Reading User Input with Scanner

Until now, your programs talked and never listened. Time to hand the user a microphone.

Every program so far has printed the same thing every time it ran, because every value was typed right into the code. Real programs ask questions. A quiz asks for your answer. A calculator asks for numbers. A game asks for your name.

In Java, the most common tool for reading what someone types into the terminal is the Scanner.

A waiter taking your order

Think of a waiter at a restaurant. They come to your table, ask a question, and then wait, notepad ready, until you answer. They don't walk off halfway through your sentence, and they don't guess.

A Scanner is your program's waiter. When you ask it to read something, your program pauses and waits until the user types an answer and presses Enter. Then the Scanner hands that answer back to your code.

Setting up a Scanner

Using a Scanner takes three steps:

import java.util.Scanner;

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

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

		System.out.println("Nice to meet you, " + name + "!");
		input.close();
	}
}

1. import java.util.Scanner; goes at the very top of the file, above the class. Scanner lives in a part of Java called java.util, and the import tells the compiler where to find it. Forget it, and you'll see error: cannot find symbol pointing at Scanner.
2. new Scanner(System.in) creates the Scanner. System.in is the keyboard, the partner of System.out that you've been printing to. The new keyword creates an object; you'll learn all about it in Classes and Objects.
3. input.nextLine() reads everything the user types up to the moment they press Enter, and gives it back as a String.

Notice the prompt uses print, not println. That keeps the cursor on the same line as the question, which looks much nicer:

What's your name? Maria
Nice to meet you, Maria!

When you're done reading input, call input.close(). Create only one Scanner for the keyboard in your program, and use it for every question.

Reading numbers

nextLine() always gives you text. For numbers, Scanner has methods that read and convert in one step:

Method	Reads	Gives back
nextLine()	the whole line	String
next()	one word (up to a space)	String
nextInt()	a whole number	int
nextDouble()	a decimal number	double
nextBoolean()	true or false	boolean

Scanner input = new Scanner(System.in);

System.out.print("How old are you? ");
int age = input.nextInt();

System.out.println("Next year you'll be " + (age + 1));

Because age is a real int, age + 1 does math. If you'd read it with nextLine(), it would be the text "17", and "17" + 1 would give you 171.

When the user types something unexpected

What if someone types seventeen when your program calls nextInt()? The Scanner can't turn that into a number, so the program crashes with an InputMismatchException:

How old are you? seventeen
Exception in thread "main" java.util.InputMismatchException

Users type unexpected things all the time. For now, write clear prompts that say what kind of answer you want, like Enter your age (a number):. In Exceptions, you'll learn to catch this problem and ask again instead of crashing.

The nextInt and nextLine trap

This one catches nearly everyone. Look at this program:

Scanner input = new Scanner(System.in);

System.out.print("How old are you? ");
int age = input.nextInt();

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

System.out.println(name + " is " + age);

You'd expect to be asked both questions. Instead, this happens:

How old are you? 17
What's your name?  is 17

The program never waited for the name. Why?

When you type 17 and press Enter, you're really sending two things: the characters 17 and an invisible newline character from the Enter key. nextInt() takes the 17 and leaves the newline sitting there. Then nextLine() comes along, sees the newline, and thinks, "That's the end of a line. Done!" It hands back an empty string without waiting.

It's like the waiter taking your order but leaving the crumpled edge of the notepad page on the table. The next waiter sees it and assumes you've already ordered.

The fix: after reading a number, call nextLine() once on its own to clear away the leftover newline:

System.out.print("How old are you? ");
int age = input.nextInt();
input.nextLine();   // clear the leftover Enter

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

Another fix that many programmers prefer: read everything with nextLine(), and convert numbers yourself with Integer.parseInt, which you learned in Type Casting and Conversion:

System.out.print("How old are you? ");
int age = Integer.parseInt(input.nextLine());

Every read consumes the whole line, Enter included, so nothing is ever left behind. That's the style the Try it program below uses.

Try it

This program asks three questions and prints a summary. It reads every answer with nextLine() and converts the numbers itself, so there's no leftover-newline trap.

Suppose the user types Maria, then 11, then 92.5. Predict exactly what the terminal will show, including the lines the user typed.

Running Java here

Running Java right in the browser is coming to Syntaxia soon, and reading keyboard input will need a real terminal anyway. On your own computer, save this program as Main.java, run it with java Main.java, and type your answers when it asks.

Check your prediction

What's your name? Maria
What grade are you in? 11
What was your last test score? 92.5

Hi, Maria!
Next year you'll be in grade 12.
You need 7.5 more points for a perfect score.

The words after each question mark are what the user typed. The empty println() prints a blank line before the summary.

Try it yourself

1. Run the program on your computer and answer with your own details.
2. Add a fourth question: "What's your favorite subject?" Print it as part of the summary.
3. Rewrite the grade question to use input.nextInt() instead. Run it and watch what happens to the next question. Then fix it with an extra input.nextLine().

Check your understanding

What must go at the top of your file before you can use Scanner?

include Scanner;

using Scanner;

Nothing, Scanner is always available

import java.util.Scanner;

What type does input.nextLine() give back?

int

String

char

It depends on what the user types

After calling nextInt(), your next nextLine() returns an empty string without waiting. Why?

nextLine is broken in newer versions of Java

The user typed too fast

nextInt left the newline from the Enter key behind, and nextLine read it

You need two Scanner objects

Up next

Your programs can now take in text from a user. What can you do with it? Plenty: measure it, search it, cut it up, and compare it. That's Working with Strings.