Persistent Storage II: File Connection and PIM API - Beginning J2ME: From Novice to Professional - page 117

Accessing File Systems
You perform read and write operations through the input and output stream classes from the
java.io package. These streams are a standard part of the CLDC, as Chapter 4 shows.
To obtain a stream to a file in the file system, however, you first need to have an opened file
connection. For this, you will need to use the File Connection optional API.
The main interface of this API is the FileConnection interface. You obtain an instance of a
FileConnection by using a method of the javax.microedition.io.Connector class. This class is
part of the Generic Connection Framework in CLDC. GCF is used primarily to make connections
through a network or a communications port (see Chapter 10).

Obtaining FileConnections from GCF
At this time, the important thing you need to know about GCF is that the following static method
of the Connector class can return a FileConnection instance:

public static Connection open(String URL, int mode) throws IOException,
IllegalArgumentException,
ConnectionNotFoundException, SecurityException

The URL to obtain a file connection will always start with “file:///”, indicating that a file is
accessed on the local host. The full syntax for the URL is actually “file://<hostname>/”, but
<hostname> is omitted when local. The mode indicates the type of access you want for the file
connection. You can use Connector.READ , Connector.WRITE , or Connector.READ_WRITE . For example,
you may get an input stream to an existing abc.txt file on an SD card using the following:

FileConnection fc = (FileConnection) Connector.open("file:///SDCard/abc.txt",
Connector.READ);
InputStream is = fc.openInputStream();

You can then use the InputStream for reading the file. The FileConnection interface has
five methods for obtaining a stream:

DataInputStream openDataInputStream()
DataOutputStream openDataOutputStream()
InputStream openInputStream()
OutputStream openOutputStream()
OutputStream openOutputStream(long Offset)

The second variation of openOutputStream() allows you to specify a byte offset to start
writing to the file.

■ Note The top-level directory of a file system is called the root. Each file system on a device may corre-

spond to a separate flash memory card or storage subsystem, and will have a different root name. The name
of the root is arbitrary and totally device dependent. For example, a vendor may choose to name the root of
an inserted SD memory card “SDCard1/”, while the device's own internal flash file system may have a root
name of “InternalFlash/”. You can obtain an enumeration of all the available roots on a device using the
FileSystemRegistry , described later in this chapter.