ObjectsFields : Structural AspectReferencesObject IdentityEffect of Basic constructorWhy create objects explicitly?Overloading ConstructorsC++ ExampleEntity, Reference, ObjectOperations on ReferencesCloning and CopyingPersistenceComposite Objects : Expanded TypesMotivation for expanded typesceg860(Prasad) L8Obj 1ObjectsRun-time Structure and Organizationceg860(Prasad) L8Obj 2FieldsFields : Structural Aspect•Basic Types (int, real, bool, char, etc.)–E.g., class Point { real x, y; }•Sub-objects–E.g, class Person { String name; …} class Car { Person owner; …}•Wastes memory space.•Fails to express sharing of objects : Update consistency problem.ceg860(Prasad) L8Obj 3References •A referencereference is a run-time value which is either null or attachedattached.•If attachedattached, it identifies a single object.–The concrete representation of a reference may include an address (pointer), type info, etc.•Every object has a unique identityunique identity, independent of its value as defined by its fields.–Object reference can serve as object identity.ceg860(Prasad) L8Obj 4Object Identity•Two objects with different identities may have identical fields.•(Cf. Java == vs equal)•(Cf. Scheme’s eq vs equal)•(Cf. Ada’s limited private type)•(Cf. Eiffel’s = vs equal vs deep_equal)•Conversely, the fields of a certain object may change during the execution; but this does not affect the object’s identity.–Objects with stateceg860(Prasad) L8Obj 5Effect of Basic constructor Cl var;Cl var = new Cl();•Create a new instance of Cl called nCl.•Initialize each field of nCl using standard default values.•Int : 0, Class : null, bool : false, etc•Attach ncl to var (a reference).ceg860(Prasad) L8Obj 6Why create objects explicitly?•Implicit creation highly inefficient (impossible for recursive structures). –Self-reference implies non-termination.•Realistic modeling can require null reference or values of two fields to be attached to the same object (sharing).–Representing Relations and Functions.ceg860(Prasad) L8Obj 7Overloading Constructors•To override language-defined defaults. •Java/C++Java/C++ : Signature-based resolution.•EiffelEiffel: Named constructors.•Ensure object satisfies consistency constraints imposed by ADT spec.•E.g., Age = Year of Birth Current Year•Support for programmer defined initializations.•E.g., Label, Color, Location, etc of a Button.ceg860(Prasad) L8Obj 8C++ Exampleclass Complex {public: Complex(double); Complex();private: double re, im;}Complex pi = 3.14;Complex e (2.71);Complex:: Complex(double r){ re = r; im = 0;}Complex:: Complex(double r): re(r), im(0) {}ceg860(Prasad) L8Obj 9EntityReferenceObject•target.feature(args)–If target = null, then an exceptionexception will be triggered at run-time.Entity, Reference, Objectceg860(Prasad) L8Obj 10Operations on References•Reference Assignment ( Cl x = y; )–Sharing of objects (Reattachment) : Entities x and y now refer to the same object.•Dynamic aliasing.–Garbage Collection (Recycling) : If the object, referred to by x initially, becomes unattached (“unreachable”) now, it may be recycled.•Reference Comparison –Equal (==) and Not Equal (=/=)ceg860(Prasad) L8Obj 11 Cloning and Copying•x.clone(y) •x is attached to an object (say, OX) that is a duplicate of the object attached to y (say, OY).•The corresponding fields of OX and OY have identical values.•Shallow copy : clone is not recursive.–Deep cloning duplicates a structure recursively without introducing any sharing of references. •x.copy(y)•Both x and y must be non-null.ceg860(Prasad) L8Obj 12Persistence•Motivation: Communication with other systems –Read/Write objects from/to files, databases, and other devices.•In practice:–Require a mechanism to store and retrieve objects containing references.•Java 1.1 Serialization and ObjectStreams •Eiffel: class Storable–Require context to determine the type of the retrieved object.ceg860(Prasad) L8Obj 13Composite Objects : Expanded TypesEiffel: expanded Cl x; vs Cl y; C++: Cl x; vs Cl* y;•The value of entity x is an instance of Cl.•Available in C++ and Ada-95.•Not available in Java.•The value of entity y is a reference to an instance of Cl.•Available in Java, C++, and Ada-95.ceg860(Prasad) L8Obj 14Motivation for expanded types•Enhanced modeling power.Class WorkStation { k: expanded KeyBoard; c: expanded CPU; m: expanded Monitor; n: Network;}–Every instance of Workstation has (contains) an instance of CPU. (Not shared.)•Can improve (space and time) efficiency.–Cyclic dependency