Admin: 1 версия

2010-06-01T05:19:09Z

1 версия

в 17:44, 31 мая 2010

2010-05-31T17:44:27Z

Новая страница

== Appending data to existing file ==



<source lang="java">
import java.io.File;
import java.io.RandomAccessFile;
public class Main {
public static void append(String fileName, String text) throws Exception {
File f = new File(fileName);
long fileLength = f.length();
RandomAccessFile raf = new RandomAccessFile(f, "rw");
raf.seek(fileLength);
raf.writeBytes(text);
raf.close();
}
public static void append(String fileName, byte[] bytes) throws Exception {
File f = new File(fileName);
long fileLength = f.length();
RandomAccessFile raf = new RandomAccessFile(f, "rw");
raf.seek(fileLength);
raf.write(bytes);
raf.close();
}
public static void main(String[] args) throws Exception {
append("c:\\tmp.txt", "Appended Data");
append("c:\\tmp.bin", "Appended Data".getBytes());
}
}</source>



== Employs RandomAccessFile to store ints and changes the value of the third int. ==



<source lang="java">
import java.io.IOException;
import java.io.RandomAccessFile;
public class MainClass {
public static void main(String[] args) {
try {
RandomAccessFile raf = new RandomAccessFile("c:\\temp\\RAFsample.txt", "rw");
raf.writeInt(10);
raf.writeInt(43);
raf.writeInt(88);
raf.writeInt(455);
// change the 3rd integer from 88 to 99
raf.seek((3 - 1) * 4);
raf.writeInt(99);
raf.seek(0); // go to the first integer
int i = raf.readInt();
while (i != -1) {
System.out.println(i);
i = raf.readInt();
}
raf.close();
} catch (IOException e) {
}
}
}</source>



== Getting FileChannel from RandomAccessFile ==



<source lang="java">
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) throws Exception {
FileChannel fc = new RandomAccessFile("data.txt", "rw").getChannel();
fc.position(fc.size());
fc.write(ByteBuffer.wrap("Some more".getBytes()));
fc.close();
}
}</source>



== RandomAccessFile Introduction ==

<OL><LI>A RandomAccessFile employs an internal pointer that points to the next byte to read.</LI><LI>This pointer is zero-based and the first byte is indicated by index 0.</LI><LI>When first created, a RandomAccessFile points to the first byte.</LI><LI>You can change the pointer"s position by invoking the seek method.</LI><LI>The skipBytes method moves the pointer by the specified number of bytes.</LI><LI>If skipping offset number of bytes would pass the end of file, the internal pointer will only move to as much as the end of file.</LI><LI>The skipBytes method returns the actual number of bytes skipped.</LI></OL>
<p>When opening a file using a RandomAccessFile, you can choose whether to open it read-only or read write.</p>



<source lang="java">
public RandomAccessFile (File file, String mode) throws FileNotFoundException
public RandomAccessFile (String filePath, String mode) throws FileNotFoundException</source>



== Seek in RandomAccessFile ==



<source lang="java">
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
public class MainClass {
public static void main(String[] args) throws IOException {
RandomAccessFile raf = new RandomAccessFile(new File("temp.tmp"), "rw");
raf.writeInt(1);
for (int i = 0; i < 10; i++) {
raf.seek(raf.length() - 4);
raf.writeInt(raf.readInt());
}
raf.close();
}
}</source>



== Test file pointer manipulation between FileChannel and RandomAccessFile objects. ==



<source lang="java">
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
public class MainClass {
public static void main(String[] argv) throws IOException {
RandomAccessFile randomAccessFile = new RandomAccessFile("test.dat", "r");
randomAccessFile.seek(1000);
FileChannel fileChannel = randomAccessFile.getChannel();
// This will print "1000"
System.out.println("file pos: " + fileChannel.position());
randomAccessFile.seek(500);
// This will print "500"
System.out.println("file pos: " + fileChannel.position());
fileChannel.position(200);
// This will print "200"
System.out.println("file pos: " + randomAccessFile.getFilePointer());
}
}</source>



== Use RandomAccessFile to reverse a file ==



<source lang="java">
import java.io.RandomAccessFile;
public class Main {
public static void main(String[] argv) throws Exception {
RandomAccessFile raf = new RandomAccessFile("a.dat", "rw");
int x, y;
for (long i = 0, j = raf.length() - 1; i < j; i++, j--) {
raf.seek(i);
x = raf.read();
raf.seek(j);
y = raf.read();
raf.seek(j);
raf.write(x);
raf.seek(i);
raf.write(y);
}
raf.close();
}
}</source>



== Use RandomAccessFile to save and read ==



<source lang="java">
import java.io.IOException;
import java.io.RandomAccessFile;
public class MainClass {
public static void main(String[] args) throws IOException {
RandomAccessFile rf = new RandomAccessFile("test.dat", "rw");
for (int i = 0; i < 10; i++)
rf.writeDouble(i * 1.414);
rf.close();
rf = new RandomAccessFile("test.dat", "rw");
rf.seek(5 * 8);
rf.writeDouble(47.0001);
rf.close();
rf = new RandomAccessFile("test.dat", "r");
for (int i = 0; i < 10; i++)
System.out.println("Value " + i + ": " + rf.readDouble());
rf.close();
}
}</source>



== Write int to RandomAccessFile using FileChannel ==



<source lang="java">
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.IntBuffer;
import java.nio.channels.FileChannel;
public class MainClass {
public static void main(String[] args) throws IOException {
FileChannel fc = new RandomAccessFile("temp.tmp", "rw").getChannel();
IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size()).asIntBuffer();
for (int i = 0; i < 10; i++)
ib.put(i);
fc.close();
}
}</source>

← Предыдущая	Версия 05:19, 1 июня 2010
(нет различий)

Java Tutorial/File/RandomAccessFile - История изменений

Admin: 1 версия

в 17:44, 31 мая 2010