Java Lesson 46: Using the RandomAccessFile class

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Java Lesson 46 by Jon Huhtala

Using the RandomAccessFile class

Overview

In the previous lesson, you learned how to uses Java's I/O streams to write data to disk and read data from disk. In both cases, the organization of the file was sequential. All file data was written or read one item after another.

Sequential files are useful, but they do not provide a way to quickly locate an item of data without reading through all the other items of data that precede it. They also do not provide a means of updating an item of data without copying the entire file.

In this lesson, you will learn the essentials of Java's RandomAccessFile class, which permits a program to directly access an item of data and replace its contents.

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The RandomAccessFile class

• Is part of the java.io package

• Is an extension of the Object class. It does NOT descend from the InputStream or OutputStream classes.

Object
	RandomAccessFile

• Is a high-level class that can be used to directly read or write primitive data items and UTF strings to a file. The file behaves like a large array of bytes. By positioning a pointer ("seeking") to a byte location within the file, a particular item of data may be either read or written.

• Has two constructors. The most frequently used requires two parameters

1. The reference of a File object that contains the pathname of the file.

2. A String that represents the mode of access. It must be either "r" (read only) or "rw" (read/write).

For example, if fd is the reference of a File object

RandomAccessFile raf = new RandomAccessFile(fd, "rw");

will construct a RandomAccessFile object for both reading and writing primitive values and UTF strings to the file. Because a checked, IOException may occur, the statement should be enclosed in a try block with an appropriate catch.

• Has many useful methods as follows:

Method	Usage
getFilePointer()	Returns the byte offset of the pointer into this file
length()	Returns the byte length of this file
readBoolean()	Reads a boolean value from the pointer location within the file
readByte()	Reads a byte value from the pointer location within the file
readChar()	Reads a char value from the pointer location within the file
readDouble()	Reads a double value from the pointer location within the file
readFloat()	Reads a float value from the pointer location within the file
readInt()	Reads a int value from the pointer location within the file
readLong()	Reads a long value from the pointer location within the file
readShort()	Reads a short value from the pointer location within the file
readUTF()	Reads a String from the pointer location within the file according to the UTF standard
seek()	Sets the pointer offset, measured from the beginning of this file, at which the next read or write will occur
writeBoolean()	Writes a specified boolean value to the pointer location within the file
writeByte()	Writes a specified byte value to the pointer location within the file
writeChar()	Writes a specified char value to the pointer location within the file
writeDouble()	Writes a specified double value to the pointer location within the file
writeFloat()	Writes a specified float value to the pointer location within the file
writeInt()	Writes a specified int value to the pointer location within the file
writeLong()	Writes a specified long value to the pointer location within the file
writeShort()	Writes a specified short value to the pointer location within the file
writeUTF()	Writes a specified String to the pointer location within the file according to the UTF standard

Notes:

1. All read and write methods automatically advance the pointer to the next data item within the file.

2. Because a checked, IOException may occur, calls to these methods should be enclosed in a try block with an appropriate catch. Consult the Java API documentation for more details.

• Example. The following program creates a file of 100 integers. The user is then allowed to directly read and change the value of selected integers within the file. The final contents of the file are then displayed.

import java.io.*;
public class App {
public static void main(String[] args) {

// Local variables and object references.

File fd;

// Get the path name from the user.

System.out.print("Enter the file's complete path name: ");
fd = new File(Keyboard.readString());

// Try to process the file.

try {

// Format the file with 100 integer values (all zero).

DataOutputStream out = new DataOutputStream(new FileOutputStream(fd));
for (int i = 0; i < 100; i++) {
out.writeInt(0);
}
out.close();
System.out.println("The file is formatted with 100 integers");

// Re-open the file for random reading and writing.

RandomAccessFile raf = new RandomAccessFile(fd, "rw");

// Allow the user to see and change any integer value within
// the file.

char again;
do {

// Ask the user for the integer's position within the file.

System.out.print("Specify an integer's position (0 to 99): ");
int position = Keyboard.readInt();

// Calculate the integer's offset (byte displacement) within
// the file.

int pointer = position * 4;

// If the offset is valid, locate the integer, read and display
// its current value, get a new value from the user, and write
// the new value back to the same location within the file.

if (pointer < raf.length()) {
raf.seek(pointer);
System.out.println("Current value: " + raf.readInt());
raf.seek(pointer);
System.out.print("New value: ");
raf.writeInt(Keyboard.readInt());
}

// If the offset is not valid, display an error message.

else {
System.out.println("Invalid position");
}

// Ask the user if they want to change another integer value
// and repeat the loop as requested.

System.out.print("Change another value? (Y/N) ");
again = Keyboard.readChar();
} while (again == 'Y' || again == 'y');

// Close the random access file.

raf.close();

// Re-open the file for sequential input and display the
// file's contents.

DataInputStream in = new DataInputStream(new FileInputStream(fd));
try {
int position = 0;
while (true) {
System.out.println(position + ": " + in.readInt());
position++;
}
}
catch (EOFException e) {
System.out.println("");
}
in.close();
System.out.println("Closed - " + fd.getPath());
}

// Catch an IOException if one is thrown.

catch (IOException e) {
System.out.println(e.getMessage());
}
}
}

Limitations and solutions

The usefulness of the RandomAccessFile class is severely restricted by its inability to read and write objects. For example, it would be difficult to use a RandomAccessFile to process a file of Customer objects.

The simplest solution to this problem is to use a Map collection (such as a HashMap) for the objects. All collections are serializable. In a single I/O operation, the entire Map can be written to disk using an ObjectOutputStream or read from disk using an ObjectInputStream. The get() and put() methods can be used to access the collection's objects while the Map is in memory.

Lab exercise for Ferris students

E-mail your answers to this assignment no later than the due date listed in the class schedule.

Review questions

1. True or False: The seek() method is overloaded to measure the offset of an item of data relative to the end of the file.

1. True

2. False

2. Assume that myFile references a RandomAccessFile containing double values and that an item of data has just been read. Which one of the following must be coded in order to sequentially read the next item of data from the file?

1. myFile.seek(RandomAccessFile.NEXT);

2. myFile.seek(myFile.getPointer() + ;

3. myFile.pointer++;

4. myFile.setPointer(myFile.getPointer() + ;

5. none of the above

3. Assume that sFile references a RandomAccessFile of short values. Which one of the following correctly positions the pointer in order to read the last short value from the file?

1. sFile.setPointer(sFile.length() - 2);

2. sFile.setPointer(sFile.length() - 4);

3. sFile.seek(sFile.length() - 2);

4. sFile.seek(sFile.length() - 4);

5. sFile.seek(RandomAccessFile.LAST);

4. If theFile is a File object containing a path name, code a single statement to construct a RandomAccessFile object named readOnly that only allows data to be read from the file.

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