A plane disappears from the air space This can occur for a number of reasons: the plane may in Java

Create QR Code in Java A plane disappears from the air space This can occur for a number of reasons: the plane may
A plane disappears from the air space This can occur for a number of reasons: the plane may
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have left the air space during its onward journey, it could have landed or, very rarely, it could have crashed This results in the disappearance of the plane object
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The plane moves its position in the sky This is normally monitored by radar equipment and it
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results in the x, y and z coordinates of the plane being updated
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Data about a plane s position is sent to a controller on the ground The object world remains the
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same; the only thing that happens is that the x, y and z co-ordinates of a plane are sent to the outside world The way in which the object world within the shaded area is altered and accessed is achieved via a mechanism known as a message An example best explains what a message looks like The line below shows a Java message being set to the aeroplane AF565; the effect of the message is to update the position of the plane to which the message is sent At this stage don t worry about how such messages are programmed in Java; the important thing you should be concentrating on at this stage in the book is the format of messages: AF565newPos(12, 444, 22); The line consist of two components The first component is an object known as the receiver object In the line above, this is the plane identified by AF565 The second component is the message The message above consists of four components The first component is known as the selector, which identifies the message that is to be sent The remaining three components are known as the arguments of the message Thus, in the example above, the receiver object AF565 receives a message identified by the selector newPos and arguments 12, 444, 22 which are associated with the x position, y position and z position of the plane Do not worry about the semicolon that is at the end of each statement In 4 in which we outline some Java programming constructs we describe the use of semicolons As we have implied above, messages are generated by events which occur in the outside world The message example above might correspond to the event of a plane moving and its new position being detected by a radar Messages either update objects, update objects and return with some value or just return with some value without updating any objects Another example of a message associated
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with the air traffic control system is: LH123getxPos(); This message is sent to the receiver object LH123 and returns with the x position of the plane Here the selector is not associated with any parameters so the brackets following it are empty This message is differentiated from the previous one that we examined in one way: it does not have any arguments A message in Java can consist of an arbitrary number of arguments ranging from zero up to some arbitrary number imposed by the Java interpreter you use Another example of a message is: AF555getyPos(); This sends the message getyPos to the receiver object AF555 and returns with the y position of the plane Another example of a message associated with the simple air traffic control system is: BA126removePlane(); This removes a plane from the air space This message would normally be sent when a plane flies outside the air space covered by the system A final example of a message is: BA133landPlane(); This would result in the receiver object (BA133) landing and disappearing from the world of plane objects It is worth pointing out that the messages which we have discussed are all associated with some Java programming code which carries out the processing associated with the message However, at this point you should not worry too much about how this code is defined You will find full details in 5 222 A more complicated air traffic control system The previous example was used to describe some of the core concepts of object-oriented technology This example provides some reinforcement of these concepts and also introduces some new properties of messages The example involves an air traffic control system in which the planes are held in a number of queues Each queue will have a collection of planes awaiting landing Normally a plane lands by being transferred from the front of one queue to the end of another queue below it The system also keeps track of the planes that have landed A snapshot of the system is shown in Figure 22 This shows that the air space has been divided into three queues with the topmost queue containing planes which have the longest time to wait before landing The instance also shows a collection of planes which have landed The important point to make about this application is that not only does it contain simple objects such as planes, but it also contains objects which themselves contain objects For example, the collection of landed planes is a set of plane objects and the queues of planes awaiting landing are also objects which in turn contain other objects (planes) Since this is a more complicated system than that described in the previous subsection it will have more messages associated with it An example of a message is shown below: queuesmoveDown(1); This takes the first plane in queue 1 and places it at the end of the queue below it (queue 2) The receiver object in this case is queues This illustrates another important point about objects The receiver object for this message is not a simple object such as a plane but a collection of objects
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