Objects

1
Objects

• Run-time Structure and Organization

2
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.

3
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.

4
Object Identity

• Two objects with different identities may have
  identical fields.
• (Cf. Java vs equal)
• (Cf. Schemes eq vs equal)
• (Cf. Adas limited private type)
• (Cf. Eiffels vs equal vs deep_equal)
• Conversely, the fields of a certain object may
  change during the execution but this does not
  affect the objects identity.
• Objects with state

5
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).

6
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.

7
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.

8
C Example

• 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)

9
Entity, Reference, Object

• target.feature(args)
• If target null, then an exception will be
  triggered at run-time.

10
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 (/)

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.

12
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 class Storable
• Require context to determine the type of the
  retrieved object.

13
Composite Objects Expanded Types

• Eiffel 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.

14
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.