Copyright © Philip M. Parker, INSEAD. Terms of Use.

CLASSESES

Specialty Definition: Class (computer science)

(From Wikipedia, the free Encyclopedia)

In object-oriented programming, a class consists of encapsulated instance variables and subprograms. Classes describe the rules by which objects behave; those objects, described by a particular class, are known as "instances" of said class. A class specifies the data which each instance contains; as well as the methods (functions) which the object can perform; such methods are sometimes described as "behavior".

Classes are sometimes described as "blueprints"; instances, of a class, will have certain aspects in common. One might describe a "class of animals" (dogs), or a "class of tangible objects" (computers). One of the benefits, of programming with classes, is that all instances of a particular class will follow the defined behaviour of said class. For example: if humans are a class; then each person is an instance of an object of the human class. Each person is generally alike; but varies in such properties as "height" and "weight". The class would list such instance variables; and also define, via subprograms, the actions which humans can perform: "marriage", sleeping, "tonsillectomy", etc.

Subclasses and Superclasses

Classes are often related in some way. The most popular of these relations is inheritance, which involves subclasses and superclasses, also known respectively as child classes (or derived classes) and parent classes (or base classes). If [car] was a class, then [Jaguar] and [Porsche] might be two sub-classes. If [Button] is a subclass of [Control], then all buttons are controls.

Some programming languages (for example C++) allow multiple inheritance - they allow a child class to have more than one parent class. This technique has been criticized by some for its unnecessary complexity and being hard to implement efficiently, though some projects have undoubtedly benefited from its use. Java, for example has no multiple inheritance, its designers feeling that this would be more trouble than it was worth.

Sub- and superclasses are considered to exist within a hierarchy.

Reasons for Implementing Classes

Classes, when used properly, can accelerate development by reducing redundant code entry, testing and bug fixing. If a class has been thoroughly tested and is known to be a solid work, it stands to reason that implementing that class or extending it will reduce if not eliminate the possibility of bugs propagating into the code. In the case of extension new code is being added so it also requires the same level of testing before it can be considered solid.

Another reason for using classes is to simplify the relationships of interrelated data. A car, for instance, has many components. These components in turn are based on other smaller components. The car has an engine and it has gears for example. To try to capture the essence of a car by describing its 1000s of individual parts would take much time and effort. Instead, by encapsulating each part by its purpose or its placement in the car we can simplify the coding of a car. We would create a gear class and an engine class and finally a car class. Each would embed information about parts internal to itself and would manage this implicitly for us. This abstraction allows developers to concentrate on the task at hand and not the internals of every problem.

Types of classes

An abstract class is one that is designed only as a parent class and from which child classes may be derived, and which is not itself suitable for instantiation. Abstract classes are often used to represent abstract concepts or entities. The incomplete features of the abstract class are then shared by a group of sibling sub-classes which add different variations of the missing pieces.

Abstract classes are superclasses which contain abstract methods and are defined such that subclasses are to extend them by implementing the methodss. The behaviors defined by such a class are "generic" and much of the class will be undefined and unimplemented. Before a class derived from an abstract class can be instantiated, it must implement particular methods for all the abstract methods of its parent classes.

Object-Based Programming

Some languages have objects, but no classes; In such "object-based languages", objects are not restricted to class structure.

C++ Examples

The first example shows how to define a C++ class; the example has no date, and performs no functions. It only contains the comment, "this is a class". The second example is of a somewhat more complex class definition.

Example 1

class example {

 // this is a class

};

Example 2

  1. include 

using std::string;

class InetMessage
{

 string m_subject, m_to, m_from;


public:

 InetMessage (const string& subject,

       const string& to,
       const string& from);

 string subject () const;
 string to () const;
 string from () const;

};

See also

class, hierarchy

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Class (computer science)."

Top     


Class (philosophy)

(From Wikipedia, the free Encyclopedia)

Classes vs. types

Philosophers sometimes distinguish classes from types and kinds. We can talk about the class of human beings, just as we can talk about the type (or natural kind), human being, or humanity. How, then, might classes differ from types? One might well think they are not actually different categories of being, but typically, while both are treated as abstract objects, classes are not usually treated as universals, whereas types usually are. Whether natural kinds ought to be considered universals is vexed; see natural kind.

There is, in any case, a difference in how we talk about types and kinds versus how we talk about classes. We say that Socrates is a token of a type, or an instance of the natural kind, human being. But notice that we say instead that Socrates is a member of the class of human beings. We would not say that Socrates is a "member" of the type or kind, human beings. He is a token (instance) of the type (kind). So the linguistic difference is: types (or kinds) have tokens (or instances); classes, on the other hand, have members.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Class (philosophy)."

Top     


Class (set theory)

(From Wikipedia, the free Encyclopedia)

In set theory and its applications throughout mathematics, a class is a collection of sets (or sometimes other mathematical objects) that can be unambiguously defined by a property that all its members share. Some classes are sets, for instance the class of all integers that are even, but others are not, for instance the class of all ordinal numbers or the class of all sets. Classes that are not sets are called proper classes.

A proper class cannot be an element of a set or a class and is not subject to the Zermelo-Fraenkel axioms of set theory; thereby a number of paradoxes of naive set theory are avoided. Instead, these paradoxes become proofss that a certain class is proper. For example, Russell's paradox becomes a proof that the class of all sets is proper, and the Burali-Forti paradox becomes a proof that the class of all ordinal numbers is proper.

The standard Zermelo-Fraenkel set theory axioms do not talk about classes; classes exist only in the metalanguage as equivalence classes of logical formulas. Another approach is taken by the von Neumann-Bernays-Gödel axioms; classes are the basic objects in this theory, and a set is then defined to be a class that is an element of some other class. The proper classes, then, are those classes that are not elements of any other class.

Several objects in mathematics are too big for sets and need to be described with classes, for instance large categories or the class-field of surreal numbers.

The word "class" is sometimes used synonymously with "set", most notably in the term "equivalence class". This usage dates from a historical period where classes and sets were not distinguished as they are in modern terminology. Many discussions of "classes" in the 19th century and earlier are really referring to sets, or perhaps to a more ambiguous concept.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Class (set theory)."

Top     


Scientific classification

(From Wikipedia, the free Encyclopedia)

Scientific classification refers to how biologists group and categorize extinct and living species of organisms. Modern classification has its roots in the system of Carl Linnaeus, who grouped species according to shared physical characteristics. These groupings have been revised since Linnaeus to improve consistency with the Darwinian principle of common descent. Genomic DNA analysis has driven many recent revisions and is likely to continue to do so. Scientific classification belongs to the science of taxonomy or biological systematics.

Early Systems

The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle.

The next major advance in developing scientific classification was by the Swiss professor, Conrad Gessner (1516 - 1565). Gessner's work was a critical compilation of life known at the time.

The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the 16th century and the beginning of the 17th careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bears a debt to the research of medical anatomists, such as Fabricius (1537 - 1619), Severinus (1580 - 1656), William Harvey (1578 - 1657), and Tyson (1649 - 1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628 - 1694), Jan Swammerdam (1637 - 1680), and Robert Hooke (1635 - 1702).

John Ray (1627 - 1705) was an English naturalist who published important works on plants, animals, and natural theology. His classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.

Linnaean taxonomy

Two years after John Ray's death Carolus Linnaeus (1707 - 1778) was born. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735). He is best known for his introduction of a method of modern classification; he created systematic zoology and botany in their present form. Linnaeus adopted Ray's conception of species, but he made the concept a practical reality by insisting that every species must have a unique Latin binomen, that is, a double name - the first half to be the name of the genus common to several species, and the second half to be a single word, which is called the specific epithet. This convention is now referred to as binomial nomenclature, and the name formed from the two parts is known as the scientific name of a species.

Before Linnaeus, long many-worded names had been used, sometimes with one additional adjective, sometimes with another, so that no true names were fixed and accepted. Linnaeus' system made it easy to identify unambiguously any given species of plant or animal. He proceeded further to introduce into his system a series of groups: genus, order, class.

The Linnaeus System works by placing each organism into a layered hierarchy of groups. Each group at a given layer is composed of a set of groups from the layer directly below. Simply knowing the two-part scientific name makes it possible to determine the other six layers.

The groupings (taxa) of taxonomy from most general to most specific are:

Several acronym mnemonics have been made for these, for instance King Phillip called out for good soup, or Kings Play Chess On Funny Green Squares.

Intermediate ranks may be created by adding prefixes, for instance:

In addition, species are often subdivided into subspecies and other infraspecific categories (see subspecies). The term varieties is sometimes used in place of subspecies. In horticulture, for example, it refers to populations modified by selective breeding, for instance the Peace Rose, a hybrid Tea Rose.

In husbandry, horticulture and other activities outside scientific biology, people still assume the truth of the traditional Linnaean system.

Modern developments

The approach Linnaeus took to classifying species and the majority of his taxonomic groupings remained the standard in biology for at least two centuries. Since the 1960s, however, a trend called cladism or cladistic taxonomy, has emerged and is expected to supplant Linnaean classification. In classifying species, cladists place a priority in achieving coherence with the Darwinian principle of common descent.

Meanwhile, at the top of the hierarchy of classification, there has movement towards a three domain system. The domains originally were replacements for the different kingdoms, but many scientists regard them as a groupings above the formerly paramount kingdom level.

Cladistics

In grouping species, cladists look for "derived similarities," meaning those aspects that species can be expected to share by virtue of a common evolutionary ancestry. This approach differs from that of phenetics, which does not address ancestry and associates species based on overall similarity, and it differs also from classification based on ad hoc "key characters." Cladists avail themselves of all types of information available, including DNA sequences and hybridization studies, biochemistry, and traditional morphology. They often make use of computers to identify the most likely phylogeny or "family tree" that relates the species they are considering.

Cladistics requires taxa (groups of species) to be clades. A formal code of phylogenetic nomenclature, the Phylocode[1], is currently under development for a cladistic taxonomy that abandons the Linnaean structure.

More at: cladistics.

Could add a description of the difficulty in classifying microbes: their features are derived from direct visual observation, but include such procedural characteristics as Gram stain type, motility, ability to form spores, etc. However, given an unknown bacterium with a given set of characteristics, it is in general not possible to predict its phylogeny, toxicity, etc. Other methods, using genes, their DNA, and several types of RNA, are under development.

Examples

The usual classifications of three species follow: The fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans, and the cucumber tree.

Fruit Fly (Drosophila)

KingdomAnimalia
PhylumArthropoda
ClassInsecta
OrderDiptera
FamilyDrosophilidae
GenusDrosophila
Speciesmelanogaster

Human (Homo sapiens)

KingdomAnimalia
PhylumChordata
SubphylumVertebrata
ClassMammalia
SubclassEutheria
OrderPrimates
SuborderCatarrhini
FamilyHominidae
GenusHomo
Speciessapiens

Cucumbertree (Magnolia acuminata)

KingdomPlantae
DivisionMagnoliophyta
ClassMagnoliopsida
OrderMagnoliales
FamilyMagnoliaceae
GenusMagnolia
Speciesacuminata

Note in this last example, that most of the taxa are named after the type genus, Magnolia.

Group Suffixes

Taxa above the genus level are often given names derived from the type genus. The suffixes used to form these names depend on the kingdom, and sometimes the phylum and class, as follows:

TaxonPlantsAlgaeFungiAnimals
Division/Phylum-phyta-phyta-mycota
Subdivision/Subphylum-phytina-phytina-mycotina
Class-opsida-phyceae-mycetes
Subclass-idae-phycidae-mycetidae
Order-ales-ales-ales
Suborder-ineae-ineae-ineae
Superfamily-acea-acea-acea-oidea
Family-aceae-aceae-aceae-idae
Subfamily-oideae-oideae-oideae-inae
Tribe-eae-eae-eae-ini
Subtribe-inae-inae-inae-ina

See also:

External Links:

Top     


Social class

(From Wikipedia, the free Encyclopedia)

A social class is a group of people that have similar social and economic status.

At various times the division of society into classes has had various levels of support in law. At one extreme we find old Indian classes - castes, which one could neither enter after birth, nor leave. (Though this applied only in relatively recent history.) On the other extreme there exist classes in modern Western societies which appear very fluid and have little support in law. When sociologistss speak of "class" they usually mean economically based classes in modern or near pre-modern society.

In Marxist terms a class is a group of people with a specific relationship to the means of production (social production). Marxists explain history in terms of a war of classes between those who control social production and those who produce social goods. In the Marxist view of capitalism this is a conflict between capitalists (bourgeoisie) and workers (proletariat).

Modern usage of the word "class" generally considers only the relative wealth of individuals or social groups, and not the ownership of the means of production.

The sociologist Max Weber formulated a three-component theory of stratification, with class, status and party (or politics) as conceptually distinct elements.

All three dimensions have consequences for what Weber called "life chances".

See also:

The concept of "caste" differs from that of "class", and refers to rigid status groupings, the membership of which is usually inherited. Particular caste groups include: For other meanings of the word class, see Class.

Further reading

Top     



INDEX

1. Bibliography


  

Copyright © Philip M. Parker, INSEAD. Terms of Use.