1http://www.tutorialspoint.com/java/java_generics.htmJava -
GenericsIt would be nice if we could write a single sort method
that could sort the elements in an Integer array, aString array or
an array of any type that supports ordering.Java Generic methods
and generic classes enable programmers to specify, with a single
method declaration,a set of related methods or, with a single class
declaration, a set of related types, respectively.Generics also
provide compile-time type safety that allows programmers to catch
invalid types at compiletime.Using Java Generic concept we might
write a generic method for sorting an array of objects, then invoe
thegeneric method with Integer arrays, !ouble arrays, String arrays
and so on, to sort the array elements.Generic Methods:"oucan write
a singlegeneric methoddeclaration that canbecalledwith arguments of
differenttypes.#ased on the types of the arguments passed to the
generic method, the compiler handles each method callappropriately.
$ollowing are the rules to define Generic %ethods& 'll generic
method declarations have a type parameter section delimited by
angle bracets () and*+ that precedes the method,s return type ( ) -
* in the ne.t e.ample+. -ach type parameter section contains one or
more type parameters separated by commas. ' typeparameter, also
nown as a type variable, is an identifier that specifies a generic
type name. /he type parameters can be used to declare the return
type and act as placeholders for the types ofthe arguments passed
to the generic method, which are nown as actual type arguments. '
generic method,s body is declared lie that of any other method.
0ote that type parameters canrepresent only reference types not
primitive types (lie int, double and char+. Example:$ollowing
e.ample illustrate how we can print array of different type using a
single Generic method&public class GenericMethodTest{ //
generic method printArray public static < E > void
printArray( E[ inputArray ! {// "isplay array elements #or ( E
element $ inputArray !{%ystem&out&print#( '(s ') element
!*2 + %ystem&out&println(!*+public static void main( %tring
args[ !{// ,reate arrays o# -nteger) "ouble and ,haracter-nteger[
intArray . { /) 0) 1) 2) 3 +*"ouble[ doubleArray . { /&/)
0&0) 1&1) 2&2 +*,haracter[ charArray . { 454) 4E4) 464)
464) 474 +*%ystem&out&println( 'Array integerArray
contains$' !*printArray( integerArray !* // pass an -nteger
array%ystem&out&println( '8nArray doubleArray contains$'
!*printArray( doubleArray !* // pass a "ouble
array%ystem&out&println( '8nArray characterArray contains$'
!*printArray( characterArray !* // pass a ,haracter array+ +/his
would produce following result&Array integerArray contains$/ 0
1 2 3 9Array doubleArray contains$/&/ 0&0 1&1 2&2
Array characterArray contains$5 E 6 6 7Bounded Type
Parameters:/here may be times when you,ll want to restrict the inds
of types that are allowed to be passed to a typeparameter. $or
e.ample, a method that operates on numbers might only want to
accept instances of 0umberor its subclasses. /his is what bounded
type parameters are for./o declare a bounded type parameter, list
the type parameter,s name, followed by the e.tends eyword,followed
by its upper bound.Example:$ollowing e.ample illustrate how e.tends
is used in a general sense to mean either 1e.tends1 (as in
classes+or 1implements1 (as in interfaces+. /his e.ample is Generic
method to return the largest of three2omparable objects&public
class Ma:imumTest{ // determines the largest o# three ,omparable
ob;ects public static T ma:imum(T :) T y) T = !{ ma: . y* // y is
the largest so #ar+i# ( = !{ ma: . +return ma:* // returns the
largest ob;ect + public static void main( %tring args[ !
{%ystem&out&print#( 'Ma: o# (d) (d and (d is (d8n8n') 1) 2)
3) ma:imum( 1) 2) 3 ! !* %ystem&out&print#( 'Ma:m o#
(&/#)(&/# and (&/# is (&/#8n8n') 9&9) ?&?)
@&@) ma:imum( 9&9) ?&?) @&@ ! !*
%ystem&out&print#( 'Ma: o# (s) (s and (s is (s8n')'pear')
'apple') 'orange') ma:imum( 'pear') 'apple') 'orange' ! !* ++/his
would produce following result&Ma:imum o# 1) 2 and 3 is
3Ma:imum o# 9&9) ?&? and @&@ is ?&?Ma:imum o# pear)
apple and orange is pearGeneric Classes:' generic class declaration
loos lie a non-generic class declaration, e.cept that the class
name is followedby a type parameter section.'s withgenericmethods,
thetypeparameter sectionof agenericclasscanhaveoneor
moretypeparametersseparatedbycommas.
/heseclassesarenownasparameteri3edclassesorparameteri3edtypes
because they accept one or more parameters.Example:$ollowing
e.ample illustrate how we can define a generic class&public
class Ao: {private T t*public void add(T t! {this&t . t*+public
T get(! {return t*4+public static void main(%tring[ args! { Ao:
integerAo: . ne> Ao:(!* Ao: stringAo: . ne> Ao:(!*
integerAo:&add(ne> -nteger(/=!!* stringAo:&add(ne>
%tring('5ello Borld'!!* %ystem&out&print#('-nteger Calue
$(d8n8n') integerAo:&get(!!* %ystem&out&print#('%tring
Calue $(s8n') stringAo:&get(!!*++/his would produce following
result&-nteger Calue $/=%tring Calue $5ello Borld
