Java Reference
In-Depth Information
Because classesareinvolved,Javareferstothiskindofinheritance as
implementation
inheritance
.
Javasupportsmultipleinheritanceonlyinaninterfacecontext,inwhichaclassinher-
itsbehaviortemplatesfromoneormoreinterfacesthroughinterfaceimplementation,or
in which an interface inherits behavior templates from one or more interfaces through
interfaceextension.Becauseinterfacesareinvolved,Javareferstothiskindofinherit-
ance as
interface inheritance
. (I discuss interfaces later in this chapter.)
ThissectionintroducesyoutoJava'ssupportforimplementationinheritancebyfirst
focusingonclassextension.Itthenintroducesyoutoaspecialclassthatsitsatthetopof
Java'sclasshierarchy.Afterintroducingyoutocomposition,whichisanalternativeto
implementation inheritance for reusing code, this section shows you how composition
can be used to overcome problems with implementation inheritance.
Extending Classes
Java provides the reserved word
extends
for specifying a hierarchical relationship
betweentwoclasses.Forexample,supposeyouhavea
Vehicle
classandwanttoin-
cement these relationships.
Listing 2-21.
Relating classes via
extends
class Vehicle
{
// member declarations
}
class Car extends Vehicle
{
// member declarations
}
class Truck extends Vehicle
{
// Member declarations
}
Listing 2-21
codifies relationships that are known as “is-a” relationships: a car or a
truck is a kind of vehicle. In this relationship,
Vehicle
is known as the
base class
,
