Table of Contents
Just as you would like to format output data (using the Format family, or the printf family), you may also have a need to read in formatted input data.
There are two classes that are useful for this:
(Package java.util)
Object
StringTokenizer
(Package java.io)
Object
StreamTokenizer
While they don’t provide full parsing capabilities they are still extremely useful.
Note: If you really would like to do some more serious parsing in Java, see http://java-source.net/open-source/parser-generators for how to find more capable parsers, such as ANTLR, JavaCC, and CUP, etc.
The StringTokenizer Class
public class StringTokenizer implements Enumeration
The StringTokenizer class allows an application to break a string into tokens.
The set of delimiters (the characters that separate tokens) may be specified either at StringTokenizer creation time or on a per-token basis.
An instance of StringTokenizer behaves in one of two ways, depending on whether it was created with the returnTokens flag having the value true or false:
- If the flag is false, delimiter characters serve to separate tokens. A token is a maximal sequence of consecutive characters that are not delimiters.
- If the flag is true, delimiter characters are also considered to be tokens. A token is either one delimiter character, or a maximal sequence of consecutive characters that are not delimiters.
Here’s a simple example:
package inputOutput;
import java.util.StringTokenizer;
public class TokenizeString
{
public static void main(String[] args)
{
StringTokenizer st = new StringTokenizer("Java is great!");
while (st.hasMoreTokens())
{
System.out.println(st.nextToken());
}
}
}
which prints out:
Java is great!
Note that the default delimiter characters are:
- space (‘ ‘)
- tab (‘\t’)
- newline (‘\n’)
- carriage return (‘\r’)
Here’s another, modified, version, where we specify the delimiters explicitly:
package inputOutput;
import java.util.StringTokenizer;
public class TokenizeString
{
public static void main(String[] args)
{
String delimiters = ".;!?;:";
String paragraph =
"The spirit of Java is alive in the world! " +
"How can you be ignorant of it? " +
"Parse, compile, and assimilate; " +
"perhaps the world will become your oyster! " +
"An example: 'Write once, run anywhere.'";
StringTokenizer st =
new StringTokenizer(paragraph, delimiters);
while (st.hasMoreTokens())
{
System.out.println(st.nextToken());
}
}
}
It outputs the following:
The spirit of Java is alive in the world How can you be ignorant of it Parse, compile, and assimilate perhaps the world will become your oyster An example 'Write once, run anywhere '
Note that, in the above example, we are considering tokens to be sentences (well, kind of — actually, we’re just using the punctuation as delimiters). In other words, tokens do not have to be words, or single characters only. A token may be any sequence of characters.
If we want the delimiter characters also to be returned as tokens, we can make a slight further modification:
package inputOutput;
import java.util.StringTokenizer;
public class TokenizeString
{
public static void main(String[] args)
{
String delimiters = ".;!?;:";
String paragraph =
"The spirit of Java is alive in the world! " +
"How can you be ignorant of it? " +
"Parse, compile, and assimilate; " +
"perhaps the world will become your oyster! " +
"An example: 'Write once, run anywhere.'";
StringTokenizer st =
new StringTokenizer(paragraph, delimiters, true);
while (st.hasMoreTokens())
{
System.out.println(st.nextToken());
}
}
}
which outputs:
The spirit of Java is alive in the world ! How can you be ignorant of it ? Parse, compile, and assimilate ; perhaps the world will become your oyster ! An example : 'Write once, run anywhere . '
Perhaps we’d like to handle things inside single quotes a little better:
package inputOutput;
import java.util.StringTokenizer;
public class TokenizeString
{
public static void main(String[] args)
{
String delimiters = ".;!?;:'"; // Added single quote
String paragraph =
"The spirit of Java is alive in the world! " +
"How can you be ignorant of it? " +
"Parse, compile, and assimilate; " +
"perhaps the world will become your oyster! " +
"An example: 'Write once, run anywhere.'";
StringTokenizer st =
new StringTokenizer(paragraph, delimiters, true);
while (st.hasMoreTokens())
{
String token = st.nextToken();
if (token.equals("'"))
{
if (st.hasMoreTokens())
token += st.nextToken("'"); // Use different delimiter set.
if (st.hasMoreTokens())
token += st.nextToken();
}
System.out.println(token);
}
}
}
which outputs:
The spirit of Java is alive in the world ! How can you be ignorant of it ? Parse, compile, and assimilate ; perhaps the world will become your oyster ! An example : 'Write once, run anywhere.'
The StreamTokenizer Class
public class StreamTokenizer
The StreamTokenizer class takes an input stream and parses it into “tokens”, allowing the tokens to be read one at a time. The parsing process is controlled by a table and a number of flags that can be set to various states. The stream tokenizer can recognize identifiers, numbers, quoted strings, and various comment styles.
Each byte read from the input stream is regarded as a character in the range ‘\u0000’ through ‘\u00FF’. The character value is used to look up five possible attributes of the character:
- white space
- alphabetic
- numeric
- string quote
- comment character.
Each character can have zero or more of these attributes.
In addition, an instance has four flags. These flags indicate:
- Whether line terminators are to be returned as tokens or treated as white space that merely separates tokens.
- Whether C-style comments are to be recognized and skipped.
- Whether C++-style comments are to be recognized and skipped.
- Whether the characters of identifiers are converted to lowercase.
A typical application first constructs an instance of this class, sets up the syntax tables, and then repeatedly loops calling the nextToken method in each iteration of the loop until it returns the value TT_EOF.
Here’s an example, which parses Java files:
package inputOutput;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.StreamTokenizer;
public class TokenizeStream
{
/**
* Main entry point.
* This expects a single argument:
* the name of the file to be tokenized.
*/
public static void main(String[] args)
{
String fileName = args[0];
System.out.println("Parsing file: " + fileName);
try
{
BufferedReader reader =
new BufferedReader(
new FileReader(fileName) );
StreamTokenizer st = new StreamTokenizer(reader);
st.slashSlashComments(true);
st.slashStarComments(true);
int token = StreamTokenizer.TT_NUMBER; // Just not TT_EOF
while (token != StreamTokenizer.TT_EOF)
{
token = st.nextToken();
switch (token)
{
case StreamTokenizer.TT_NUMBER:
System.out.println("Number: " + st.nval);
break;
case StreamTokenizer.TT_WORD:
System.out.println("Word: " + st.sval);
break;
case StreamTokenizer.TT_EOL:
System.out.println("End of line");
break;
case StreamTokenizer.TT_EOF:
System.out.println("End of Stream");
break;
default:
System.out.println("Character: " +
(char)(token));
break;
}
}
}
catch (FileNotFoundException e)
{
e.printStackTrace();
}
catch (IOException e)
{
e.printStackTrace();
}
}
}
which, when told to parse itself, outputs (I’ve removed lots of lines in the middle, or it would get far too long):
Parsing file: TokenizeStream.java
Word: package
Word: inputOutput
Character: ;
Word: import
Word: java.io.BufferedReader
Character: ;
Word: import
Word: java.io.FileReader
Character: ;
Word: import
Word: java.io.FileNotFoundException
Character: ;
Word: import
Word: java.io.IOException
Character: ;
Word: import
Word: java.io.StreamTokenizer
Character: ;
Word: public
Word: class
Word: TokenizeStream
Character: {
...
Word: catch
Character: (
Word: FileNotFoundException
Word: e
Character: )
Character: {
Word: e.printStackTrace
Character: (
Character: )
Character: ;
Character: }
Word: catch
Character: (
Word: IOException
Word: e
Character: )
Character: {
Word: e.printStackTrace
Character: (
Character: )
Character: ;
Character: }
Character: }
Character: }
End of Stream
You can use the StreamTokenizer class relatively simply, as above. However, many applications demand more sophisticated usage, where you specify the attributes of word characters, etc. Details are left to the imaginative reader…