Reading Keyboard Input with Scanner

New words in this topic, in plain English. Come back here whenever one feels fuzzy.

Input

Data that comes into a program from outside: typed on a keyboard, read from a file, or received over a network.

Standard input

The program's default input stream, System.in. In a terminal it is the keyboard; it can also be fed from a file.

Scanner

A class in java.util that reads text and hands it to you piece by piece, converting pieces to numbers when asked.

Token

One piece of input between delimiters, such as Asha or 42 in Asha 42.

Delimiter

The separator between tokens. For Scanner it is any whitespace by default: spaces, tabs and line breaks.

Whitespace

Characters you can't see but that make space: the space bar, tab and the line break from Enter.

Blocking

Waiting. A blocking call pauses the program until the data it needs arrives.

Import

A line at the top of a file, like import java.util.Scanner;, that lets you use a class from another package by its short name.

Exception

An error object Java throws while the program runs. If nothing catches it, the program stops and prints it (Phase 7).

1. 1

   Run the keyboard program on your machine

   Copy the first walkthrough program into Main.java on your computer, compile it with javac Main.java and run java Main in a terminal. Type your own name, age and height. Then run it again and type twelve for the age, and read the exception.

   terminalcopy cmd

   $ java Main

   ── expected output ──

   Your name: Asha Kumar

   Your age: twelve

   Exception in thread "main" java.util.InputMismatchException

   at java.base/java.util.Scanner.throwFor(Scanner.java:947)

   at java.base/java.util.Scanner.next(Scanner.java:1602)

   at java.base/java.util.Scanner.nextInt(Scanner.java:2267)

   at java.base/java.util.Scanner.nextInt(Scanner.java:2221)

   at Main.main(Main.java:9)

2. 2

   Change the fake keyboard

   In 'Read numbers until the input ends', change the String to "10 20 oops 30". Predict the count and sum before running. The loop stops at oops because hasNextInt() is false there, so you get count 2 and sum 30; 30 is never read.

3. 3

   Reproduce the trap

   In the trap example, delete the second name = in.nextLine(); and its print. Now the program reports an empty name, exactly what happens to a real user who typed their age and then never got asked for their name.

Errors are the best teachers. Make each change, read the error, guess what went wrong, then reveal the answer.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("How old are you? ");
        int age = in.nextInt();
        System.out.println("Next year you will be " + (age + 1));
    }
}

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner("5 8");
        int a = in.nextInt();
        int b = in.nextInt();
        int c = in.nextInt();
        System.out.println(a + b + c);
    }
}

1. 1

   Create one with Scanner in = new Scanner(System.in); after import java.util.Scanner;. System.in is the program's standard input, an InputStream of raw bytes from the keyboard. Scanner turns those bytes into characters and the characters into tokens: pieces of text separated by whitespace (spaces, tabs, line breaks), which is Scanner's default delimiter.

2. 2

   Token methods: next() returns the next token as a String, nextInt(), nextLong(), nextDouble() and nextBoolean() read the next token and convert it. They skip any whitespace first, so 42 reads as 42, and numbers on one line or on separate lines work the same way. nextLine() is different: it returns everything up to the end of the current line (without the line break) and moves past it.

3. 3

   Every next... call blocks: the program waits until the user has typed enough and pressed Enter. If the token doesn't match, nextInt throws InputMismatchException and leaves the bad token in place; if the input has ended, it throws NoSuchElementException. The hasNextInt(), hasNextDouble(), hasNext() and hasNextLine() methods look ahead without consuming anything, so you can check first and react.

4. 4

   The classic trap: nextInt() reads 25 but leaves the Enter key's line break behind. A following nextLine() finds that empty rest-of-line and returns "" immediately, so the user never gets to type their name. Fix it by calling nextLine() once to throw the rest away, or by reading every line with nextLine() and converting with Integer.parseInt(line.strip()) (Topic 1.6).

5. 5

   Numbers are read using the default locale: on a German system nextDouble() expects 1,45, not 1.45. Call in.useLocale(Locale.US) when the input format is fixed. Create one Scanner for System.in and reuse it; closing it also closes System.in, after which no part of the program can read the keyboard again.

6. 6

   Scanner can read from other sources too: new Scanner("12 34") scans a fixed String (perfect for testing, and what this page's runnable examples do, because the browser has no keyboard), and new Scanner(Path.of("data.txt")) scans a file (Phase 12). For very large inputs, such as programming contests, BufferedReader is much faster (see the DSA course at /dsa).

• ↔

  Scanner's typed methods (nextInt, nextDouble) are convenient and validate input, but they are slow and their mix with nextLine is error-prone; reading whole lines and parsing them yourself is more predictable.

• ↔

  BufferedReader is fast and reports I/O errors, but gives you only lines; you split and convert them yourself.

• ↔

  Command-line arguments (the args array of main) suit values known when the program starts and scripts; interactive input suits programs that talk to a person.

• I can create a Scanner for the keyboard and for a fixed String, with the right import.

• I can read Strings, ints, doubles and booleans and explain what a token is.

• I can explain and fix the nextInt then nextLine trap.

• I validate input with hasNextInt and discard bad tokens with next.

• I know why to use one Scanner for System.in and not close it.

• I can recognise InputMismatchException and NoSuchElementException and say what caused each.