Sealed classes: Developers were excited when this new feature was released in JDK15! They have been in use in other programming languages. Now it’s Java’s turn! See: https://xperti.io/blogs/sealed-classes-java-feature/

1. BLOG
Java Feature Spotlight: Sealed Classes
NOVEMBER 23, 2020
Java technologyis known forits regularand frequentreleases,with something new
developers can lookforward to each time. In September2020, itintroduced anotherone of
these exciting newJava features.The release ofJava SE 15 included “sealed classes”(JEP
360)as a previewfeature. Itis a prominentJava feature.This is becausethe introduction of
sealed classes in Java holdsthe solution to a problemJava has had fromits initial 1.0version,
released 25 years ago.
SeeAlso: Looking BackOn 25 Years OfJava: MajorMilestones
EXPERTS CLIENTS HOW IT WORKS
Table ofContents
1. Context
2. Sealed Classes
3. How To Define A Sealed Class
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2. Context
Aswe all know, one ofthefundamentals ofobject-oriented programming is inheritance. Java
programmers have been reusing fields and methods ofexisting classes byinheriting themto
newclasses. Itsavestime and theydon’thavetowritethe code again. Butthings change ifa
developerdoes notwantto allowanyrandomclassto extend his/hercreated class.This
developercan seal a class ifhe/she doesn’twantanyclients ofhis/herlibrarydeclaring any
more primitives. Bysealing a class, Java Developers can nowspecifywhich classes are
allowed to extend, restricting anyotherarbitraryclassfromdoing so.
Sealed Classes
Sealed classes in Java primarilyprovide restrictions in extending subclasses.A sealed class is
abstractbyitself. Itcannotbe instantiated directly. Butitcan have abstractmembers.
Restriction is nota newconceptin Java.We are all aware ofa Java feature called “final
classes.”Ithas alreadybeen offering restricting extension butthe addition ofsealed classes in
Java can be considered a generalization offinality. Restriction primarilyofferstwo advantages:
Developers get better control over the code as he or she can now control all the
implementations, and
The compiler can also better reason about exhaustiveness just like it is done in
switch statements or cast conversion.
4. Features Offered By Sealed Classes
4.1. · Extensibility And Control
4.2. · Simplification
4.3. · Protection
4.4. · More Accessibility
4.5. · Exhaustiveness
5. Significance Of Sealed Classes
6. Sum And Product Types
7. Conflict With Encapsulation?
8. Sealed Classes And Records
9. Records And Pattern Matching
10. Conclusion
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3. How To Define A Sealed Class
To seal a class,we need to add the sealed modifierto its declaration.Afterthat, a permits
clause is added.This clause specifiesthe classesthatare allowed to be extended. Itmustbe
added afteranyextends and implements clauses.
Belowis averysimple demonstration ofa sealed class in Java.
1. public sealed class Vehicle permits Car, Truck, Motorcycle {...}
2. final class Car extends Vehicle {...}
3. final class Boat extends Vehicle {...}
4. final class Plane extends Vehicle {...}
In the example above, ‘Vehicle’ isthe name ofa sealed class,which specifiesthree permitted
subclasses;Car, Boatand Plane.
There are certain conditionsfordeclaring a sealed classthatmustbefulfilled bythe
subclasses:
1. The subclasses must be in the same package or module as the base class. It is also
possible to define them in the same source file as the base class. In such a
situation, the “permits” clause will not be required.
2. The subclasses must be declared either final, sealed or non-sealed.
The permits listis defined to mention the selected classesthatcan implement‘Vehicle’. In this
case,these classes are ‘Car’, ‘Boat’ and ‘Plane’.A compilation errorwill be received ifanyother
class orinterface attemptsto extend the ‘Vehicle’ class.
Features Offered By Sealed Classes
· Extensibility And Control
Sealed classes offera newwayto declare all available subclasses ofa class orinterface. It’s a
handyJava feature. Especiallyifa developerwishesto make superclasses accessiblewhile
restricting unintended extensibility. Italso allows classes and interfacesto have more control
overtheirpermitted subtypes.This can be useful formanyapplications including general
domain modelling and forbuilding more secure and stable platformlibraries.
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4. · Simplification
Anothernoticeable advantage ofintroducing sealed classes in Java isthe simplification of
code. Itgreatlysimplifies code byproviding an option to representthe constraints ofthe
domain. NowJava coders are notrequired to use a defaultsection in a switch ora catch-all
‘else’ blockto avoid getting an unknown type.Additionally, italso seems useful forserialization
ofdata to and fromstructured data formats, like XML. Sealed classes also allowdevelopersto
knowall possible subtypesthatare supported in the given format. (And yes,the subtypes are
nothidden.Thiswill be discussed in detail laterin the article.)
· Protection
Sealed classes can also be used as a layerofadditional protection againstinitialization of
unintended classes during polymorphic deserialization. Polymorphic deserialization has been
one ofthe primarysources ofattacks in such frameworks.Theseframeworks can take
advantage ofthe information ofthe complete setofsubtypes, and in case ofa potential attack,
theycan stop before even trying to load the class.
· More Accessibility
Thetypical process ofcreating a newclass orinterface includes deciding which scope
modifiermustbe used. Itis usuallyverysimple untilthe developers come across a project
where using a defaultscope modifieris notrecommended bythe official style guide.With
sealed classes, developers nowgetbetteraccessibility, using inheritancewith a sealed scope
modifierwhile creating newclasses. In otherwords,with the entryofsealed classes in Java, ifa
developerneedsthe superclassto bewidelyaccessible butnotarbitraryextensible, he/she
has a straightforward solution.
Sealed classes also allowJava libraries’ authorsto decouple accessibilityfromextensibility. It
providesfreedomand flexibilityto developers. Buttheymustuse itlogicallyand notoveruse it.
Forinstance, ‘List’ cannotbe sealed, as users should havethe abilityto create newkinds of
‘Lists’. Itmakes sensefordevelopersto notseal it.
· Exhaustiveness
Sealed classes also carryoutan exhaustive listofpossible subtypes,which can be used by
both programmers and compilers. Forexample, in the defined class above, a compilercan
extensivelyreason aboutthevehicles class (notpossible withoutthis list).This information can
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5. be used bysome othertools aswell. Forinstance,the Javadoc tool liststhe permitted subtypes
in the generated documentation pagefora sealed class.
Significance Of Sealed Classes
Sealed classes rankhighlyamong the otherfeatures released in Java 15. Jonathan Harley, a
Software DevelopmentTeamLeaderprovided an excellentexplanation on Quora abouthow
importantsealed classes can beforJava Developers: “Thefactthatinterfaces, aswell as
classes, can be sealed is importantto understand howdevelopers can usethemto improve
theircode. Until now, ifyou wanted to expose an abstraction tothe restofan application while
keeping the implementation private,youronlychoiceswereto expose an interface (which can
always be extended)oran abstractclasswith a package-private constructorwhich you hope
will indicateto usersthattheyshould notinstantiate itthemselves. Buttherewas nowayto
restricta userfromadding theirconstructorswith differentsignatures oradding theirpackage
with the same name asyours.”
Hefurtherexplained, “Sealed types allowyou to expose a type (interface orclass)to othercode
while still keeping full control ofsubtypes, and theyalso allowyou to keep abstractclasses
completelyprivate…”
Sum And Product Types
The example mentioned earlierin the article makes a statementabouthowa ‘Vehicle’ can only
eitherbe a:
Car
Boat or a
Plane
Itmeansthatthe setof allVehicles is equaltothe setof all Cars, all Boats and all Planes
combined.This iswhysealed classes are also known as “sumtypes.” Becausetheirvalue setis
the sumofthevalue sets ofa fixed listofothertypes. Sumtypes, and sealed classes are new
forJava butnotin the largerscale ofthings.Scala and manyotherhigh-level programming
languages have been using sealed classestoo, aswell as sumtypesforquite sometime.

6. Conflict With Encapsulation?
Object-oriented modelling has always encouraged developersto keep the implementation of
an abstracttype hidden.
Butthen whyisthis newJava feature contradicting this rule?
When developers are modelling awell-understood and stable domain, encapsulation can be
neglected because userswill notbe benefitting fromthe application ofencapsulation in this
case. In theworstpossible scenario, itmayeven make itdifficultforclientstoworkwith avery
simple domain.
This does notmean thatencapsulation is a mistake. Itjustmeansthatata higherand complex
level ofprogramming, developers are aware ofthe consequences. Sotheycan makethe callto
go a bitoutoflineto getsomeworkdone.
Sealed Classes And Records
Sealed classesworkwellwith records (JEP384). Record is a relativelynewJava featurethatis
a formofproducttype. Records are a newkind oftype declaration in Java similarto Enum. Itis
a restricted formofclass. Records are implicitlyfinal, so a sealed hierarchywith records is
slightlymore concise.To explain thisfurther,we can extend the previouslymentioned example
using recordsto declarethe subtypes:
1. sealed interface Vehicle permits Car, Boat, Plane {
2. record Car (float speed, string mode) implements Vehicle {...}
3. record Boat (float speed, string mode) implements Vehicle {...}
4. record Plane (float speed, string mode) implements Vehicle {...}
}
This example shows howsumand record (producttypes)worktogether;we can saythata car,
a plane ora boatis defined byits speed and its mode.
In anotherapplication, itcan also be used forselecting which othertypes can bethe
subclasses ofthe sealed class.
Forexample, simple arithmetic expressionswith records and sealed typeswould be likethe
code mentioned below:
1. sealed interface Arithmetic {...}
2. record MakeConstant (int i) implements Arithmetic {...}
3. record Addition (Arithmetic a, Arithmetic b) implements Arithmetic {...}
4. record Multiplication (Arithmetic a, Arithmetic b) implements Arithmetic

7. {...}
5. record Negative (Arithmetic e) implements Arithmetic {...}
Herewe havetwo concretetypes: addition and multiplication which hold two subexpressions,
and two concretetypes, MakeConstantand Negativewhich holds one subexpression. Italso
declares a supertypeforarithmetic and capturesthe constraintthatthese are
the only subtypes ofarithmetic.
The combination ofsealed classes and records is also known as “algebraic datatypes”. Records
allowusto express producttypes, and sealed classes allowusto express sumtypes.
Records And Pattern Matching
Both records and sealed types have an association with pattern matching. Records admiteasy
decomposition intotheircomponents, and sealed types providethe compilerwith
exhaustiveness information sothata switch thatcovers allthe subtypes need notprovide a
defaultclause.
A limited formof pattern matching has been previouslyintroduced in Java SE 14which will
hopefullybe extended in thefuture.This initialversion ofJava feature allows Java developers
to usetype patternsin “instanceof.”
Forexample, let’stake a lookatthe code snippetbelow:
1. if (vehicle instanceof Car c) {
2. // compiler has itself cast vehicle to the car and bound it to c
3. System.out.printf("The speed of this car is %d%n", c.speed());
}
Conclusion
Although manyothertoolswere released with sealed classes, itremainsthe mostprominent
Java feature ofthe release.We are still notcertain aboutthefinal representation ofsealed
classes in Java. (Ithas been released as a previewin Java 15). Butsofarsealed classes are
offering awide range ofuses and advantages.Theyproveto be useful as a domain modelling
technique,when developers need to capture an exhaustive setofalternatives in the domain
model. Sealed types become a natural complementto records, astogethertheyformcommon
patterns. Both ofthemwould also be a natural fitfor pattern matching. Itis obviousthat
sealed classes serve as a quite useful improvementin Java.And,with the overwhelming

8. JAVA FEATURE SEALED CLASSES SEALED CLASSES IN JAVA  0    
Java Security Vulnerabilities:
Case Commentary
DECEMBER 15, 2020
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DECEMBER 10, 2020
Getting Comfortable With
FPGAs
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responsefromthe Java community,we can expectthata better, more refined version of
sealed classes in Javawill soon be released.
AUTHOR
Shaharyar Lalani
Shaharyar Lalani is a developer with a strong interest in business analysis, project management, and UX design.
He writes and teaches extensively on themes current in the world of web and app development, especially in Java
technology.
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