User Input

What if we want to prompt the user to enter something? For example, prompt the user to enter their name, and print a welcome message.

To achieve this, we use a class called Scanner. First, you create an instance of Scanner using its constructor (we'll dive into constructors once we get to object-oriented programming) then pass an argument System.in(). Your beginner Java journey will involve a lot of this, so get used to it!

Here's a basic example:

import java.util.Scanner;

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

        System.out.print("Please enter your name: ");
        String name = in.nextLine();
        System.out.printf("Hello %s!", name);
    }
}

Please enter your name: John
Hello John!              ^^ we typed in 'John' here

In this example, we entered the name 'John' into the console, saved it into a variable called name using a built-in method nextLine(), then printed it.

info

nextLine() is used to take Strings as input. There are several other methods for other types, like nextInt(), nextDouble(), and so on. However, these differ from nextLine(), and we'll go over that soon.

Scanner(System.in) is Java's one-size-fits-all solution for reading from stdin. Although there are better solutions for reading other streams, such as files, Scanner can read files as well.

nextLine() vs next()

There's an important distinction to be made here. nextLine() reads an entire line that you entered into the console. For example, typing Hello world is one line. However, there is a method called next() that reads one item at a time, seperated by a delimiter. By default, the delimiter is a space. Using next() on Hello world will only read Hello, and you'll have to call next() again to read world.

import java.util.Scanner;

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

        System.out.print("Please enter your first and last name: ");
        String fName = in.next();
        String lName = in.next();

        System.out.printf("Hello %s %s", fName,lName);
    }
}

Please enter your first and last name: John Smith
Hello John Smith

next() reads Strings, and there are other methods to read other data types: nextDouble(), nextInt(), and so on..

Scanner.in() can read any primitive data type, but remember that Java is a typed language. You must specify the type of your variables. When reading different data types, always ensure that you're assigning them to the proper type!

Scanner in = new Scanner(System.in);

        int myNum = in.nextInt();
        double myDouble = in.nextDouble();
        byte myByte = in.nextByte();
        int wrong = in.nextDouble(); // error

Reading Multiple Values

What if you want to read a first and last name at once, for example?

By passing in multiple different tokens separated by spaces, you can use the next() method to assign one token at a time.

This has significant use cases. Two of our favourite are: reading structured files, and reading command line arguments. These topics are advanced, so for now, let's quickly go over a very simple example that represents reading a file.

Let's say you have a file that stores customer info. Each line looks something like this: John Smith 23 [email protected]. Each line takes the form firstName lastName age email. You can read this information in the following way:

import java.util.Scanner;

public class Learn {
    public static void main(String[] args) {
        System.out.println("Enter your first name, last name, age, and email: ");
        Scanner in = new Scanner(System.in);

        String fName = in.next();
        String lName = in.next();
        int age = in.nextInt();
        String email = in.next();

        System.out.printf("Your line was: %s %s %d %s", fName,lName, age, email);
    }
}

Enter your first name, last name, age, and email:
John Smith 23 [email protected]
Your line was: John Smith 23 [email protected]

Note that we didn't actually read a file, we just entered the information seperated by spaces. File-reading is a bit more advanced, and there are other things that you should learn first.

This example just illustrated how you can read multiple pieces of information at once.