The Java Connections FrameWork is a group of class or method and interfacs in the java.util package. Its main purpose is to provide a unified framework for implementing common data structure. A collections is a object that contains other objects,which are called that elements of the collections . The JCF specifies four general types of collections: List, Queue, Set and Map.  A list is a sequence of elements.A Queue is a first-in-first-out collections line waitingline. A set is an unstructured collections of distinct elements. A map is a collection of components pairs that works like an index or dictionary.

The JCF implements the interfaces with several different data structures. The simplest structure is an indexed structure, that is, an array, which is used by the ArrayList, the ArrayDeque,the HashSet, and the HashMapclasses. Other implementations use a linked structure. The LinkedListclass uses a doubly linked linear structure, the PriorityQueueclass uses a heap tree, the TreeSetand TreeMapclasses use a linked binary search tree, and the LinkedHashSet, andLinkedHashMapclasses use a hybrid linked array structure. The specialized EnumSetand EnumMapclasses use a bit string. These twelve implementations are summarized in Table 4.1.