The Line interface represents a mono or multi-channel audio feed. A line is an element of the digital audio "pipeline," such as a mixer, an input or output port, or a data path into or out of a mixer.

A line can have controls, such as gain, pan, and reverb. The controls themselves are instances of classes that extend the base Control class. The Line interface provides two accessor methods for obtaining the line's controls: getControls returns the entire set, and getControl returns a single control of specified type.

Lines exist in various states at different times. When a line opens, it reserves system resources for itself, and when it closes, these resources are freed for other objects or applications. The isOpen() method lets you discover whether a line is open or closed. An open line need not be processing data, however. Such processing is typically initiated by subinterface methods such as SourceDataLine.write and TargetDataLine.read.

You can register an object to receive notifications whenever the line's state changes. The object must implement the LineListener interface, which consists of the single method update. This method will be invoked when a line opens and closes (and, if it's a DataLine, when it starts and stops).

An object can be registered to listen to multiple lines. The event it receives in its update method will specify which line created the event, what type of event it was (OPEN, CLOSE, START, or STOP), and how many sample frames the line had processed at the time the event occurred.

Certain line operations, such as open and close, can generate security exceptions if invoked by unprivileged code when the line is a shared audio resource.

Opens the line, indicating that it should acquire any required system resources and become operational. If this operation succeeds, the line is marked as open, and an OPEN event is dispatched to the line's listeners.

Note that some lines, once closed, cannot be reopened. Attempts to reopen such a line will always result in an LineUnavailableException.

Some types of lines have configurable properties that may affect resource allocation. For example, a DataLine must be opened with a particular format and buffer size. Such lines should provide a mechanism for configuring these properties, such as an additional open method or methods which allow an application to specify the desired settings.

This method takes no arguments, and opens the line with the current settings. For SourceDataLine and TargetDataLine objects, this means that the line is opened with default settings. For a Clip, however, the buffer size is determined when data is loaded. Since this method does not allow the application to specify any data to load, an IllegalArgumentException is thrown. Therefore, you should instead use one of the open methods provided in the Clip interface to load data into the Clip.

For DataLine's, if the DataLine.Info object which was used to retrieve the line, specifies at least one fully qualified audio format, the last one will be used as the default format.