Java Feature Spotlight: Sealed Classes
- 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 Type and hit enter... RECENT POSTS Java Security Vulnerabilities: Case Commentary What is TornadoVM? Getting Comfortable With FPGAs Java Feature Spotlight: Sealed Classes The New Java Roadmap: What’s In Store For The Future? ARCHIVES December 2020 November 2020 October 2020 September 2020 August 2020 July 2020 June 2020 BECOME AN XPERTI HIRE AN XPERTI
- 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 May 2020 April 2020 March 2020 February 2020 CATEGORIES Articles Blog Press Release Uncategorized CONNECT & FOLLOW Subscribe to newsletter now! Your email address.. SUBSCRIBE CHECK OUR TWEETS NEWSLETTER
- 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. Whoever said the path to career success is long and stony never tried Xperti. Excellent career opportunities for Am… https://t.co/fJdkbHaKHB 5 DAYS Enlightened, ambitious, and ready for a great challenge! Xperti wishes you a joyous Hanukkah Festival 2020!… https://t.co/Cf9DmnHj6W 6 DAYS Excellent opportunities for your technology career, only with Xperti, America’s community of top 1% tech talent. Si… https://t.co/EkofL2C0gE 7 DAYS @XPERTI1 LATEST POSTS Java Security Vulnerabilities: Case Commentary DECEMBER 15, 2020 What is TornadoVM? DECEMBER 10, 2020 Getting Comfortable With FPGAs DECEMBER 2, 2020 Java Feature Spotlight:
- 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 Sealed Classes NOVEMBER 23, 2020 INSTAGRAM View on Instagram CATEGORIES Articles (1) Blog (45) Press Release (1) Uncategorized (1)
- 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 What is TornadoVM? DECEMBER 10, 2020 Getting Comfortable With FPGAs DECEMBER 2, 2020 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. Name Email Website RELATED POSTS WRITE A COMMENT
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