Download Java Generics: Bounded/Unbounded Wildcards, Raw Types, Ensure & Restrictions and more Slides Java Programming in PDF only on Docsity!
Bounded
Generic
Type
public^ static
void^ main(String[]
args^ )^ {
ou ded Ge e c
ype
p^
(^ g[]^ g
) { Rectangle^ rectangle
=^ new^ Rectangle(2,
2);
Circle9^ circle
=^ new^ Circle9(2); System out println("Same area? " + equalArea(rectangle
circle));
System.out.println( Same
area?^ +^ equalArea(rectangle
,^ circle));
} bli^ t ti^
E^ t^ d^ G^
t i Obj^ t^ b
l
public^ static
boolean equalArea(E^ object1,
E^ object2)^ { return^ object1.getArea()
==^ object2.getArea(); }
Raw Type and backward Compatibility Raw^ Type // raw type
and^ backward
Compatibility
//^ raw^ type ArrayList^ list
=^ new^ ArrayList();
This^ is^ roughly
equivalent^
to ArrayList
list^ =^ new
ArrayList();
Make^ it^ Sa
fe
//^ Max1.java:
Find^ a^ maximum
object
a e t Sa epublic class Max1 { public^ class
Max1^ { /**^ Return
the^ maximum
between^ two
objects^ */
public^ static
>
E
p^
p max(E^ o1,^
E^ o2)^ { if^ (o1.compareTo(o2)
>^ 0) return^ o1;elsereturn o2;return^ o2; } } Max.max("Welcome", 23);
18
}
Wildcards Why^ wildcards
are^ necessary?
See^ this^ example.
Wildcards^ WildCardDemo1WildCardDemo1?
unbounded
wildcard
?^ t^ d^
T^ b
d d^
ild^ d
?^ extends
T^ b
ounded^ wildcard
?^ super^ T^
lower^ bound
wildcard
WildCardDemo2WildCardDemo
WildCardDemo3WildCardDemo
Avoiding
Unsafe^ Raw
Types
o d^ Use
g U sa e
a^ ypes
new^ ArrayList() Instead^ of new^ ArrayList();
TestArrayListNewTestArrayListNew
RunRun
Ensure and Restrictions on Generics Generics^ are
implemented
using^ an^ approach
called
Ensure^ and
Restrictions
on^ Generics
type^ erasure
.^ The^ compiler
uses^ the^ generic
type
information
to^ compile
the^ code,
but^ erases
it
ft^ d^
S^ th^
i^ i f^
ti^ i^ t
il bl
afterwards.
So^ the^ generic
information
is^ not^ available
at^ run^ time.
This^ approach
enables^ the
generic^ code
to
be backward
compatible with the legacy code that
be^ backward
‐compatible
with^ the^ legacy
code^ that
uses^ raw^ types.
22 docsity.com
Important
Facts
It^ is^ important
to^ note^ that
a^ generic
class^ is
h^ d b^
ll it^ i^ t^
dl^ f it
t^ l
po ta t^
acts
shared^ by
all^ its^ instances
regardless
of^ its^ actual
generic^ type.^ GenericStack
stack1^ =^ new
GenericStack();
GenericStack
stack2^ =^ new
GenericStack();GenericStack();
Although^ GenericStack and
GenericStack b^ h^ i l^
l^ G^ i S
k
are^ two^ types,
but^ there^ i
s^ only^ one^
class^ GenericStack
loaded^ into
the^ JVM.
Retractions
on^ Generics
^ Restriction
1:^ Cannot^
Create^ an^ Instance
of^ a^ Generic
Type (i e^
new E())
et act o s o
Ge e cs
Type.^ (i.e.,^
new^ E()). ^ Restriction 2: Generic Array Creation is Not Allowed^ Restriction
2:^ Generic
Array^ Creation
is^ Not^ Allowed
(i.e.,^ new^ E[100]). Restriction
3:^ A^ Generic
Type^ Parameter
of^ a^ Class^ Is
Not^ Allowed
in^ a^ Static^
Context. ^ Restriction
4:^ Exception
Classes^ Cannot
be^ Generic.^25
UML^ DiagramU^ ag a^ GenericMatrix^ # add(element1:E, element2: E): E^ # multiply(element1: E, element2: E): E
IntegerMatrix
multiply(element1:
E, element2: E): E
zero(): E +addMatrix(matrix1: E[][], matrix2: E[][]): E[][]+multiplyMatrix(matrix1: E[][], matrix2: E[][]): E[][]+printResult(m1: Number[][], m2: Number[][],printResult(m1: Number[][], m2: Number[][],m3: Number[][], op: char): void
RationalMatrix^27
Generic^
Type Objective:^ This^ example
gives^ two^ programs
that
yp utilize the^ GenericMatrix
class^ for^ integer
matrix
arithmetic^
and^ rational
matrix^ arithmetic. TestIntegerMatrixTestIntegerMatrix^
RunRun
IntegerMatrixIntegerMatrix
TestRationalMatrixTestRationalMatrix
RunRun
RationalMatrixRationalMatrix
