Java Reference
In-Depth Information
File Input and Output
The new file I/O capabilities introduced in Java 1.4 provide the potential for substantially improved
performance at the cost of some slight increase in complexity over the I/O facilities of previous releases.
There are three kinds of objects involved in reading and writing files using the new I/O capability:
A
file stream object
that encapsulates the physical file that you are working with. We saw how
to create
FileOutputStream
objects at the end of the previous chapter and you use these
for files to which you want to write. In the next chapter we will be using
FileInputStream
objects for files that we want to read.
❑
One or more
buffer objects
in which you put the data to be written to a file, or from which
you get the data that has been read. We will learn about buffer objects in the next section.
❑
A
channel object
that provides the connection to the file and does the reading or writing of the
data using one or more buffer objects. We will see how to obtain a channel from a file stream
object later in this chapter.
❑
The way in which these types of object work together is illustrated in the diagram below:
File Stream
Object
The channel transfers
data between the
buffers and the file
stream.
Channel
Object
Buffer
Objects
The process for writing and reading files is basically quite simple. To write to a file, you load data into one or
more buffers that you have created, and then call a method for the channel object to write the data to the file
that is encapsulated by the file stream. To read from a file, you call a method for the channel object to read
data from the file into one or more buffers, and then retrieve the data from the buffers.
There are four classes defined in the
java.io
package that we will be using when we are working with
files. As we have said, the
FileInputStream
and
FileOutputStream
classes define objects that
provide access to a file for reading or writing respectively. You use an object of type
RandomAccessFile
when you want to access a file randomly, or when you want to use a single
channel to both read from and write to a file. We will be exploring this, along with the
FileInputStream
class in the next chapter. You will see from the SDK documentation for the
FileInputStream
,
FileOutputStream
, and
RandomAccessFile
classes that they each provide
methods for I/O operations. However, we will ignore these, as we will be using the services of a file
channel to perform operations with objects of these stream classes. The only method from these classes
that we will be using is the
close()
method, that closes the file and any associated channel.
