Objects of type
Scanner
are useful for breaking down formatted input into tokens and translating individual tokens according to their data type.
By default, a scanner uses white space to separate tokens. (White space characters include blanks, tabs, and line terminators. For the full list, refer to the documentation for
Character.isWhitespace
.) To see how scanning works, let's look at
ScanXan
, a program that reads the individual words in xanadu.txt
and prints them out, one per line.
import java.io.*; import java.util.Scanner; public class ScanXan { public static void main(String[] args) throws IOException { Scanner s = null; try { s = new Scanner( new BufferedReader( new FileReader("xanadu.txt"))); while (s.hasNext()) { System.out.println(s.next()); } } finally { if (s != null) { s.close(); } } } }
Notice that ScanXan
invokes Scanner
's close
method when it is done with the scanner object. Even though a scanner is not a stream, you need to close it to indicate that you're done with its underlying stream.
The output of ScanXan
looks like this:
In Xanadu did Kubla Khan A stately pleasure-dome ...
To use a different token separator, invoke useDelimiter()
, specifying a regular expression. For example, suppose you wanted the token separator to be a comma, optionally followed by white space. You would invoke,
s.useDelimiter(",\\s*");
The ScanXan
example treats all input tokens as simple String
values. Scanner
also supports tokens for all of the Java language's primitive types (except for char
), as well as BigInteger
and BigDecimal
. Also, numeric values can use thousands separators. Thus, in a US
locale, Scanner
correctly reads the string "32,767" as representing an integer value.
We have to mention the locale, because thousands separators and decimal symbols are locale specific. So, the following example would not work correctly in all locales if we didn't specify that the scanner should use the US
locale. That's not something you usually have to worry about, because your input data usually comes from sources that use the same locale as you do. But this example is part of the Java Tutorial and gets distributed all over the world.
The
ScanSum
example reads a list of double
values and adds them up. Here's the source:
import java.io.FileReader; import java.io.BufferedReader; import java.io.IOException; import java.util.Scanner; import java.util.Locale; public class ScanSum { public static void main(String[] args) throws IOException { Scanner s = null; double sum = 0; try { s = new Scanner( new BufferedReader( new FileReader("usnumbers.txt"))); s.useLocale(Locale.US); while (s.hasNext()) { if (s.hasNextDouble()) { sum += s.nextDouble(); } else { s.next(); } } } finally { s.close(); } System.out.println(sum); } }
And here's the sample input file,
usnumbers.txt
8.5 32,767 3.14159 1,000,000.1
The output string is "1032778.74159". The period will be a different character in some locales, because System.out
is a PrintStream
object, and that class doesn't provide a way to override the default locale. We could override the locale for the whole program or we could just use formatting, as described in the next topic,
Formatting.