Collections Framework Overview
The Java 2 platform includes a collections
framework. A collection is an object that represents a
group of objects (such as the familiar Vector class). A
collections framework is a unified architecture for representing and
manipulating collections, allowing them to be manipulated
independently of the details of their representation.
The primary advantages of a collections framework are that it:
- Reduces programming effort by providing useful
data structures and algorithms so you don't have to write them
- Increases performance by providing
high-performance implementations of useful data structures and
algorithms. Because the various implementations of each interface are
interchangeable, programs can be easily tuned by switching
- Provides interoperability between unrelated APIs
by establishing a common language to pass collections back and forth.
- Reduces the effort required to learn APIs by
eliminating the need to learn multiple ad hoc collection APIs.
- Reduces the effort required to design and implement
APIs by eliminating the need to produce ad hoc collections
- Fosters software reuse by providing a standard
interface for collections and algorithms to manipulate them.
The collections framework consists of:
- Collection Interfaces - Represent different types
of collections, such as sets, lists and maps. These interfaces form
the basis of the framework.
- General-purpose Implementations - Primary
implementations of the collection interfaces.
- Legacy Implementations - The collection classes
from earlier releases, Vector and Hashtable, have
been retrofitted to implement the collection interfaces.
- Special-purpose Implementations - Implementations
designed for use in special situations. These implementations
display nonstandard performance characteristics, usage restrictions, or
- Concurrent Implementations - Implementations
designed for highly concurrent use.
- Wrapper Implementations - Add functionality,
such as synchronization, to other implementations.
- Convenience Implementations - High-performance
"mini-implementations" of the collection interfaces.
- Abstract Implementations - Partial
implementations of the collection interfaces to facilitate custom
- Algorithms - Static methods that perform
useful functions on collections, such as sorting a list.
- Infrastructure - Interfaces that provide essential
support for the collection interfaces.
- Array Utilities - Utility functions for arrays of
primitives and reference objects. Not, strictly speaking, a part of the
Collections Framework, this functionality was added to the Java
platform at the same time and relies on some of the same infrastructure.
There are nine collection interfaces. The most basic interface
is Collection. Five interfaces extend Collection:
Set, List, SortedSet, Queue, and
The other three collection interfaces, Map, SortedMap, and
ConcurrentMap do not extend Collection, as they represent
mappings rather than true collections. However, these interfaces contain
collection-view operations, which allow them to be manipulated
All of the modification methods in the collection interfaces are labeled
optional. Some implementations may not perform one or more of these
operations, throwing a runtime exception
(UnsupportedOperationException) if they are attempted.
Implementations must specify in their documentation which optional
operations they support. Several terms are introduced to aid in this
- Collections that do not support any modification operations (such as
add, remove and clear) are referred to as
unmodifiable. Collections that are not unmodifiable are referred to
- Collections that additionally guarantee that no change in the
Collection object will ever be visible are referred to as
immutable. Collections that are not immutable are referred to
- Lists that guarantee that their size remains constant even though
the elements may change are referred to as fixed-size. Lists
that are not fixed-size are referred to as variable-size.
- Lists that support fast (generally constant time) indexed element
access are known as random access lists. Lists that do not support
fast indexed element access are known as sequential access lists.
marker interface is provided to allow lists to advertise the fact that they
support random access. This allows generic algorithms to alter their behavior
to provide good performance when applied to either random or sequential
Some implementations may restrict what elements (or in the case of
Maps, keys and values) may be stored. Possible restrictions
include requiring elements to:
- Be of a particular type.
- Be non-null.
- Obey some arbitrary predicate.
Attempting to add an element that violates an implementation's
restrictions results in a runtime exception, typically a
ClassCastException, an IllegalArgumentException or a
NullPointerException. Attempting to remove or test for the
presence of an element that violates an implementation's restrictions
may result in an exception, though some "restricted collections" may
permit this usage.
Classes that implement the collection interfaces typically have names of
general purpose implementations are summarized in the table below:
The general-purpose implementations support all of the optional
operations in the collection interfaces, and have no restrictions on the
elements they may contain. They are unsynchronized, but the
Collections class contains static factories called
synchronization wrappers that may be used to add synchronization
to any unsynchronized collection. All of the new implementations have
fail-fast iterators, which detect illegal concurrent modification, and
fail quickly and cleanly (rather than behaving erratically).
The AbstractCollection, AbstractSet,
AbstractList, AbstractSequentialList and
AbstractMap classes provide skeletal implementations of the
core collection interfaces, to minimize the effort required to
implement them. The API documentation for these classes describes
precisely how each method is implemented so the implementer knows
which methods should be overridden, given the performance of the
"basic operations" of a specific implementation.
The main design goal was to produce an API that was reasonably
small, both in size, and, more importantly, in "conceptual
weight." It was critical that the new functionality not seem
alien to current Java programmers; it had to augment current
facilities, rather than replacing them. At the same time, the new API
had to be powerful enough to provide all the advantages described
To keep the number of core interfaces small, the interfaces do not
attempt to capture such subtle distinctions as mutability,
modifiability, and resizability. Instead, certain calls in the core
interfaces are optional, allowing implementations to throw an
UnsupportedOperationException to indicate that they do not
support a specified optional operation. Of course, collection
implementers must clearly document which optional operations are
supported by an implementation.
To keep the number of methods in each core interface small, an
interface contains a method only if either:
- It is a truly fundamental operation: a basic operations in
terms of which others could be reasonably defined,
- There is a compelling performance reason why an important
implementation would want to override it.
It was critical that all reasonable representations of collections
interoperate well. This included arrays, which cannot be made to
implement the Collection interface directly without changing
the language. Thus, the framework includes methods to allow
collections to be dumped into arrays, arrays to be viewed as
collections, and maps to be viewed as collections.