Scanner in Java

In this tutorial, we will learn about Java Scanner and its methods with the help of examples.

ParameterDetails
SourceSource could be either one of String, File or any kind of InputStream

General Pattern that does most commonly asked about tasks

The following is how to properly use the java.util.Scanner class to interactively read user input from System.in correctly( sometimes referred to as stdin, especially in C, C++ and other languages as well as in Unix and Linux). It idiomatically demonstrates the most common things that are requested to be done.

package com.stackoverflow.scanner;
import javax.annotation.Nonnull;
import java.math.BigInteger;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.; import java.util.regex.Pattern; import static java.lang.String.format; public class ScannerExample { private static final Set EXIT_COMMANDS; private static final Set HELP_COMMANDS; private static final Pattern DATE_PATTERN; private static final String HELP_MESSAGE; static { final SortedSet ecmds = new TreeSet(String.CASE_INSENSITIVE_ORDER); ecmds.addAll(Arrays.asList("exit", "done", "quit", "end", "fino")); EXIT_COMMANDS = Collections.unmodifiableSortedSet(ecmds); final SortedSet hcmds = new TreeSet(String.CASE_INSENSITIVE_ORDER); hcmds.addAll(Arrays.asList("help", "helpi", "?")); HELP_COMMANDS = Collections.unmodifiableSet(hcmds); DATE_PATTERN = Pattern.compile("\d{4}([-\/])\d{2}\1\d{2}"); // http://regex101.com/r/xB8dR3/1 HELP_MESSAGE = format("Please enter some data or enter one of the following commands to exit %s", EXIT_COMMANDS); } /*
Using exceptions to control execution flow is always bad.
That is why this is encapsulated in a method, this is done this
way specifically so as not to introduce any external libraries
so that this is a completely self contained example.
@param s possible url
@return true if s represents a valid url, false otherwise
/
private static boolean isValidURL(@Nonnull final String s)
{
try { new URL(s); return true; }
catch (final MalformedURLException e) { return false; }
}
private static void output(@Nonnull final String format, @Nonnull final Object… args)
{
System.out.println(format(format, args));
}
public static void main(final String[] args)
{
final Scanner sis = new Scanner(System.in);
output(HELP_MESSAGE);
while (sis.hasNext())
{
if (sis.hasNextInt())
{
final int next = sis.nextInt();
output("You entered an Integer = %d", next);
}
else if (sis.hasNextLong())
{
final long next = sis.nextLong();
output("You entered a Long = %d", next);
}
else if (sis.hasNextDouble())
{
final double next = sis.nextDouble();
output("You entered a Double = %f", next);
}
else if (sis.hasNext("\d+"))
{
final BigInteger next = sis.nextBigInteger();
output("You entered a BigInteger = %s", next);
}
else if (sis.hasNextBoolean())
{
final boolean next = sis.nextBoolean();
output("You entered a Boolean representation = %s", next);
}
else if (sis.hasNext(DATE_PATTERN))
{
final String next = sis.next(DATE_PATTERN);
output("You entered a Date representation = %s", next);
}
else // unclassified
{
final String next = sis.next();
if (isValidURL(next))
{
output("You entered a valid URL = %s", next);
}
else
{
if (EXIT_COMMANDS.contains(next))
{
output("Exit command %s issued, exiting!", next);
break;
}
else if (HELP_COMMANDS.contains(next)) { output(HELP_MESSAGE); }
else { output("You entered an unclassified String = %s", next); }
}
}
}
/*
This will close the underlying Readable, in this case System.in, and free those resources.
You will not be to read from System.in anymore after this you call .close().
If you wanted to use System.in for something else, then don't close the Scanner.
*/
sis.close();
System.exit(0);
}
}

Using custom delimiters

You can use custom delimiters (regular expressions) with Scanner, with .useDelimiter(“,”), to determine how the input is read. This works similarly to String.split(…). For example, you can use Scanner to read from a list of
comma separated values in a String:

Related Article: BufferedWriter in Java

Scanner scanner = null;
try{
    scanner = new Scanner("i,like,unicorns").useDelimiter(",");;
    while(scanner.hasNext()){
        System.out.println(scanner.next());
    }
}catch(Exception e){
    e.printStackTrace();
}finally{
    if (scanner != null)
        scanner.close();
}

This will allow you to read every element in the input individually. Note that you should not use this to parse CSV data, instead, use a proper CSV parser library, see CSV parser for Java for other possibilities.

Reading system input using Scanner

Scanner scanner = new Scanner(System.in); //Scanner obj to read System input
String inputTaken = new String();
while (true) {
      String input = scanner.nextLine(); // reading one line of input
      if (input.matches("\s+")) // if it matches spaces/tabs, stop reading
           break;
      inputTaken += input + " ";
      }
System.out.println(inputTaken);

The scanner object is initialized to read input from keyboard. So for the below input from keyboar, it’ll produce the
output as Reading from keyboard

Reading
from
keyboard
//space

Reading file input using Scanner

Scanner scanner = null;
try {
     scanner = new Scanner(new File("Names.txt"));
     while (scanner.hasNext()) {
         System.out.println(scanner.nextLine());
     }
} catch (Exception e) {
      System.err.println("Exception occurred!");
} finally {
      if (scanner != null)
          scanner.close();
}

Here a Scanner object is created by passing a File object containing the name of a text file as input. This text file will be opened by the File object and read in by the scanner object in the following lines. scanner.hasNext() will check to see if there is a next line of data in the text file. Combining that with a while loop will allow you to iterate through every line of data in the Names.txt file. To retrieve the data itself, we can use methods such as nextLine(),nextInt(),nextBoolean(), etc. In the example above, scanner.nextLine()is used. nextLine() refers to the following line in a text file, and combining it with a scanner object allows you to print the contents of the line. To close a scanner object, you would use .close().

Using try with resources (from Java 7 onwards), the above mentioned code can be written elegantly as below.

try (Scanner scanner = new Scanner(new File("Names.txt"))) {
     while (scanner.hasNext()) {
          System.out.println(scanner.nextLine());
     }
} catch (Exception e) {
     System.err.println("Exception occurred!");
}

Read the entire input as a String using Scanner

You can use Scanner to read all of the text in the input as a String, by using \Z (entire input) as the delimiter. For example, this can be used to read all text in a text file in one line:

String content = new Scanner(new File("filename")).useDelimiter("\Z").next();
System.out.println(content);

Remember that you’ll have to close the Scanner, as well as catch the IoException this may throw, as described in the example Reading file input using Scanner.

Carefully Closing a Scanner

it can happen that you use a scanner with the System.in as parameter for the constructor, then you need to be aware that closing the scanner will close the InputStream too giving as next that every try to read the input on that (Or any other scanner object) will throw an java.util.NoSuchElementException or an
java.lang.IllegalStateException

example:

Scanner sc1 = new Scanner(System.in);
Scanner sc2 = new Scanner(System.in);
int x1 = sc1.nextInt();
sc1.close();
// java.util.NoSuchElementException
int x2 = sc2.nextInt();
// java.lang.IllegalStateException
x2 = sc1.nextInt();

Read an int from the command line

import java.util.Scanner;

Scanner s = new Scanner(System.in);
int number = s.nextInt();

If you want to read an int from the command line, just use this snippet. First of all, you have to create a Scanner object, that listens to System.in, which is by default the Command Line, when you start the program from the command line. After that, with the help of the Scanner object, you read the first int that the user passes into the command line and store it in the variable number. Now you can do whatever you want with that stored int.

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