Object-Oriented Techniques - Java - page 306

Items that can't be switched must nonetheless be in the database, for various purposes (audit-
ing, insurance, etc.). In the method that turns things off, the code is careful to check whether
each object in the database is an instance of the PowerSwitchable interface. If so, the object
is casted to PowerSwitchable so that its powerDown() method can be called. If not, the ob-
ject is skipped, thus preventing any possibility of turning out the emergency lights or shutting
off a machine that is busy running Seti@Home , downloading a big MP3 playlist, or perform-
ing system backups. The following code shows this set of classes in action:

public public class class BuildingManagement BuildingManagement {

Asset things [] = new new Asset [ 24 ];

int int numItems = 0 ;

/** Scenario: goodNight() is called from a timer Thread at 2200, or when
* we get the "shutdown" command from the security guard.
*/
public

public void void goodNight () {

for for ( int int i = 0 ; i < things . length ; i ++)

iif ( things [ i ] instanceof instanceof PowerSwitchable )

(( PowerSwitchable ) things [ i ]). powerDown ();

}

// goodMorning() would be the same, but call each one's powerUp().

/** Add a Asset to this building */
public

public void void add ( Asset thing ) {

System . out . println ( "Adding " + thing );
things [ numItems ++] = thing ;

}

/** The main program */
public

public static void main ( String [] av ) {

BuildingManagement b1 = new

static void

new BuildingManagement ();

b1 . add ( new new RoomLights ( 101 )); // control lights in room 101

b1 . add ( new new EmergencyLight ( 101 )); // and emerg. lights.

// add the computer on desk#4 in room 101
b1 . add ( new new ComputerCPU ( 10104 ));

// and its monitor
b1 . add ( new

new ComputerMonitor ( 10104 ));

// time passes, and the sun sets...
b1 . goodNight ();