Input and Output Streams

Using Streams to Concatenate Files (notes)

The SequenceInputStream creates a single input stream from multiple input sources. This example program, Concatenate, uses SequenceInputStream to implement a concatenation utility that sequentially concatenates files together in the order they are listed on the command line.

This is the controlling class of the Concatenate utility:

import java.io.*;

class Concatenate {
    public static void main(String[] args) {
        ListOfFiles mylist = new ListOfFiles(args);

        try {
            SequenceInputStream s = new SequenceInputStream(mylist);
            int c;

            while ((c = s.read()) != -1) {
               System.out.write(c);
            }

            s.close();
        } catch (IOException e) {
            System.err.println("Concatenate: " + e);
        }
    }
}

The first thing that the Concatenate utility does is create a ListOfFiles object named mylist which is initialized from the command line arguments entered by the user. The command line arguments list the files to be concatenated together. mylist is used to initialize the SequenceInputStream which uses mylist to get a new InputStream for every filename listed by the user.

import java.util.*;
import java.io.*;

class ListOfFiles implements Enumeration {

    String[] listOfFiles;
    int current = 0;

    ListOfFiles(String[] listOfFiles) {
        this.listOfFiles = listOfFiles;
    }

    public boolean hasMoreElements() {
        if (current < listOfFiles.length)
            return true;
        else
            return false;
    }

    public Object nextElement() {
        InputStream is = null;

        if (!hasMoreElements())
            throw new NoSuchElementException("No more files.");
        else {
            try {
                String nextElement = listOfFiles[current];
                current++;
                is = new FileInputStream(nextElement);
            } catch (FileNotFoundException e) {
                System.out.println("ListOfFiles: " + e);
            }
        }
        return is;
    }
}

ListOfFiles implements the Enumeration interface. You'll see how this comes into play as we walk through the rest of the program.

After the main method creates the SequenceInputStream, it reads from it one byte at a time. When the SequenceInputStream needs an InputStream from a new source (such as for the first byte read or when it runs off the end of the current input stream), it calls nextElement on the Enumeration object to get the next InputStream. ListOfFiles creates FileInputStream objects lazily, meaning that whenever SequenceInputStream calls nextElement, ListOfFiles opens a FileInputStream on the next filename in the list and returns the stream. When the ListOfFiles runs out of files to read (it has no more elements), nextElement returns null, and the call to SequenceInputStream's read method returns -1 to indicate the end of input.

Concatenate simply echos all the data read from the SequenceInputStream to the standard output.

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Try this: Try running Concatenate on the farrago.txt and words.txt files which are used as input to other examples in this lesson.

Input and Output Streams