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description= Covering literally everything there is to know about Java 8 s default methods.;
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Text of the page (random words):
a method to an interface without a compile error and hints at the method call resolution strategy every class which implements comparator will now contain the public method thencomparing comparator without having to implement it itself it comes for free so to speak explicit calls to default methods further below we will see some reasons why one might want to explicitly call a default implementation of a method from some specific superinterface if the need arises this is how it s done class stringcomparator implements comparator string override public comparator string thencomparing comparator super string other log call to thencomparing return comparator super thencomparing other note how the name of the interface is used to specify the following super which would otherwise refer to the superclass in this case object this is syntactically similar to how the reference to the outer class can be accessed from a nested class it is not possible to call a method from an interface that is not mentioned in the implements clause if for example our stringcomparator were to implement objectcomparator t extends comparator t the call comparator super thencomparing would cause a compile error when implementing two interfaces where one extends the other comparator super causes a different compile error together this means that it is not possible to explicitly call overridden or reabstracted default methods thanks to charlie for pointing me into the right direction resolution strategy so let s consider an instance of a type which implements an interface with default methods what happens if a method is called for which a default implementation exists note that a method is identified by its signature which consists of the name and the parameter types rule 1 classes win over interfaces if a class in the superclass chain has a declaration for the method concrete or abstract you re done and defaults are irrelevant rule 2 more specific interfaces win over less specific ones where specificity means subtyping a default from list wins over a default from collection regardless of where or how or how many times list and collection enter the inheritance graph rule 3 there s no rule 3 if there is not a unique winner according to the above rules concrete classes must disambiguate manually brian goetz mar 3 2013 formatting mine first of all this clarifies why these methods are called default methods and why they must be started off with the keyword default such an implementation is a backup in case a class and none of its superclasses even consider the method i e provide no implementation and are not declaring it as abstract see rule 1 equivalently a default method of interface x is only used when the class does not also implement an interface y which extends x and declares the same method either as default or abstract see rule 2 while these rules are simple they do not prevent developers from creating complex situations this post gives an example where the resolution is not trivial to predict and arguments that this feature should be used with care the resolution strategy implies several interesting details conflict resolution rule 3 or rather its absence means that concrete classes must implement each method for which competing default implementations exist otherwise the compiler throws an error if one of the competing implementations is appropriate the method body can just explicitly call that method this also implies that adding default implementations to an interface can lead to compile errors if a class a implements the unrelated interfaces x and y and a default method which is already present in x is added to y class a will not compile anymore what happens if a x and y are not compiled together and the jvm stumbles upon this situation interesting question to which the answer seems somewhat unclear looks like the jvm will throw an incompatibleclasschangeerror re abstracting methods if an abstract class or interface a declares a method as abstract for which a default implementation exists in some superinterface x the default implementation of x is overridden hence all concrete classes which subtype a must implement the method this can be used as an effective tool to enforce the reimplementation of inappropriate default implementations this technique is used throughout the jdk e g on concurrentmap link which re abstracts a number of methods for which map link note that concrete classes can not explicitly call the overridden default implementation overriding methods on object it is not possible for an interface to provide default implementations for the methods in object trying to do so will result in a compile error why well first of all it would be useless since every class inherits from object rule 1 clearly implies that those methods would never be called but that rule is no law of nature and the expert group could have made an exception the mail which also contains the rules brian goetz gives many reasons why they didn t the one i like best formatting mine at root the methods from object such as tostring equals and hashcode are all about the object s state but interfaces do not have state classes have state these methods belong with the code that owns the object s state the class modifiers note that there are a lot of modifiers you can not use on default methods the visibility is fixed to public as on other interface methods the keyword synchronized is forbidden as on abstract methods the keyword final is forbidden as on abstract methods of course these features were requested and comprehensive explanations for their absence exist e g for final and synchronized the arguments are always similar this is not what default methods were intended for and introducing those features will result in more complex and error prone language rules and or code you can use static though which will reduce the need for plural form utility classes a little context now that we know all about how to use default methods let s put that knowledge into context interface evolution the expert group which introduced default methods can often be found stating that their goal was to allow interface evolution the purpose of default methods is to enable interfaces to be evolved in a compatible manner after their initial publication brian goetz sep 2013 before default methods it was practically impossible excluding some organizational patterns see this nice overview to add methods to interfaces without breaking all implementations while this is irrelevant for the vast majority of software developers which also control those implementations it is a crucial problem for api designers java always stayed on the safe side and never changed interfaces after they were released but with the introduction of lambda expressions this became unbearable imagine the collective pain of always writing stream of mylist foreach because foreach could no be added to list so the expert group which introduced lambdas decided to find a way to enable interface evolution without breaking any existing implementations their focus on this goal explains the characteristics of default methods this not only allows to add methods like the jdk did it is also opens up the possibility to refactor or remove interface methods in a backwards compatible manner if clients can be expected to update their code in a transition phase it is even possible to formulate cookbook rules for that process where the group deemed it possible without degrading usability of this primary use case they also enabled the use of default methods to create traits or rather something close to them still they were frequently attacked for not going all the way to mixins and traits to which the often repeated answer was yes because that is was not our goal ousting utility classes the jdk and especially common auxiliary libraries like guava and apache commons are full of utility classes their name is usually the plural form of the interface they are providing their methods for e g collections or sets the primary reason for their existence is that those utility methods could not be added to the original interface after its release with default methods this becomes possible all those static methods which take an instance of the interface as an argument can now be transformed into a default method on the interface as an example look at the static collections sort list link which as of java 8 simply delegates to the new instance default method list sort comparator link another example is given in my post on how to use default methods to improve the decorator pattern other utility methods which take no arguments usually builders can now become static default methods on the interface while removing all interface related utility classes in a code base is possible it might not be advisable the usability and cohesiveness of the interface should remain the main priority not stuffing every imaginable feature in there my guess is that it only makes sense to move the most general of those methods to the interface while more obscure operations could remain in one or more utility classes or remove them entirely if you re into that classification in his argument for new javadoc tags brian goetz weakly classifies the default methods which were introduced into the jdk so far formatting mine 1 optional methods this is when the default implementation is barely conformant such as the following from iterator default void remove throw new unsupportedoperationexception remove it adheres to its contract because the contract is explicitly weak but any class that cares about removal will definitely want to override it 2 methods with reasonable defaults but which might well be overridden by implementations that care enough for example again from iterator default void foreach consumer super e consumer while hasnext consumer accept next this implementation is perfectly fine for most implementations but some classes e g arraylist might have the chance to do better if their maintainers are sufficiently motivated to do so the new methods on map e g putifabsent are also in this bucket 3 methods where its pretty unlikely anyone will ever override them such as this method from predicate default predicate t and predicate super t p objects requirenonnull p return t t test t p test t brian goetz jan 31 2013 i call this classification weak because it naturally lacks hard rules about where to place a method that does not make it useless though quite the opposite i consider it a great help in communicating about them and a good thing to keep in mind while reading or writing default methods documentation note that default methods were the primary reason to introduce the new unofficial javadoc tags apinote implspec and implnote the jdk makes frequent use of them so it is important to understand their meaning a good way to learn about them is to read my last post smooth right which covers them in all detail inheritance and class building different aspects of inheritance and how it is used to build classes often come up in discussions about default methods let s take a closer look at them and see how they relate to the new language feature multiple inheritance of what with inheritance a type can assume characteristics of another type three kinds of characteristics exist type i e by subtyping a type is another type behavior i e a type inherits methods and thus behaves the same way as another type state i e a type inherits the variables defining the state of another type since classes subtype their superclass and inherit all methods and variables class inheritance clearly covers all three of those characteristics at the same time a class can only extend one other class so this is limited to single inheritance interfaces are different a type can inherit from many interfaces and becomes a subtype of each so java has been supporting this kind of multiple inheritance from day 1 but before java 8 an implementing class only inherited the interface s type yes it also inherited the contract but not its actual implementation so it had to provide its own behavior with default methods this changes so from version 8 on java supports multiple inheritance of behavior as well java still provides no explicit way to inherit the state of multiple types something similar can be achieved with default methods though either with an evil hack or the virtual field pattern the former is dangerous and should never be used the latter also has some drawbacks especially regarding encapsulation and should be used with great care default methods vs mixins and traits when discussing default methods they are sometimes compared to mixins and traits this article can not cover those in detail but will give a rough idea how they differ from interfaces with default methods a helpful comparison of mixins and traits can be found on stackoverflow mixins mixins allow to inherit their type behavior and state a type can inherit from several mixins thus providing multiple inheritance of all three characteristics depending on the language one might also be able to add mixins to single instances at runtime as interfaces with default methods allow no inheritance of state they are clearly no mixins traits similar to mixins traits allow types and instances to inherit from multiple traits they also inherit their type and behavior but unlike mixins conventional traits do not define their own state this makes traits similar to interfaces with default methods the concepts are still different but those differences are not entirely trivial i might come back to this in the future and write a more detailed comparison but until then i will leave you with some ideas as we ve seen method call resolution is not always trivial which can quickly make the interaction of different interfaces with default methods a complexity burden traits typically alleviate this problem one way or another traits allow certain operations which java does not fully support see the bullet point list after selection of operations in the wikipedia article about traits the paper trait oriented programming in java 8 explores a trait oriented programming style with default methods and encounters some problems so while interfaces with default methods are no traits the similarities allow to use them in a limited fashion like they were this is in line with the expert group s design goal which tried to accommodate this use case wherever it did not conflict with their original goal namely interface evolution and ease of use default methods vs abstract classes now that interfaces can provide behavior they inch into the territory of abstract classes and soon the question arises which to use in a given situation language differences let s first state some of the differences on the language level while interfaces allow multiple inheritance they fall short on basically every other aspect of class building default methods are never final can not be synchronized and can not override ...
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