Java Reference
In-Depth Information
Abstract classes support implementation, but can be genuinely abstract (
Listing
the upper levels of class hierarchies.
Interfaces and abstract classes can be used together. For example, the Collections
Framework's
java.util
package provides
List
,
Map
, and
Set
interfaces; and
AbstractList
,
AbstractMap
, and
AbstractSet
abstract classes that
provide skeletal implementations of these interfaces.
Theskeletalimplementations makeiteasyforyoutocreateyourowninterfaceim-
plementations,toaddressyouruniquerequirements.Iftheydonotmeetyourneeds,
you can optionally have your class directly implement the appropriate interface.
Collecting Garbage
Objectsarecreatedviareservedword
new
,buthowaretheydestroyed?Withoutsome
way to destroy objects, they will eventually fill up the heap's available space and the
application will not be able to continue. Java does not provide the developer with the
ability to remove them from memory. Instead, Java handles this task by providing a
garbage collector
, which is code that runs in the background and occasionally checks
forunreferencedobjects.Whenthegarbagecollectordiscoversanunreferencedobject
(ormultipleobjectsthatreferenceeachother,andwheretherearenootherreferencesto
eachother—onlyAreferencesBandonlyBreferencesA,forexample),itremovesthe
object from the heap, making more heap space available.
An
unreferenced object
isanobjectthatcannotbeaccessedfromanywherewithinan
application.Forexample,
new Employee("John", "Doe");
isanunreferenced
objectbecausethe
Employee
referencereturnedby
new
isthrownaway.Incontrast,
a
referenced object
isanobjectwheretheapplicationstoresatleastonereference.For
example,
Employee emp = new Employee("John", "Doe");
isareferenced
object because variable
emp
contains a reference to the
Employee
object.
A referenced object becomes unreferenced when the application removes its last
storedreference.Forexample,if
emp
isalocalvariablethatcontainstheonlyreference
toan
Employee
object, thisobject becomes unreferenced whenthemethod inwhich
emp
isdeclaredreturns.Anapplicationcanalsoremoveastoredreferencebyassigning
null
toitsreferencevariable.Forexample,
emp = null;
removesthereferenceto
the
Employee
object that was previously stored in
emp
.
Java's garbage collector eliminates a form of memory leakage in C++ implementa-
tions that do not rely on a garbage collector. In these C++ implementations, the deve-
lopermustdestroydynamicallycreatedobjectsbeforetheygooutofscope.Iftheyvan-
