Reading User Input - Java

Java reads keyboard input through the Scanner class, which lives in the java.util package. You need to import it at the top of your file before you can use it.

Java

Open in playground

import java.util.Scanner;

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

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

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

        System.out.println("Hello " + name + ", next year you will be " + (age + 1));
        in.close();
    }
}

A sample run:

text

Enter your name: Kabir Singh
Enter your age: 20
Hello Kabir Singh, next year you will be 21

Useful Scanner methods

nextLine() reads a whole line of text, including spaces.
next() reads a single word.
nextInt(), nextDouble() and nextBoolean() read a value of that type.
hasNextInt() checks whether the next input is a valid int, without reading it.

A common trap

nextInt() reads the number but leaves the Enter key (a newline) waiting. A following nextLine() then reads that empty leftover line. Call nextLine() once to clear it:

Java

Open in playground

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Roll number: ");
        int roll = in.nextInt();
        in.nextLine();                 // throw away the rest of the line
        System.out.print("Full name: ");
        String name = in.nextLine();
        System.out.println(roll + " - " + name);
    }
}

Validating input

Java

Open in playground

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter a whole number: ");
        while (!in.hasNextInt()) {
            System.out.print("That is not a number, try again: ");
            in.next();                 // discard the bad word
        }
        int n = in.nextInt();
        System.out.println("Its square is " + n * n);
    }
}