Chapter 8

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Chapter 8. Object Lifetime Exam Objectives

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import java.io.*; class WellbehavedClass { // ... void wellbehavedMethod() { File aFile; long[] bigArray = new long[20000]; // ... uses local variables ... // Does cleanup (before starting something extensive) aFile = null; // (1) bigArray = null; // (2) // Start some other extensive activity // ... } // ... } +

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Page 7 of 30 Return from creating list Y X: item3: recycled. X: item2: recycled. X: item1: recycled. Return from creating HeavyItem Return from creating HeavyItem Return from creating HeavyItem Three local item: recycled. Three: recycled. Two local item: recycled. Return from creating HeavyItem One local item: recycled. One: recycled. Return from main().

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Page 9 of 30 static int population; protected int blobId; BasicBlob() { blobId = idCounter++; ++population; } protected void finalize() throws Throwable { --population; super.finalize(); }

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Page 11 of 30 class BasicBlob { /* See Example 8.2. */ } class Blob extends BasicBlob { /* See Example 8.2.*/ } public class MemoryCheck { public static void main(String[] args) { // (6) int blobsRequired, blobSize; try { blobsRequired = Integer.parseInt(args[0]); blobSize = Integer.parseInt(args[1]); } catch(IndexOutOfBoundsException e) { System.err.println( "Usage: MemoryCheck "); return; } Runtime environment = Runtime.getRuntime(); // System.out.println("Total memory: " + environment.totalMemory());// System.out.println("Free memory before blob creation: " + environment.freeMemory()); // for (int i=0; i
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Total memory: 2031616 Free memory before blob creation: 1773192 0: Hello 1: Hello 2: Hello 1: Bye 2: Bye 3: Hello 0: Bye 3: Bye 4: Hello Free memory after blob creation: 818760 4: Bye Free memory after requesting GC: 1619656 0 blobs alive 6

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public int initMaxGuests() { // (4) System.out.println("occupancyPerRoom: " + occupancyPerRoom); System.out.println("maxNoOfGuests: " + noOfRooms * occupancyPerRoom); return noOfRooms * occupancyPerRoom; } public int getMaxGuests() { return maxNoOfGuests; }

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public int getOccupancy() { return occupancyPerRoom; }

// (6)

}

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public class TestOrder { public static void main(String[] args) { Hotel hotel = new Hotel(); // (7) System.out.println("After object creation: "); System.out.println("occupancyPerRoom: " + hotel.getOccupancy()); // (8) System.out.println("maxNoOfGuests: " + hotel.getMaxGuests()); // (9) } } <

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class RoomOccupancyTooHighException extends RuntimeException {} class TooManyHotelsException extends Exception {}

// (1) Unchecked Exception // (2) Checked Exception

class Hotel { // Static Members private static int noOfHotels = 12; private static Hotel[] hotelPool = createHotelPool(); private static Hotel[] createHotelPool() { try { if (noOfHotels > 10) throw new TooManyHotelsException();

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// (3) // (4)

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Page 18 of 30 } catch (TooManyHotelsException e) { noOfHotels = 10; System.out.println("No. of hotels adjusted to " + noOfHotels); } return new Hotel[noOfHotels]; } // Instance private int private int private int

Members noOfRooms = 215; occupancyPerRoom = 5; maxNoOfGuests = initMaxGuests();

// (5)

private int initMaxGuests() { // (6) if (occupancyPerRoom > 4) throw new RoomOccupancyTooHighException(); return noOfRooms * occupancyPerRoom; } } public class ExceptionsInInitializers { public static void main(String[] args) { try { new Hotel();} catch (RoomOccupancyTooHighException exception) { exception.printStackTrace(); } } } <

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No. of hotels adjusted to 10 RoomOccupancyTooHighException at Hotel.initMaxGuests(ExceptionsInInitializers.java:29) at Hotel.(ExceptionsInInitializers.java:25) at ExceptionsInInitializers.main(ExceptionsInInitializers.java:36)

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static int sf1 = sf2 = 30; static int sf2; int if1 = 5;

// (7) Static field. Assignment to sf2 allowed // (8) Static field // (9) Non-static field

static { // (10) Static initializer block int d = 2 * sf1; // (11) OK. Read operation after declaration int e = sf1 = 50; // (12) } public static void main(String[] args) { System.out.println("sf1: " + StaticForwardReferences.sf1); System.out.println("sf2: " + StaticForwardReferences.sf2); } }

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class BankrupcyException extends RuntimeException {} class TooManyHotelsException extends Exception {}

// (1) Unchecked Exception // (2) Checked Exception

class Hotel { // Static Members private static boolean bankrupt = true; private static int noOfHotels = 11; private static Hotel[] hotelPool; static { try {

// (3) Static block // (4) Handles checked exception

if (noOfHotels > 10) throw new TooManyHotelsException(); } catch (TooManyHotelsException e) { noOfHotels = 10; System.out.println("No. of hotels adjusted to " + noOfHotels); } hotelPool = new Hotel[noOfHotels]; } static { if (bankrupt) throw new BankrupcyException(); } // ...

// (5) Static block // (6) Throws unchecked exception

} public class ExceptionInStaticInitBlocks { public static void main(String[] args) { new Hotel(); } } <

3

No. of hotels adjusted to 10 Exception in thread "main" java.lang.ExceptionInInitializerError

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Page 21 of 30 at ExceptionInStaticInitBlocks.main(ExceptionInStaticInitBlocks.java:33) Caused by: BankrupcyException at Hotel.(ExceptionInStaticInitBlocks.java:26)

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(1) (2) (3) (4) (5) (6)

Instance initializer block OK. Assignment to nsf1 allowed OK. Static field access in non-static context Not OK. Read operation before declaration OK. Assignment to nsf1 allowed OK. Not accessed by simple name

} int nsf1 = nsf2 = 30; int nsf2; static int sf1 = 5; {

// (7) Non-static field. Assignment to nsf2 allowed // (8) Non-static field // (9) Static field

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}

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public static void main(String[] args) { NonStaticForwardReferences objRef = new NonStaticForwardReferences(); System.out.println("nsf1: " + objRef.nsf1); System.out.println("nsf2: " + objRef.nsf2); } } <

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class RoomOccupancyTooHighException extends Exception {} class BankrupcyException extends RuntimeException {}

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// (1) Checked exception // (2) Unchecked exception

class Hotel { // Instance Members private boolean bankrupt = true; private int noOfRooms = 215; private int occupancyPerRoom = 5; private int maxNoOfGuests; {

// (3) Instance block try { if (occupancyPerRoom > 4) // (4) Handles checked exception throw new RoomOccupancyTooHighException(); } catch (RoomOccupancyTooHighException exception) { System.out.println("ROOM OCCUPANCY TOO HIGH: " + occupancyPerRoom); occupancyPerRoom = 4; } maxNoOfGuests = noOfRooms * occupancyPerRoom;

} {

}

// (5) Instance initializer block if (bankrupt) throw new BankrupcyException(); // ...

// (6) Throws unchecked exception

} public class ExceptionsInInstBlocks { public static void main(String[] args) { new Hotel(); } } <

3

ROOM OCCUPANCY TOO HIGH: 5 Exception in thread "main" BankrupcyException

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Page 24 of 30 at Hotel.(ExceptionsInInstBlocks.java:26) at ExceptionsInInstBlocks.main(ExceptionsInInstBlocks.java:32)

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// (4) System.out.println("Instance initializer block in SubclassB"); value = 2; // (5)

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Page 25 of 30 } int value = initializerExpression();

// (6)

private int initializerExpression() { // (7) System.out.println("Instance initializer expression in SubclassB"); return 1; } } public class ObjectConstruction { public static void main(String[] args) { SubclassB objRef = new SubclassB(); // (8) System.out.println("value: " + objRef.value); } } <

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Page 26 of 30 this.superValue = 911; System.out.println("superValue: " + this.superValue); } } class SubclassB extends SuperclassA { private int value = 800; // (5) SubclassB() { // (6) System.out.println("Constructor in SubclassB"); this.doValue(); System.out.println("superValue: " + this.superValue); } void doValue() { // (7) System.out.println("value: " + this.value); } } public class ObjectInitialization { public static void main(String[] args) { System.out.println("Creating an object of SubclassB."); new SubclassB(); // (8) } } <

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Page 27 of 30

private static int count = 5; final static int STEP = 10; boolean alive; // INSERT STATIC INITIALIZER BLOCK HERE } # static { alive = true; count = 0; } static { STEP = count; } static { count += STEP; } $ static ; static {;} % static { count = 1; } ,

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Page 28 of 30

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public class Initialization { private static String msg(String msg) { System.out.println(msg); return msg; } public Initialization() { m = msg("1"); } { m = msg("2"); } String m = msg("3"); public static void main(String[] args) { Object obj = new Initialization(); } } # #

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Page 29 of 30

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public class Initialization { private static String msg(String msg) { System.out.println(msg); return msg; } static String m = msg("1"); { m = msg("2"); } static { m = msg("3"); } public static void main(String[] args) { Object obj = new Initialization(); } } # #

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