Collection interfaces - The primary means by
which collections are manipulated.
- A group of objects. No assumptions are made about the order of
the collection (if any) or whether it can contain duplicate
Set - The
familiar set abstraction. No duplicate elements permitted. May or
may not be ordered. Extends the Collection interface.
Ordered collection, also known as a sequence. Duplicates are
generally permitted. Allows positional access. Extends the
Queue - A
collection designed for holding elements before processing. Besides
basic Collection operations, queues provide additional
insertion, extraction, and inspection operations.
Deque - A
double ended queue, supporting element insertion and
removal at both ends. Extends the Queue interface.
Map - A
mapping from keys to values. Each key can map to one value.
- A set whose elements are automatically sorted, either in their
natural ordering (see the Comparable
interface) or by a Comparator
object provided when a SortedSet instance is created.
Extends the Set interface.
- A map whose mappings are automatically sorted by key, either
using the natural ordering of the keys or by a comparator
provided when a SortedMap instance is created. Extends the
- A SortedSet extended with navigation methods reporting
closest matches for given search targets. A NavigableSet
may be accessed and traversed in either ascending or descending
- A SortedMap extended with navigation methods returning
the closest matches for given search targets. A
NavigableMap can be accessed and traversed in either
ascending or descending key order.
- A Queue with operations that wait for the queue to
become nonempty when retrieving an element and that wait for space
to become available in the queue when storing an element. (This
interface is part of the java.util.concurrent
- A Deque with operations that wait for the deque to
become nonempty when retrieving an element and wait for space to
become available in the deque when storing an element. Extends both
the Deque and BlockingQueue interfaces. (This
interface is part of the java.util.concurrent
- A Map with atomic putIfAbsent, remove,
and replace methods. (This interface is part of the
General-purpose implementations - The primary
implementations of the collection interfaces.
HashSet - Hash
table implementation of the Set interface. The best
all-around implementation of the Set interface.
- Red-black tree implementation of the NavigableSet
- Hash table and linked list implementation of the Set
interface. An insertion-ordered Set implementation that
runs nearly as fast as HashSet.
Resizable array implementation of the List interface (an
unsynchronized Vector). The best all-around implementation
of the List interface.
Efficient, resizable array implementation of the Deque
- Doubly-linked list implementation of the List interface.
Provides better performance than the ArrayList
implementation if elements are frequently inserted or deleted
within the list. Also implements the Deque interface. When
accessed through the Queue interface, LinkedList
acts as a FIFO queue.
- Heap implementation of an unbounded priority queue.
HashMap - Hash
table implementation of the Map interface (an
unsynchronized Hashtable that supports null keys
and values). The best all-around implementation of the Map
Red-black tree implementation of the NavigableMap
- Hash table and linked list implementation of the Map
interface. An insertion-ordered Map implementation that
runs nearly as fast as HashMap. Also useful for building
Wrapper implementations -
Functionality-enhancing implementations for use with other
implementations. Accessed solely through static factory methods.
Returns an unmodifiable view of a specified collection that throws
an UnsupportedOperationException if the user attempts to
- Returns a synchronized collection that is backed by the specified
(typically unsynchronized) collection. As long as all accesses to
the backing collection are through the returned collection, thread
safety is guaranteed.
Collections.checkedInterface - Returns
a dynamically type-safe view of the specified collection, which
throws a ClassCastException if a client attempts to add an
element of the wrong type. The generics mechanism in the language
provides compile-time (static) type checking, but it is possible to
bypass this mechanism. Dynamically type-safe views eliminate this
Adapter implementations - Implementations that
adapt one collections interface to another:
newSetFromMap(Map) - Creates a general-purpose
Set implementation from a general-purpose Map
nCopies - Returns an immutable list consisting of n
copies of a specified object.
Legacy implementations - Older collection
classes were retrofitted to implement the collection interfaces.
Synchronized resizable array implementation of the List
interface with additional legacy methods.
- Synchronized hash table implementation of the Map
interface that does not allow null keys or values, plus
additional legacy methods.
- An implementation of the Map interface that stores only
references to its keys. Storing only weak references
enables key-value pairs to be garbage collected when the key is no
longer referenced outside of the WeakHashMap. This class
is the easiest way to use the power of weak references. It is
useful for implementing registry-like data structures, where the
utility of an entry vanishes when its key is no longer reachable by
- Identity-based Map implementation based on a hash table.
This class is useful for topology-preserving object graph
transformations (such as serialization or deep copying). To perform
these transformations, you must maintain an identity-based "node
table" that keeps track of which objects have already been seen.
Identity-based maps are also used to maintain
object-to-meta-information mappings in dynamic debuggers and
similar systems. Finally, identity-based maps are useful in
preventing "spoof attacks" resulting from intentionally perverse
equals methods. (IdentityHashMap never invokes the equals
method on its keys.) An added benefit of this implementation is
that it is fast.
- A List implementation backed by an copy-on-write array.
All mutative operations (such as add, set, and
remove) are implemented by making a new copy of the array.
No synchronization is necessary, even during iteration, and
iterators are guaranteed never to throw
ConcurrentModificationException. This implementation is
well-suited to maintaining event-handler lists (where change is
infrequent, and traversal is frequent and potentially
- A Set implementation backed by a copy-on-write array.
This implementation is similar to CopyOnWriteArrayList.
Unlike most Set implementations, the add,
remove, and contains methods require time
proportional to the size of the set. This implementation is well
suited to maintaining event-handler lists that must prevent
EnumSet - A
high-performance Set implementation backed by a bit
vector. All elements of each EnumSet instance must be
elements of a single enum type.
EnumMap - A
high-performance Map implementation backed by an array.
All keys in each EnumMap instance must be elements of a
single enum type.
Concurrent implementations - These
implementations are part of java.util.concurrent.
- A highly concurrent, high-performance ConcurrentMap
implementation based on a hash table. This implementation never
blocks when performing retrievals and enables the client to select
the concurrency level for updates. It is intended as a drop-in
replacement for Hashtable. In
addition to implementing ConcurrentMap, it supports all of
the legacy methods of Hashtable.
Algorithms - The Collections
class contains these useful static methods.
- Sorts a list using a merge sort algorithm, which provides average
case performance comparable to a high quality quicksort, guaranteed
O(n*log n) performance (unlike quicksort), and stability
(unlike quicksort). A stable sort is one that does not reorder
Iterators - Similar to the familiar Enumeration
interface, but more powerful, and with improved method names.
- In addition to the functionality of the Enumeration
interface, enables the user to remove elements from the backing
collection with well-defined, useful semantics.
- Iterator for use with lists. In addition to the functionality of
the Iterator interface, supports bidirectional iteration,
element replacement, element insertion, and index retrieval.
- Imparts a natural ordering to classes that implement it.
The natural ordering can be used to sort a list or maintain order
in a sorted set or map. Many classes were retrofitted to implement
- Represents an order relation, which can be used to sort a list or
maintain order in a sorted set or map. Can override a type's
natural ordering or order objects of a type that does not implement
the Comparable interface.
ConcurrentModificationException - Thrown by iterators
and list iterators if the backing collection is changed
unexpectedly while the iteration is in progress. Also thrown by
sublist views of lists if the backing list is changed
- Marker interface that lets List implementations indicate
that they support fast (generally constant time) random access.
This lets generic algorithms change their behavior to provide good
performance when applied to either random or sequential access
Contains static methods to sort, search, compare, hash, copy,
resize, convert to String, and fill arrays of primitives