Input

So far, we’ve hardcoded all the values in our programs. Time to let the user type something. In Java, we read input through a tool called Scanner

import java.util.Scanner;

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

        System.out.print("What's your name? ");
        String name = scanner.nextLine();

        System.out.println("Hello, " + name + "!");
    }
}

Run the code, type something, press Enter

Two new things appeared

• import java.util.Scanner; at the top, before the public class. This tells Java: "I want to use the Scanner tool from Java’s standard library"
• Scanner scanner = new Scanner(System.in); creates a new scanner that reads from standard input (the keyboard)

The actual reading happens with scanner.nextLine(): it waits for the user to type a line and press Enter, then gives back the typed text as a String

For numbers, Scanner has different methods

import java.util.Scanner;

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

        System.out.print("Your age: ");
        int age = scanner.nextInt();

        System.out.println("Next year you will be " + (age + 1));
    }
}

scanner.nextInt() reads a whole number directly and hands it to us as an int, with no conversion needed on our side

For decimal numbers, there’s scanner.nextDouble()

public class Main {
    public static void main(String[] args) {
        double height = scanner.nextDouble();
    }
}

A small gotcha that catches everyone. If you mix nextInt() and nextLine(), things get weird

public class Main {
    public static void main(String[] args) {
        int age = scanner.nextInt();
        String name = scanner.nextLine();   // this looks empty!
    }
}

Why? Because nextInt() reads the number but leaves the newline character behind. Then nextLine() picks up that empty newline and immediately returns an empty string

The fix: add an extra scanner.nextLine() to "eat" the leftover newline

public class Main {
    public static void main(String[] args) {
        int age = scanner.nextInt();
        scanner.nextLine();   // consume the leftover newline
        String name = scanner.nextLine();
    }
}

Annoying, but you only need to remember it once

To keep things simple in this exercise, we’ll read the name first, then the age

Mission: Crew Check-In

Every crew member arriving at the station must scan in at the terminal. The system reads their name and age, then prints a welcome line.

Create a Scanner, read the name first (first line), then the age (second line). Then build and print the welcome message using +.

Read the input directly, don't print a question before reading. The check-in terminal stays silent until the welcome line.

Input (typed by the user when the program runs):

• First line: crew member's name
• Second line: crew member's age

Example

If the user types

Cortez
60

your program should print

Hello Cortez, you are 60 years old. Next year you will be 61