Java: TextFileReader

TextFileReader.java

import java.io.IOException;
import java.util.Iterator;
import java.io.FileReader;
import java.io.BufferedReader;

/**
 * Abstraction for reading the context of a text file into a String. Implements the
 * Iterator pattern to read the content of a text file a single line at a time.
 *
 * @author      <a href='mailto:[email protected]'>[email protected]</a>
 * @version     0.120090705.071455
 * @since       0.120090705.071455
 */
public class TextFileReader implements Iterable<String> {

    /**
     * The full name of the text file from which the content will be read.
     */
	private final String fileName;

    /**
     * Create a new instance of the text file reader to get the content of the
     * file represented by the specified {@code fileName}.
     *
     * @param fileName      the full path and file name of the file to be
     *                      read. Should not be {@code null}.
     * @throws AssertionError
     *                     if the {@code fileName} is {@code null} and
     *                     the {@literal -ea} flag on the vm has been set.
     */
	public TextFileReader(final String fileName) {
		assert fileName != null;
		this.fileName = fileName;
	} 

    /**
     * Create a new iterator to iterate through the contents of the file.
     *
     * @return              an iterator to return the content of the file
     *                      line by line as a String. Never {@code null}.
     */
    @Override
	public Iterator<String> iterator() {
		return new TextFileIterator();
	}

    /**
     * An implementation of the Iterator pattern that iterates through the
     * contents of a text file returning each line in the file in turn.
     */
	private final class TextFileIterator implements Iterator<String> {

        /**
         * The input stream used to read the content of the text file.
         */
        final BufferedReader in;
        
        /**
         * The next line of text to be returned on the next call to {@link #next()}.
         */
        String nextline;

        /**
         * Create a new instance of the iterator class to read the content from
         * the {@link TextFileReader#fileName specified text file}.
         * This constructor is responsible for opening a stream to read the content 
         * as well as reading the first line of text from the file.
         *
         * @throws IllegalArgumentException
         *                     if an error occurs opening the input stream or
         *                     reading the first line of text from the file.
         */
        public TextFileIterator() {
            try {
                in = new BufferedReader(new FileReader(fileName));
                nextline = in.readLine(); // We peek ahead like this for the benefit of hasNext( ).
            } catch (IOException e) {
                throw new IllegalArgumentException(e);
            }
        }

        /**
         * Check if there is another line of text to be returned from this
         * iterator.
         *
         * @return              {@code true} if there is another line to
         *                      be returned, {@code false} otherwise.
         */
        @Override
        public boolean hasNext() {
            return nextline != null;
        }

        /**
         * Return the next line of text from the iterator. This method will
         * first read the line of text following the next line of text to be
         * returned. If no line follows the next line to be returned, this
         * method will close the input stream being used to read text.
         *
         * @throws IllegalArgumentException
         *                     if an error occurs trying to read from the
         *                     file or attempting to close the file after
         *                     all lines of text have been read.
         */
        public String next() {
            try {
                String result = nextline;
                // If we haven't reached EOF yet
                if (nextline != null) {
                    nextline = in.readLine();   // Read another line
                    if (nextline == null) {
                        in.close();     // And close on EOF
                    }
                }
                // Return the line we read last time through.
                return result;

            } catch (IOException e) {
                throw new IllegalArgumentException(e);
            }
        }

        /**
         * This method is not supported since the file is read-only we 
         * do not allow lines to be removed from the file.
         *
         * @throws UnsupportedOperationsException
         *                     always.
         */
        public void remove() {
            throw new UnsupportedOperationException();
        }        

	}

    /**
     * A simple demonstration of how the class can be used to read the content of a text file
     * and print the content to the screen.
     *
     * @param args          takes an optional single argument that is the full path to the
     *                      text file to be read. If no argument is supplied then this demo
     *                      will attempt to read the context of this java file.
     */
    public static void main(String[] args) {
        final String fileName = (args.length > 0) ? args[0] : "./TextFileReader.java";

        for (final String line : new TextFileReader(fileName)) {
            System.out.println(line);
        }
    }

}

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