The class demonstrates the use of java.io.RandomAccessFile
/*
* 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.
}
}
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