8

C H A P T E R

Classes

class 1. The noun

class

 derives from

Medieval French and French

classe

 from Latin

classis

,

probably originally a summons,

hence a summoned collection of persons,

a group liable to be summoned:

perhaps for

callassis

 from

calare

,

to call, hence to summon.

 Eric Partridge

Origins: A Short Etymological Dictionary of Modern English

C

LASS declarations define new reference types and describe how they are

implemented ( 8.1).

The name of a class has as its scope all type declarations in the package in

which the class is declared ( 8.1.1). A class may be declared

abstract

 ( 8.1.2.1)

and must be declared

abstract

 if it is incompletely implemented; such a class

cannot be instantiated, but can be extended by subclasses. A class may be declared

final

 ( 8.1.2.2), in which case it cannot have subclasses. If a class is declared

public

, then it can be referred to from other packages.

Each class except

Object

 is an extension of (that is, a subclass of) a single

existing class ( 8.1.3) and may implement interfaces ( 8.1.4).

The body of a class declares members (fields and methods), static initializers,

and constructors ( 8.1.5). The scope of the name of a member is the entire decla 

ration of the class to which the member belongs. Field, method, and constructor

declarations may include the access modifiers ( 6.6)

public

,

protected

, or

private

. The members of a class include both declared and inherited members

( 8.2). Newly declared fields can hide fields declared in a superclass or superinter 

face. Newly declared methods can hide, implement, or override methods declared

in a superclass or superinterface.

Field declarations ( 8.3) describe class variables, which are incarnated once,

and instance variables, which are freshly incarnated for each instance of the class.

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