Download
1 / 14
140 likes | 261 Views
Objects. Run-time Structure and Organization. Fields : 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.
E N D
Objects Run-time Structure and Organization L8Obj
Fields : 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. L8Obj
References • A reference is a run-time value which is either null or attached. • If attached, it identifies a single object. • The concrete representation of a reference may include an address (pointer), type info, etc. • Every object has a unique identity, independent of its value as defined by its fields. • Object reference can serve as object identity. L8Obj
Object Identity • Two objects with different identities may have identical fields. • (Cf. Java ==vsequal) • (Cf. Scheme’s eqvsequal) • (Cf. Ada’s limited private type) • (Cf. Eiffel’s =vs equalvsdeep_equal) • Conversely, the fields of a certain object may change during the execution; but this does not affect the object’s identity. • Objects with state L8Obj
Effect 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). L8Obj
Why 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. L8Obj
Overloading Constructors • To override language-defined defaults. • Java/C++ : Signature-based resolution. • Eiffel: 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. L8Obj
class 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) {} C++ Example L8Obj
Reference Entity Object Entity, Reference, Object • target.feature(args) • If target = null, then an exception will be triggered at run-time. L8Obj
Operations 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 (=/=) L8Obj
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. L8Obj
Persistence • 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: classStorable • Require context to determine the type of the retrieved object. L8Obj
Composite Objects : Expanded Types Eiffel:expanded Cl x; vs Cl y; C++: Cl x; vs Cl* y; • The value of entity xis an instance ofCl. • Available in C++ and Ada-95. • Not available in Java. • The value of entityy isa reference to an instance ofCl. • Available in Java, C++, and Ada-95. L8Obj
Motivation 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 prohibited. L8Obj
