WordList.java Source code

Introduction

Here is the source code for WordList.java

Source

/*
 * Copyright (c) 2004 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 3nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose,
 * including teaching and use in open-source projects.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book, 
 * please visit http://www.davidflanagan.com/javaexamples3.
 */

import java.io.IOException;
import java.io.RandomAccessFile;

/**
 * This class represents a list of strings saved persistently to a file, along
 * with an index that allows random access to any string in the list. The static
 * method writeWords() creates such an indexed list in a file. The class
 * demostrates the use of java.io.RandomAccessFile
 */
public class WordList {
    // This is a simple test method
    public static void main(String args[]) throws IOException {
        // Write command line arguments to a WordList file named "words.data"
        writeWords("words.data", args);

        // Now create a WordList based on that file
        WordList list = new WordList("words.data");
        // And iterate through the elements of the list backward
        // This would be very inefficient to with sequential-access streams
        for (int i = list.size() - 1; i >= 0; i--)
            System.out.println(list.get(i));
        // Tell the list we're done with it.
        list.close();
    }

    // This static method creates a WordList file
    public static void writeWords(String filename, String[] words) throws IOException {
        // Open the file for read/write access ("rw"). We only need to write,
        // but have to request read access as well
        RandomAccessFile f = new RandomAccessFile(filename, "rw");

        // This array will hold the positions of each word in the file
        long wordPositions[] = new long[words.length];

        // Reserve space at the start of the file for the wordPositions array
        // and the length of that array. 4 bytes for length plus 8 bytes for
        // each long value in the array.
        f.seek(4L + (8 * words.length));

        // Now, loop through the words and write them out to the file,
        // recording the start position of each word. Note that the
        // text is written in the UTF-8 encoding, which uses 1, 2, or 3 bytes
        // per character, so we can't assume that the string length equals
        // the string size on the disk. Also note that the writeUTF() method
        // records the length of the string so it can be read by readUTF().
        for (int i = 0; i < words.length; i++) {
            wordPositions[i] = f.getFilePointer(); // record file position
            f.writeUTF(words[i]); // write word
        }

        // Now go back to the beginning of the file and write the positions
        f.seek(0L); // Start at beginning
        f.writeInt(wordPositions.length); // Write array length
        for (int i = 0; i < wordPositions.length; i++)
            // Loop through array
            f.writeLong(wordPositions[i]); // Write array element
        f.close(); // Close the file when done.
    }

    // These are the instance fields of the WordList class
    RandomAccessFile f; // the file to read words from

    long[] positions; // the index that gives the position of each word

    // Create a WordList object based on the named file
    public WordList(String filename) throws IOException {
        // Open the random access file for read-only access
        f = new RandomAccessFile(filename, "r");

        // Now read the array of file positions from it
        int numwords = f.readInt(); // Read array length
        positions = new long[numwords]; // Allocate array
        for (int i = 0; i < numwords; i++)
            // Read array contents
            positions[i] = f.readLong();
    }

    // Call this method when the WordList is no longer needed.
    public void close() throws IOException {
        if (f != null)
            f.close(); // close file
        f = null; // remember that it is closed
        positions = null;
    }

    // Return the number of words in the WordList
    public int size() {
        // Make sure we haven't closed the file already
        if (f == null)
            throw new IllegalStateException("already closed");
        return positions.length;
    }

    // Return the string at the specified position in the WordList
    // Throws IllegalStateException if already closed, and throws
    // ArrayIndexOutOfBounds if i is negative or >= size()
    public String get(int i) throws IOException {
        // Make sure close() hasn't already been called.
        if (f == null)
            throw new IllegalStateException("already closed");
        f.seek(positions[i]); // Move to the word position in the file.
        return f.readUTF(); // Read and return the string at that position.
    }
}