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Using Streams to Implement Pipes

The java.io package contains two classes, PipedInputStream(in the API reference documentation) and PipedOutputStream(in the API reference documentation), that implement the input and output components of a pipe. Pipes are used to channel the output from one program (or thread) into the input of another.

Piped input and output streams are convenient for methods that produce output to be used as input by someone else. For example, suppose that you are writing a class that implements various text utilities such as sorting and reversing text. It would be nice if the output of one of these methods could be used as the input for another so you could string a series of these methods together to perform some function. The pipe shown here uses reverse, sort, and reverse on a list of words to create a list of rhyming words:

Without piped streams, you would have to create a temporary file between each step:

Let's look at a program that implements the reverse and sort methods described above using piped streams, and then uses the reverse and sort methods in the pipe shown above to generate a list of rhyming words.

First, the RhymingWords class contains three methods: main, reverse, and sort. The main method provides the code for the main program, which opens an input file, uses the other two methods to reverse, sort, and reverse the words in the input file, and then writes the results to the standard output stream.

reverse and sort are designed to be used in a pipe. Both reverse and sort read data from an InputStream, process it (either reversing the strings or sorting them), and produce a PipedInputStream suitable for another method to read. Let's look in detail at reverse; the sort method is very similar to reverse and doesn't warrant its own discussion.

public static InputStream reverse(InputStream source) {
    PipedOutputStream pos = null;
    PipedInputStream pis = null;

    try {
        DataInputStream dis = new DataInputStream(source);

        pos = new PipedOutputStream();
        pis = new PipedInputStream(pos);
        PrintStream ps = new PrintStream(pos);

        new WriteReversedThread(ps, dis).start();

    } catch (Exception e) {
        System.out.println("RhymingWords reverse: " + e);
    }
    return pis;
}
The reverse method takes an InputStream called source that contains a list of strings to be reversed. reverse maps a DataInputStream onto the source InputStream so that it can use the DataInputStream's readLine method to read each line from the file. DataInputStream is a filtered stream which must be attached to (mapped onto) an InputStream whose data is to be filtered when read. Working with Filtered Streams talks about this.

Next reverse creates a PipedOutputStream and connects a PipedInputStream to it. Remember that a PipedOutputStream must be connected to a PipedInputStream. Then reverse maps a PrintStream onto the PipedOutputStream so that it can use the PrintStream's println method to write strings to the PipedOutputStream.

Now reverse creates a WriteReversedThread thread object, hands it to two streams--the PrintStream attached to the PipedOutputStream and the DataInputStream attached to source--and starts it. The WriteReversedThread object read words from the DataInputStream, reverses them, and writes the output to the PrintStream (thereby writing the output to a pipe). The Thread object allows each end of the pipe to run independently of one another and prevents the main method from locking up if one end of a pipe blocks waiting for an I/O call to complete.

Here's the run method for WriteReversedThread:

public void run() {
    if (ps != null && dis != null) {
        try {
	    String input;
	    while ((input = dis.readLine()) != null) {
	        ps.println(reverseIt(input));
	        ps.flush();
	    }
	    ps.close();
        } catch (IOException e) {
	    System.out.println("WriteReversedThread run: " + e);
        }
    }
} 

Because the PipedOutputStream is connected to the PipedInputStream, all data written to the PipedOutputStream flows into the PipedInputStream. The data can be read from the PipedInputStream by another program or thread. reverse returns the PipedInputStream for use by the calling program.

The sort method follows the same pattern:

Calls to reverse and sort can be cascaded together so that the output from one method can be the input for the next method. In fact, the main method does just that. It cascades calls to reverse, sort, and then reverse to generate a list of rhyming words:
InputStream rhymedWords = reverse(sort(reverse(words)));
When you run RhymingWords on this file of words you will see this output:
Java
interface
image
language
communicate
integrate
native
string
network
stream
program
application
animation
exception
primer
container
user
graphics
threads
tools
class
bolts
nuts
object
applet
environment
development
argument
component
input
output
anatomy
security
If you look closely you can see that rhyming words such as environment, development, argument, and component are grouped together.

See Also

java.io.PipedInputStream
java.io.PipedOutputStream


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