Networking: The Generic Connectionn Framework - Java 2 Micro Edition Application Development - page 142

for reading data from the connection: openInputStream() returns an InputStream , and
openDataInputStream() returns a DataInputStream . Analogously, the
OutputConnection provides two methods, openOutputStream() and
openDataOutputStream() , returning the corresponding streams for writing data.

Probably the most important connection interface is the StreamConnection , combining the
InputConnection and OutputConnection in one interface. It is used directly or as base
interface for most connection types. It is the base interface for the ContentConnection , which
adds functionality for accessing the meta-information available for HTTP connections. For datagram
connections, the GCF provides the DatagramConnection interface, representing an endpoint for
datagrams. Finally, the GCF supports the interface StreamConnectionNotifier that
corresponds to J2SE ServerSocket s. This interface consists of only one acceptAndOpen()
method. This method returns a StreamConnection when a client has successfully established a
connection.

For filesystem and serial port connectivity, the PDAP provides the FileConnection and
CommConnection interfaces. These connections are optional to a PDAP implementation depending
on the underlying hardware and operating system.

We have already shown the simple open() method of the Connector class, taking a URI string as
input and returning an instance of a class implementing the Connection interface. However, this is
not the only open() method available; the Connector class provides two additional open()
methods. The second variant of open() takes an additional mode integer constant that determines
whether the connection is read-only, write-only, or a read-write connection. The corresponding
constants are listed in Table 6.2 . The last variant of the open() method takes a third parameter,
indicating that the protocol may throw an InterruptedIOException in the case of a device-
specific timeout. Without using additional parameters, the behavior of the open() method is to open a
connection in READ_WRITE mode, without throwing timeout exceptions.

The optional parameters depend on the protocol to which they are passed. For instance, the connection
mode WRITE might result in an IllegalArgumentException if it is used to open a protocol for
accessing a bar code scanner allowing read-only access.

Table 6.2. Possi b le Access Modes for Opening a Connection Using the GCF
Mode Description

READ Read-only connections

READ_WRITE Connections using read and write access

WRITE Write-only connections

We already mentioned that a concrete instance of a class implementing the Connection interface is
returned when a connection is established successfully. In order to access the functionality of the
concrete type of connection, the returned Connection needs to be typecast to the corresponding sub-
interface. For example, when opening an HTTP connection, it usually makes sense to cast the returned
object to the more specific HttpConnection interface:

HttpConnection httpConnection =
(HttpConnection) Connector.open ("http://java.sun.com");

In the following section, we will take a closer look at the different connection types that are listed in the
CLDC specification.

Connection Types