Collections Framework Overview


The Java platform includes a collections framework. A collection is an object that represents a group of objects (such as the classic ArrayList class). A collections framework is a unified architecture for representing and manipulating collections, enabling collections to be manipulated independently of implementation details.

The primary advantages of a collections framework are that it:

The collections framework consists of:

Collection Interfaces

The collection interfaces are divided into two groups. The most basic interface, java.util.Collection, has the following descendants:

The other collection interfaces are based on java.util.Map and are not true collections. However, these interfaces contain collection-view operations, which enable them to be manipulated as collections. Map has the following offspring:

Many of the modification methods in the collection interfaces are labeled optional. Implementations are permitted to not perform one or more of these operations, throwing a runtime exception (UnsupportedOperationException) if they are attempted. The documentation for each implementation must specify which optional operations are supported. Several terms are introduced to aid in this specification:

Some implementations restrict what elements (or in the case of Maps, keys and values) can be stored. Possible restrictions include requiring elements to:

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 can result in an exception. Some restricted collections permit this usage.

Collection Implementations

Classes that implement the collection interfaces typically have names in the form of <Implementation-style><Interface>. The general purpose implementations are summarized in the following table:

General purpose implementations
Interface Hash Table Resizable Array Balanced Tree Linked List Hash Table + Linked List
Set HashSet   TreeSet   LinkedHashSet
List   ArrayList   LinkedList  
Deque   ArrayDeque   LinkedList  
Map HashMap   TreeMap   LinkedHashMap

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 can be used to add synchronization to many unsynchronized collections. All of the new implementations have fail-fast iterators, which detect invalid concurrent modification, and fail quickly and cleanly (rather than behaving erratically).

The AbstractCollection, AbstractSet, AbstractList, AbstractSequentialList and AbstractMap classes provide basic 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 must be overridden, given the performance of the basic operations of a specific implementation.

Concurrent Collections

Applications that use collections from more than one thread must be carefully programmed. In general, this is known as concurrent programming. The Java platform includes extensive support for concurrent programming. See Java Concurrency Utilities for details.

Collections are so frequently used that various concurrent friendly interfaces and implementations of collections are included in the APIs. These types go beyond the synchronization wrappers discussed previously to provide features that are frequently needed in concurrent programming.

These concurrent-aware interfaces are available:

The following concurrent-aware implementation classes are available. See the API documentation for the correct usage of these implementations.

Design Goals

The main design goal was to produce an API that was small in size and, more importantly, in "conceptual weight." It was critical that the new functionality not seem too different to current Java programmers; it had to augment current facilities, rather than replace them. At the same time, the new API had to be powerful enough to provide all the advantages described previously.

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, enabling implementations to throw an UnsupportedOperationException to indicate that they do not support a specified optional operation. 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 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 enable collections to be moved into arrays, arrays to be viewed as collections, and maps to be viewed as collections.

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