JAVA 问题

A simple text scanner which can parse primitive types and strings using regular
expressions.
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
System.in:
Scanner sc = new Scanner(System.in);
int i = sc.nextInt();

As another example, 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();
}

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 java.lang.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 primitive-type 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 findInLine,
findWithinHorizon,
and skip
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 java.lang.Readable.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 java.util.Locale.getDefault
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 and 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.

Since:
1.5
Version:
%I%, %G%