Reading Input from the Console

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java.util.Scanner

We can create a Scanner object to read input from the console (represented by System.in):

package org.wecancodeit.fundamentals;

import java.util.Scanner;

public class NamePromptApp {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        System.out.println("What is your name?");
        String name = input.nextLine();
        System.out.println("Hello, " + name);

        input.close();
    }

}

Let's run it!

If I type Ford Prefect, then this is what I will see in my console:

$ What is your name?
Ford Prefect
$ Hello, Ford Prefect

Let's Break It Down

Rather than a primitive like an int or a boolean, our Scanner is an object. (Strings are objects, too.) Scanner is a class representing our input variable's type.

Scanner input = new Scanner(System.in);

Remember, primitive types are in all lowercase, while class names are CapitalizedCamelCase:

Scanner input = new Scanner(System.in);

Creating the scanner

We use the new operator to create an object. The other operators we've seen have used one (+, -, =) or more (++, !=) symbols, but new is a legit operator, too:

Scanner input = new Scanner(System.in);

It is followed by a special method called a constructor. The job of a constructor is to construct an object. A constructor's name is the same as the name of the type of thing it's constructing:

Scanner input = new Scanner(System.in);

This constructor accepts System.in as a parameter in order to "scan" input from the console. System.in is the complement of System.out, representing console input:

Scanner input = new Scanner(System.in);

More on classes, objects, and constructors later.

Dat Message Do

Think of methods as messages we send to objects. We have been using the main method:

public static void main(String[] args) {
  // your code here
}

main is the message that the Java Virtual Machine (JVM) sends to our application's class to tell it to do something. You may hear a class containing a main method called a "main class".

Can Ya Do Something For Me?

We call the nextLine method when we want to ask our Scanner for the next line of user input. This is like saying, "Hey input! Can ya give me the next line?":

String name = input.nextLine();

See that dot lurking in there? It's an operator, too. Whimsically, we refer to it as the dot operator:

String name = input.nextLine();

We use the dot operator to send messages to objects.

Ponder System.out.println("my message");.

What's happening there?

Call Me

If we look at the Scanner class, we can find out what that nextLine() method looks like. The part of the method before the opening curly bracket ({) is called its signature. The signature of the nextLine() method is:

public String nextLine()

If it wasn't public, we wouldn't be able to see it, so we wouldn't be able to send that message:

public String nextLine()

String indicates the method's return type. This is how it responds when we talk to it. When we call nextLine(), its response is a String:

public String nextLine()

Other Methods

Scanner offers us other methods, as well. (Its capabilities are far beyond our simple needs.) We have been using nextLine(), which reads an entire line of input. (Until you hit <Enter>.)

Used as we're using it, Scanner breaks up input by blank spaces and line breaks. Its other methods don't read entire lines. They read up to the first space or line break.

Here are some of those you may find useful:

method	what it does	return type
nextLine()	reads the next `String` from the input until the line break	`String`
nextInt()	reads the next `int` from the input until the next space or line break	`int`
nextDouble()	reads the next `double` from the input until the next space or line break	`double`
next()	reads the next `String` from the input until the next space or line break	`String`

Demo

If we do this:

Scanner input = new Scanner(System.in);

System.out.println("Enter street address:");

int houseNumber = input.nextInt();
String street = input.next();

System.out.println("house number is " + houseNumber);
System.out.println("street is " + street);

Then we can do this:

$ Enter street address:
43 Brown St
$ house number is 43
$ street is Brown