Reading input with Scanner

Written Explanation

Where the keyboard lives: System.in

You already know System.out, the place a program prints to. Its twin is System.in, the place a program reads from. By default, System.in is connected to the keyboard (strictly, to standard input; when you run a program in a terminal you can redirect it from a file or another program). System.in is a low-level stream of raw bytes, awkward to use directly. Scanner wraps it and turns the raw input into text, numbers, and lines.

Three lines to get started

import java.util.Scanner;                 // 1. make the class available

public class Greeter {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);   // 2. create a scanner on the keyboard
        // ... use it ...
    }
}

1. import java.util.Scanner; goes at the top of the file, before the class. Scanner lives in the package java.util, and Java only finds classes from that package when you import them. (Classes in java.lang, such as String and System, are imported automatically.)
2. new Scanner(System.in) builds a scanner attached to the keyboard. The variable in is a reference variable, like the String variables from BEG-009: the box holds a pointer to a Scanner object. in is just a name; scanner or input are equally common.

You create the scanner once and reuse it for the whole program. Creating several scanners on the same System.in is a classic way to lose input, because each one may read ahead and hold characters the others never see.

Prompt, then read

A running program that waits silently looks frozen. Always print a prompt first, using print (not println) so the cursor stays on the same line:

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

The program stops at nextLine() until the user types something and presses Enter. This pause is called blocking: the program is waiting for input and does nothing else.

The four reading methods

There are siblings for the other primitive types (nextLong(), nextBoolean(), and so on). These four are enough for now.

The Javadoc says Scanner breaks its input into tokens using a delimiter, and that by default the delimiter is whitespace: spaces, tabs, and line endings. So next(), nextInt(), and nextDouble() each skip any whitespace in front of them, read one token, and stop right after it. nextLine() works differently: it returns whatever is left on the current line, spaces included, and moves to the start of the next line.

The difference matters as soon as the text has a space in it:

So for names, addresses, and sentences use nextLine(). For single numbers use nextInt() or nextDouble().

What a number reader needs

nextInt() accepts only text that looks like a whole number, for example 7, -3, or +5. If the user types abc, or 3.5, or a number too large for an int, Java throws an InputMismatchException and the program stops. Exceptions have their own module later. For this lesson, remember the rule: call the method that matches what you expect, and the user must play along. Lessons 3 and 5 show how to survive users who do not.

nextDouble() has a quirk worth knowing early. It reads numbers using the locale of the computer, meaning the language and region settings. On a machine set up with a dot as the decimal separator, 2.5 works and 2,5 throws. On a machine set up with a comma, it can be the other way round. If your decimals are rejected on your own computer, this is the first thing to suspect.

Closing the scanner

in.close() tells the scanner you are done. For a scanner attached to System.in there is a catch the Javadoc points out: closing a scanner closes the underlying source if it can be closed, and System.in can. After that, nothing in the program can read the keyboard again, even through a new Scanner. So the rule is: create one scanner, use it for the whole program, and close it at the very end (or, in small exercises, simply let the program finish). Never close it in the middle and then try to read again.

The anatomy of a small input program

1. import java.util.Scanner;
2. create the scanner once
3. for each question: print a prompt, read, store in a variable
4. use the variables (calculate, print)
5. close the scanner

Every program in this module follows this pattern.