9: OBJECT LIFETIME in Java

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CHAPTER 9: OBJECT LIFETIME
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expression at (5) is evaluated as (sqSide = (height = 20)) Every object of the class NonStaticInitializers will have the field height set to the value 20 The declaration-before-reading rule is equally applicable to static initializer expressions when static fields are referenced by their simple names Example 94 shows why the order of field initializer expressions can be important The initializer expressions in Example 94 are calls to methods defined in the class Methods are not subject to the same access rules as initializer expressions The call at (2) to the method initMaxGuests() defined at (4) is expected to return the maximum number of guests However, the field occupancyPerRoom at (3) will not have been explicitly initialized; therefore, its default value 0 will be used in the method initMaxGuests(), which will return an incorrect value The program output shows that after object creation, the occupancy per room is correct, but the maximum number of guests is wrong
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Example 94
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Initializer Expression Order and Method Calls
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class Hotel { private int noOfRooms = 12; private int maxNoOfGuests = initMaxGuests(); private int occupancyPerRoom = 2; // (1) // (2) Bug // (3)
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public int initMaxGuests() { // (4) Systemoutprintln("occupancyPerRoom: " + occupancyPerRoom); Systemoutprintln("maxNoOfGuests: " + noOfRooms * occupancyPerRoom); return noOfRooms * occupancyPerRoom; } public int getMaxGuests() { return maxNoOfGuests; } // (5)
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public int getOccupancy() { return occupancyPerRoom; } // (6) } //________________________________________________________________________ public class TestOrder { public static void main(String[] args) { Hotel hotel = new Hotel(); // (7) Systemoutprintln("After object creation: "); Systemoutprintln("occupancyPerRoom: " + hotelgetOccupancy()); // (8) Systemoutprintln("maxNoOfGuests: " + hotelgetMaxGuests()); // (9) } }
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occupancyPerRoom: 0 maxNoOfGuests: 0 After object creation: occupancyPerRoom: 2 maxNoOfGuests: 0
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98: FIELD INITIALIZER EXPRESSIONS
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Exception Handling and Initializer Expressions
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Initializer expressions in named classes and interfaces must not result in any uncaught checked exception (see Section 66, p 243) If any checked exception is thrown during execution of an initializer expression, it must be caught and handled by code called from the initializer expression This restriction does not apply to instance initializer expressions in anonymous classes Example 95 illustrates exception handling for initializer expressions in named classes The static initializer expression at (3) calls the static method createHotelPool() at (4), which catches and handles the checked TooManyHotelsException defined at (2) If the method createHotelPool() were to use the throws clause to specify the checked exception, instead of catching and handling it within a try-catch block, the initializer expression at (3), which called the method, would have to handle the exception However, the initializer expression cannot specify any exception handling, as the compiler would complain The instance initializer expression at (5) calls the method initMaxGuests() at (6), which can throw the unchecked RoomOccupancyTooHighException If thrown, this exception will be caught and handled in the main() method Program output confirms that an unchecked RoomOccupancyTooHighException was thrown during program execution
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Example 95
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Exceptions in Initializer Expressions
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class RoomOccupancyTooHighException extends RuntimeException {} // (1) Unchecked Exception class TooManyHotelsException extends Exception {} // (2) Checked Exception //_____________________________________________________________________________ class Hotel { // Static Members private static int noOfHotels = 12; private static Hotel[] hotelPool = createHotelPool(); // (3) private static Hotel[] createHotelPool() { // (4) try { if (noOfHotels > 10) throw new TooManyHotelsException(); } catch (TooManyHotelsException e) { noOfHotels = 10; Systemoutprintln("No of hotels adjusted to " + noOfHotels); } return new Hotel[noOfHotels]; } // Instance Members private int noOfRooms = 215; private int occupancyPerRoom = 5; private int maxNoOfGuests = initMaxGuests(); // (5)
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410 private int initMaxGuests() { if (occupancyPerRoom > 4) throw new RoomOccupancyTooHighException(); return noOfRooms * occupancyPerRoom; }
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CHAPTER 9: OBJECT LIFETIME // (6)
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} //_____________________________________________________________________________ public class ExceptionsInInitializers { public static void main(String[] args) { try { new Hotel(); } catch (RoomOccupancyTooHighException exception) { exceptionprintStackTrace(); } } }
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No of hotels adjusted to 10 RoomOccupancyTooHighException at HotelinitMaxGuests(ExceptionsInInitializersjava:28) at Hotel<init>(ExceptionsInInitializersjava:24) at ExceptionsInInitializersmain(ExceptionsInInitializersjava:35)
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