BINARY I/O - Introduction to Java Programming: Comprehensive Version - page 697

17.26

If you serialize two objects of the same type, will they take the same amount of
space? If not, give an example.

17.27

Is it true that any instance of java.io.Serializable can be successfully serial-
ized? Are the static variables in an object serialized? How do you mark an instance
variable not to be serialized?

17.28

Can you write an array to an ObjectOutputStream ?

17.29

Is it true that DataInputStream / DataOutputStream can always be replaced by
ObjectInputStream / ObjectOutputStream ?

17.30

What will happen when you attempt to run the following code?

import java.io.*;

public class Test {
public static void main(String[] args) throws IOException {
try ( ObjectOutputStream output =
new ObjectOutputStream( new FileOutputStream( "object.dat" )); ) {
output.writeObject( new A());
}
}
}

class A implements Serializable {
B b = new B();
}

class B {
}

17.7 Random-Access Files
Java provides the RandomAccessFile class to allow data to be read from and
written to at any locations in the file.

Key
Point

All of the streams you have used so far are known as read-only or write-only streams. These
streams are called sequential streams . A file that is opened using a sequential stream is called
a sequential-access file . The contents of a sequential-access file cannot be updated. However,
it is often necessary to modify files. Java provides the RandomAccessFile class to allow
data to be read from and written to at any locations in a file. A file that is opened using the
RandomAccessFile class is known as a random-access file .
The RandomAccessFile class implements the DataInput and DataOutput interfaces,
as shown in Figure 17.18. The DataInput interface (see Figure 17.9) defines the methods
for reading primitive-type values and strings (e.g., readInt , readDouble , readChar ,
readBoolean , readUTF ) and the DataOutput interface (see Figure 17.10) defines the
methods for writing primitive-type values and strings (e.g., writeInt , writeDouble ,
writeChar , writeBoolean , writeUTF ).
When creating a RandomAccessFile , you can specify one of two modes: r or rw . Mode
r means that the stream is read-only, and mode rw indicates that the stream allows both read
and write. For example, the following statement creates a new stream, raf , that allows the
program to read from and write to the file test.dat :

read-only
write-only
sequential-access file

random-access file

RandomAccessFile raf = new RandomAccessFile( "test.dat" , "rw" );

If test.dat already exists, raf is created to access it; if test.dat does not exist, a new file named
test.dat is created, and raf is created to access the new file. The method raf.length()
returns the number of bytes in test.dat at any given time. If you append new data into the file,
raf.length() increases.