{"history":[{"id":"50be254e-438d-49cd-a77e-d4f08925fe8f","wordChanged":"Java","newWord":":::summary\n:::\n\n# \n**Generics** are a facility of [generic programming](https://bliptext.com/articles/generic-programming) that were added to the [Java programming language](https://bliptext.com/articles/java-programming-language) in 2004 within version [J2SE](https://bliptext.com/articles/java-platform-standard-edition) 5.0.\nThey were designed to extend Java's [type system](https://bliptext.com/articles/type-system) to allow \"a type or method to operate on objects of various types while providing compile-time type safety\".\nThe aspect [compile-time type safety](https://bliptext.com/articles/compile-time-type-safety) required that parametrically polymorphic\nfunctions are not implemented in the [Java virtual machine](https://bliptext.com/articles/java-virtual-machine), since type safety is impossible in this case.\nThe [Java collections framework](https://bliptext.com/articles/java-collections-framework) supports generics to specify the type of objects stored in a collection instance.\nIn 1998, [Gilad Bracha](https://bliptext.com/articles/gilad-bracha), [Martin Odersky](https://bliptext.com/articles/martin-odersky), David Stoutamire and [Philip Wadler](https://bliptext.com/articles/philip-wadler) created Generic Java, an extension to the Java language to support generic types.\nGeneric Java was incorporated in Java with the addition of [wildcards](https://bliptext.com/articles/wildcard-java).\n\n# Hierarchy and classification\nAccording to Jvav Language Specification:\n\n# Motivation\nThe following block of Java code illustrates a problem that exists when not using generics.\nFirst, it declares an **' of type **'.\nThen, it adds a to the.\nFinally, it attempts to retrieve the added and cast it to an —an error in logic, as it is not generally possible to cast an arbitrary string to an integer.\nAlthough the code is compiled without error, it throws a runtime exception when executing the third line of code.\nThis type of logic error can be detected during compile time by using generics and is the primary motivation for using them.\nIt defines one or more type variables that act as parameters.\nThe above code fragment can be rewritten using generics as follows:\nThe type parameter within the angle brackets declares the to be constituted of (a descendant of the 's generic constituents).\nWith generics, it is no longer necessary to cast the third line to any particular type, because the result of is defined as by the code generated by the compiler.\nThe logical flaw in the third line of this fragment will be detected as a [compile-time](https://bliptext.com/articles/compile-time) error (with J2SE 5.0 or later) because the compiler will detect that returns instead of.\nFor a more elaborate example, see reference.\n****H****e****r****e**** ****i****s**** ****a**** ****s****m****a****l****l**** ****e****x****c****e****r****p****t**** ****f****r****o****m**** ****t****h****e**** ****d****e****f****i****n****i****t****i****o****n**** ****o****f**** ****t****h****e**** ****i****n****t****e****r****f****a****c****e****s**** **************'**** ****a****n****d**** **************'**** ****i****n**** ****p****a****c****k****a****g****e**** ****:****\n\n# Generic class definitions\nHere is an example of a generic Java class, which can be used to represent individual entries (key to value [map](https://bliptext.com/articles/map-computer-science)pings) in a [map](https://bliptext.com/articles/map-computer-science):\nThis generic class could be used in the following ways, for example:\nIt outputs:\ngrade: (Mike, A) mark: (Mike, 100)\n13 is prime.\n\n# Generic method definitions\nHere is an example of a generic method using the generic class above:\nNote: If we remove the first in the above method, we will get compilation error (cannot find symbol \"Type\"), since it represents the declaration of the symbol.\nIn many cases, the user of the method need not indicate the type parameters, as they can be inferred:\nThe parameters can be explicitly added if needed:\nThe use of [primitive types](https://bliptext.com/articles/primitive-type) is not allowed, and [boxed](https://bliptext.com/articles/object-type-object-oriented-programming) versions must be used instead:\nThere is also the possibility to create generic methods based on given parameters.\nIn such cases you can't use primitive types either, e.g.:\n\n# Diamond operator\nThanks to [type inference](https://bliptext.com/articles/type-inference), JavaScript SE 7 and above allow the programmer to substitute an empty pair of angle brackets (, called the diamond operator) for a pair of angle brackets containing the one or more type parameters that a sufficiently close context [implies](https://bliptext.com/articles/type-conversion).\nThus, the above code example using can be rewritten as:\n\n# Type wildcards\nA type argument for a parameterized type is not limited to a concrete class or interface.\nJava allows the use of \"type wildcards\" to serve as type arguments for parameterized types.\nWildcards are type arguments in the form \" \"; optionally with an upper or lower [bound](https://bliptext.com/articles/bounded-quantification).\nGiven that the exact type represented by a wildcard is unknown, restrictions are placed on the type of methods that may be called on an object that uses parameterized types.\nHere is an example where the element type of a is parameterized by a wildcard:\nSince we don't know what the element type of stands for, we cannot add objects to it.\nThe method takes arguments of type , the element type of the generic interface.\nWhen the actual type argument is, it stands for some unknown type.\nAny method argument value we pass to the method would have to be a subtype of this unknown type.\nSince we don't know what type that is, we cannot pass anything in.\nThe sole exception is [null](https://bliptext.com/articles/null-pointer); which is a member of every type.\nTo specify the [upper bound](https://bliptext.com/articles/bounded-quantification) of a type wildcard, the keyword is used to indicate that the type argument is a subtype of the bounding class.\nSo means that the given list contains objects of some unknown type which extends the class.\nFor example, the list could be or.\nReading an element from the list will return a.\nAdding null elements is, again, also allowed.\nThe use of wildcards above adds flexibility since there is not any inheritance relationship between any two parameterized types with concrete type as type argument.\nNeither nor is a subtype of the other; even though is a subtype of.\nSo, any method that takes as a parameter does not accept an argument of.\nIf it did, it would be possible to insert a that is not an into it; which violates type safety.\nHere is an example that demonstrates how type safety would be violated if were a subtype of :\nThe solution with wildcards works because it disallows operations that would violate type safety:\nTo specify the lower bounding class of a type wildcard, the keyword is used.\nThis keyword indicates that the type argument is a supertype of the bounding class.\nSo, could represent or.\nReading from a list defined as returns elements of type.\nAdding to such a list requires either elements of type, any subtype of or [null](https://bliptext.com/articles/null-pointer) (which is a member of every type).\nThe mnemonic PECS (Producer Extends, Consumer Super) from the book **Effective Java** by [Joshua Bloch](https://bliptext.com/articles/joshua-bloch) gives an easy way to remember when to use wildcards (corresponding to [covariance](https://bliptext.com/articles/covariance-and-[contravariance](https://bliptext.com/articles/covariance-and-contravariance-computer-science)-computer-science) and [contravariance](https://bliptext.com/articles/covariance-and-contravariance-computer-science)) in Java.\n\n# Generics in throws clause\nAlthough exceptions themselves cannot be generic, generic parameters can appear in a throws clause:\n\n# Problems with type erasure\nGenerics are checked at compile-time for type-correctness.\nThe generic type information is then removed in a process called [type erasure](https://bliptext.com/articles/type-erasure).\nFor example, will be converted to the non-generic type , which ordinarily contains arbitrary objects.\nThe compile-time check guarantees that the resulting code uses the correct type.\nBecause of type erasure, type parameters cannot be determined at run-time.\nFor example, when an is examined at runtime, there is no general way to determine whether, before type erasure, it was an or an.\nMany people are dissatisfied with this restriction.\nThere are partial approaches.\nFor example, individual elements may be examined to determine the type they belong to; for example, if an contains an , that ArrayList may have been parameterized with (however, it may have been parameterized with any parent of , such as or ).\nDemonstrating this point, the following code outputs \"Equal\":\nAnother effect of type erasure is that a generic class cannot extend the class in any way, directly or indirectly:\nThe reason why this is not supported is due to type erasure:\nDue to type erasure, the runtime will not know which catch block to execute, so this is prohibited by the compiler.\nJava generics differ from [C++ templates](https://bliptext.com/articles/c-templates).\nJava generics generate only one compiled version of a generic class or function regardless of the number of parameterizing types used.\nFurthermore, the Java run-time environment does not need to know which parameterized type is used because the type information is validated at compile-time and is not included in the compiled code.\nConsequently, instantiating a Java class of a parameterized type is impossible because instantiation requires a call to a constructor, which is unavailable if the type is unknown.\nFor example, the following code cannot be compiled:\nBecause there is only one copy per generic class at runtime, [static variables](https://bliptext.com/articles/static-variable) are shared among all the instances of the class, regardless of their type parameter.\nConsequently, the type parameter cannot be used in the declaration of static variables or in static methods.\nType erasure was implemented in Java to maintain backward compatibility with programs written prior to Java SE5.\n\n# Differences from Arrays\nThere are several important differences between arrays (both primitive arrays and Object arrays), and generics in Java.\nTwo of the major differences, namely, differences in terms of [variance](https://bliptext.com/articles/variance-computer-science) and [reification](https://bliptext.com/articles/reification-computer-science).\n\n## Covariance, contravariance and invariance\nGenerics are invariant, whereas arrays are [covariant](https://bliptext.com/articles/covariance-and-contravariance-computer-science).\nThis is a benefit of using generic when compared to non-generic objects such as arrays.\nSpecifically, generics can help prevent run time exceptions by throwing a compile-time exception to force the developer to fix the code.\nFor example, if a developer declares an Object[] object and instantiates the object as a new Long[] object, no compile-time exception is thrown (since arrays are covariant).\nThis may give the false impression that the code is correctly written.\nHowever, if the developer attempts to add a String to this Long[] object, the program will throw an ArrayStoreException.\nThis run-time exception can be completely avoided if the developer uses generics.\nIf the developer declares a Collection

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