Java IO.pdf - Nguyen Dang Binh

Java I/O
serial port events: when the runtime detects a change in state at a monitored parallel port, it
fires a parallel port event to the registered parallel port listener. A parallel port event signals
some sort of activity on the parallel port, either an error or an empty output buffer. Parallel
port events are represented by instances of the javax.comm.ParallelPortEvent class, a
subclass of java.util.EventObject:
public class ParallelPortEvent extends EventObject
The ParallelPortEvent class has a single public constructor, but you shouldn't use it.
Instead, the runtime creates and fires parallel port events when it wants to indicate a change
on one of the standard parallel port pins.
public ParallelPortEvent(ParallelPort src, int type, boolean oldValue,
boolean newValue)
The javax.comm.ParallelPortEvent class declares these three public methods:
public int getEventType()
public boolean getNewValue()
public boolean getOldValue()
The getEventType() method returns a named constant from the ParallelPortEvent class
that specifies what caused the event to be fired. There are two possibilities: an error and an
empty output buffer. Each parallel port event has an eventType field; its value should be one
of these mnemonic constants:
ParallelPortEvent.PAR_EV_ERROR // An error occurred on the port.
ParallelPortEvent.PAR_EV_BUFFER // The output buffer is empty.
These represent a change from one state to another, from on to off or off to on. Therefore,
there are also getNewValue() and getOldValue() methods to tell you the state of the pin
before and after the event:
public boolean getNewValue()
public boolean getOldValue()
17.5.5 Parallel Port Event Listeners
There are three steps to respond to parallel port events:
1. Implement the ParallelPortEventListener interface.
2. Register your ParallelPortEventListener object with the ParallelPort object
representing the parallel port you want to monitor.
3. Tell the parallel port the types of events you want to be notified of.
Steps 1 and 2 should be familiar from JavaBeans and the Java 1.1 AWT. Step 3 is used to
avoid getting notifications of events you're not interested in and is similar to the same step for
serial port events.
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