = sc.nextLine();
        System.out.println(age + " years old, from " + city);
    }
}

Input (what the user types)

25
New Delhi

Output

25 years old, from New Delhi

Common mistake: If you mix nextInt() and nextLine(), add an extra sc.nextLine(); after the number to clear the rest of the line.