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Java Pocket Guide by Patricia Liguori, Robert Liguori

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Chapter 12. Java Collections Framework

The Java Collections Framework is designed to support numerous collections in a hierarchical fashion. It is essentially made up of interfaces, implementations, and algorithms.

The Collection Interface

Collections are objects that group multiple elements and store, retrieve, and manipulate those elements. The interface Collection is at the root of the collection hierarchy. Subinterfaces of Collection include List, Queue, and Set. Table 12-1 shows these interfaces and whether they are ordered or allow duplicates. The interface Map is also included in the table, as it is part of the framework.

Table 12-1. Common collections

Interface

Ordered

Dupes

Notes

List

Yes

Yes

Positional access. Element insertion control.

Map

Can be

No (Keys)

Unique keys. One value mapping max per key.

Queue

Yes

Yes

Holds elements. Usually FIFO.

Set

Can be

No

Uniqueness matters.

Implementations

Table 12-2 lists commonly used collection type implementations, their interfaces, and whether or not they are ordered, sorted, and/or contain duplicates.

Table 12-2. Collection type implementations

Implementations

Interface

Ordered

Sorted

Dupes

Notes

ArrayList

List

Index

No

Yes

Fast resizable array

LinkedList

List

Index

No

Yes

Doubly linked list

Vector

List

Index

No

Yes

Legacy, synchronized

HashMap

Map

No

No

No

Key/value pairs

Hashtable

Map

No

No

No

Legacy, synchronized

LinkedHashMap

Map

Insertion,last ...

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