Java How to Program Fourth Edition - DCC

354 Arrays Chapter 7
The Elevator sends the resetButton message (message 1) to the Elevator-
Button to reset the ElevatorButton. The Elevator sends the ringBell message
(message 2) to the Bell, then opens the ElevatorDoor by passing the openDoor
message (message 3). The ElevatorDoor then opens the FloorDoor by sending an
openDoor message (message 3.1 at the top of the diagram) to that FloorDoor. The
FloorDoor informs the waitingPassenger that the FloorDoor has opened (message
3.1.1), and the waitingPassenger enters the Elevator (message
3.1.1.1). The ElevatorDoor then informs the ridingPassenger that the ElevatorDoor
has opened (message 3.2), so that the ridingPassenger may exit the
Elevator (message 3.2.1). Lastly, the Elevator informs the ElevatorShaft of
the arrival (message 4), so that the ElevatorShaft can reset the FloorButton (message
4.1) and turn on the Light (message 4.2).
Unfortunately, this design creates a problem. According to the diagram, the waitingPassenger
enters the Elevator (message 3.1.1.1) before the ridingPassenger
(message 3.2.1) exits. In “Thinking About Objects” Section 15.12, we apply
multithreading, synchronization and active classes to our collaboration diagram, to force
the waitingPassenger to wait for the ridingPassenger to exit the Elevator.
Before we correct this problem, we modify this diagram to indicate more accurately the
message passing in Section 10.22 when we discuss event handling.
SUMMARY
• Java stores lists of values in arrays. An array is a contiguous group of related memory locations.
These locations are related by the fact that they all have the same name and the same type. To refer
to a particular location or element within the array, we specify the name of the array and the subscript
(or index or position number) of the element.
• Each array a length member that is set to the number of elements in the array at the time the
program creates the array object.
• A subscript may be an integer or an integer expression. If a program uses an expression as a subscript,
the program evaluates the expression to determine the particular element of the array.
• Java arrays always begin with element 0; thus, it is important to note the difference when referring
to the “seventh element of the array” as opposed to “array element seven.” The seventh element
has a subscript of 6, while array element seven has a subscript of 7 (actually the eighth element
of the array).
• Arrays occupy space in memory and are considered to be objects. Operator new must be used to
reserve space for an array. For example, the following creates an array of 100 int values:
int b[] = new int[ 100 ];
• When declaring an array, the type of the array and the square brackets can be combined at the beginning
of the declaration to indicate that all identifiers in the declaration represent arrays, as in
double[] array1, array2;
• The elements of an array can be initialized with initializer lists in a declaration and by input.
• Java prevents referencing elements beyond the bounds of an array. If this occurs during program
execution, an ArrayIndexOutOfBoundsException occurs.
• Constant variables must be initialized with a constant expression before they are used and cannot
be modified thereafter.
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