public final Class Scanner

A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace. The resulting tokens may then be converted into values of different types using the various next methods.

For example, this code allows a user to read a number from the console.

var con = System.console();
if (con != null) {
    Scanner sc = new Scanner(con.reader());
    int i = sc.nextInt();
}

This code allows long types to be assigned from entries in a file myNumbers:

Scanner sc = new Scanner(new File("myNumbers"));
while (sc.hasNextLong()) {
    long aLong = sc.nextLong();
}

This code uses a Scanner to read lines from System#in. The Scanner uses the system property value of stdin.encoding as the Charset. Specifying the charset explicitly is important when reading from System.in, as it may differ from the default charset depending on the host environment or user configuration:

Scanner sc = new Scanner(System.in, System.getProperty("stdin.encoding"));
while (sc.hasNextLine()) {
    String aLine = sc.nextLine();
}

The scanner can also use delimiters other than whitespace. This example reads several items in from a string:

String input = "1 fish 2 fish red fish blue fish";
Scanner s = new Scanner(input).useDelimiter("\\s*fish\\s*");
System.out.println(s.nextInt());
System.out.println(s.nextInt());
System.out.println(s.next());
System.out.println(s.next());
s.close();

prints the following output:

1
    2
    red
    blue

The same output can be generated with this code, which uses a regular expression to parse all four tokens at once:

String input = "1 fish 2 fish red fish blue fish";
Scanner s = new Scanner(input);
s.findInLine("(\\d+) fish (\\d+) fish (\\w+) fish (\\w+)");
MatchResult result = s.match();
for (int i=1; i<=result.groupCount(); i++)
    System.out.println(result.group(i));
s.close();

The default whitespace delimiter used by a scanner is as recognized by Character.isWhitespace(). The reset() method will reset the value of the scanner's delimiter to the default whitespace delimiter regardless of whether it was previously changed.

A scanning operation may block waiting for input.

The next and hasNext methods and their companion methods (such as nextInt and hasNextInt) first skip any input that matches the delimiter pattern, and then attempt to return the next token. Both hasNext() and next() methods may block waiting for further input. Whether a hasNext() method blocks has no connection to whether or not its associated next() method will block. The tokens method may also block waiting for input.

The findInLine(), findWithinHorizon(), skip(), and findAll() methods operate independently of the delimiter pattern. These methods will attempt to match the specified pattern with no regard to delimiters in the input and thus can be used in special circumstances where delimiters are not relevant. These methods may block waiting for more input.

When a scanner throws an InputMismatchException, the scanner will not pass the token that caused the exception, so that it may be retrieved or skipped via some other method.

Depending upon the type of delimiting pattern, empty tokens may be returned. For example, the pattern "\\s+" will return no empty tokens since it matches multiple instances of the delimiter. The delimiting pattern "\\s" could return empty tokens since it only passes one space at a time.

A scanner can read text from any object which implements the java.lang.Readable interface. If an invocation of the underlying readable's read() method throws an java.io.IOException then the scanner assumes that the end of the input has been reached. The most recent IOException thrown by the underlying readable can be retrieved via the ioException method.

When a Scanner is closed, it will close its input source if the source implements the java.io.Closeable interface.

A Scanner is not safe for multithreaded use without external synchronization.

Unless otherwise mentioned, passing a null parameter into any method of a Scanner will cause a NullPointerException to be thrown.

A scanner will default to interpreting numbers as decimal unless a different radix has been set by using the useRadix method. The reset method will reset the value of the scanner's radix to 10 regardless of whether it was previously changed.

Localized numbers

An instance of this class is capable of scanning numbers in the standard formats as well as in the formats of the scanner's locale. A scanner's initial locale is the value returned by the Locale.getDefault(Locale.Category.FORMAT) method; it may be changed via the useLocale() method. The reset method will reset the value of the scanner's locale to the initial locale regardless of whether it was previously changed.

The localized formats are defined in terms of the following parameters, which for a particular locale are taken from that locale's DecimalFormat object, df, and its DecimalFormatSymbols object, dfs.

LocalGroupSeparator  

The character used to separate thousands groups, i.e., dfs.getGroupingSeparator()

LocalDecimalSeparator  

The character used for the decimal point, i.e., dfs.getDecimalSeparator()

LocalPositivePrefix  

The string that appears before a positive number (may be empty), i.e., df.getPositivePrefix()

LocalPositiveSuffix  

The string that appears after a positive number (may be empty), i.e., df.getPositiveSuffix()

LocalNegativePrefix  

The string that appears before a negative number (may be empty), i.e., df.getNegativePrefix()

LocalNegativeSuffix  

The string that appears after a negative number (may be empty), i.e., df.getNegativeSuffix()

LocalNaN  

The string that represents not-a-number for floating-point values, i.e., dfs.getNaN()

LocalInfinity  

The string that represents infinity for floating-point values, i.e., dfs.getInfinity()

Number syntax

The strings that can be parsed as numbers by an instance of this class are specified in terms of the following regular-expression grammar, where Rmax is the highest digit in the radix being used (for example, Rmax is 9 in base 10).

NonAsciiDigit:

A non-ASCII character c for which Character.isDigit(c) returns true

Non0Digit:

[1-Rmax] | NonASCIIDigit

Digit:

[0-Rmax] | NonASCIIDigit

GroupedNumeral:

( Non0Digit Digit? Digit?

    ( LocalGroupSeparator Digit Digit Digit)+ )

Numeral:

( ( Digit+ ) | GroupedNumeral)

Integer:

( [-+]? ( Numeral) )

| LocalPositivePrefix Numeral LocalPositiveSuffix

| LocalNegativePrefix Numeral LocalNegativeSuffix

DecimalNumeral:

Numeral

| Numeral LocalDecimalSeparator Digit*

| LocalDecimalSeparator Digit+

Exponent:

( [eE] [+-]? Digit+ )

Decimal:

( [-+]? DecimalNumeral Exponent? )

| LocalPositivePrefix DecimalNumeral LocalPositiveSuffix Exponent?

| LocalNegativePrefix DecimalNumeral LocalNegativeSuffix Exponent?

HexFloat:

[-+]? 0[xX][0-9a-fA-F]*\.[0-9a-fA-F]+ ([pP][-+]?[0-9]+)?

NonNumber:

NaN | LocalNan| Infinity | LocalInfinity

SignedNonNumber:

( [-+]? NonNumber)

| LocalPositivePrefix NonNumber LocalPositiveSuffix

| LocalNegativePrefix NonNumber LocalNegativeSuffix

Float:

Decimal | HexFloat | SignedNonNumber

Whitespace is not significant in the above regular expressions.