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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.
In computer programming, a collection is an abstract data type that is a grouping of items that can be used in a polymorphic way. Often, the items are of the same data type such as int or string . Sometimes the items derive from a common type; even deriving from the most general type of a programming language such as object or variant .
Collection classes are Java API-defined classes that can store objects in a manner similar to how data structures like arrays store primitive data types like int, double, long or char, etc., [2] but arrays store primitive data types while collections actually store objects. The primitive wrapper classes and their corresponding primitive types are:
Array, a sequence of elements of the same type stored contiguously in memory; Record (also called a structure or struct), a collection of fields . Product type (also called a tuple), a record in which the fields are not named
A list may contain the same value more than once, and each occurrence is considered a distinct item. A singly-linked list structure, implementing a list with three integer elements. The term list is also used for several concrete data structures that can be used to implement abstract lists, especially linked lists and arrays.
According to Java Language Specification: [5] A type variable is an unqualified identifier. Type variables are introduced by generic class declarations, generic interface declarations, generic method declarations, and by generic constructor declarations. A class is generic if it declares one or more type variables. [6]
In the Java Collections Framework, the class List<MyClass> represents an ordered collection of objects of type MyClass. Upper bounds are specified using extends : A List<? extends MyClass> is a list of objects of some subclass of MyClass , i.e. any object in the list is guaranteed to be of type MyClass , so one can iterate over it using a ...
/* This class has two type variables, T and V. T must be a subtype of ArrayList and implement Formattable interface */ public class Mapper < T extends ArrayList & Formattable, V > {public void add (T array, V item) {// array has add method because it is an ArrayList subclass array. add (item);}}