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Thus, often the only way to use the clone() method is if the class of an object is known, which is contrary to the abstraction principle of using the most generic type possible. For example, if one has a List reference in Java, one cannot invoke clone() on that reference because List specifies no public clone() method.
Unlike arrays (which are covariant in Java [2]), different instantiations of a generic type are not compatible with each other, not even explicitly. [2] For example, the declarations Generic<Supertype> superGeneric; Generic<Subtype> subGeneric; will cause the compiler to report conversion errors for both castings (Generic<Subtype>)superGeneric and (Generic<Supertype>)subGeneric.
More formally speaking, objects with generic type in Java are non-reifiable types. [6] A non-reifiable type is type whose representation at run-time has less information than its representation at compile-time. [6] Objects with generic type in Java are non-reifiable due to type erasure. [6] Java only enforces type information at compile-time.
The result is a type or value, depending on which sort of generic definition is applied. Generic abstraction enables generic definitions be defined by abstracting a type parameter (of a given kind). Type-indexed types are types that are indexed over the type constructors. These can be used to give types to more involved generic values.
For example, if one has a List reference in Java, one cannot invoke clone() on that reference because List specifies no public clone() method. Actual implementations of List like ArrayList and LinkedList all generally have clone() methods themselves, but it is inconvenient and bad abstraction to carry around the actual class type of an object.
PHP has hundreds of base functions and thousands more from extensions. Prior to PHP version 5.3.0, functions are not first-class functions and can only be referenced by their name, whereas PHP 5.3.0 introduces closures. [35] User-defined functions can be created at any time and without being prototyped. [35]
Collection implementations in pre-JDK 1.2 versions of the Java platform included few data structure classes, but did not contain a collections framework. [4] The standard methods for grouping Java objects were via the array, the Vector, and the Hashtable classes, which unfortunately were not easy to extend, and did not implement a standard member interface.
List comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical set-builder notation (set comprehension) as distinct from the use of map and filter functions.