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# Package java.util

Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).

See: Description

Interface Summary
Collection<E>: The root interface in the collection hierarchy.
Comparator<T>: A comparison function, which imposes a total ordering on some collection of objects.
Deque<E>: A linear collection that supports element insertion and removal at both ends.
Enumeration<E>: An object that implements the Enumeration interface generates a series of elements, one at a time.
EventListener: A tagging interface that all event listener interfaces must extend.
Formattable: The `Formattable` interface must be implemented by any class that needs to perform custom formatting using the `'s'` conversion specifier of `Formatter`.
Iterator<E>: An iterator over a collection.
List<E>: An ordered collection (also known as a sequence).
ListIterator<E>: An iterator for lists that allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the iterator's current position in the list.
Map<K,V>: An object that maps keys to values.
Map.Entry<K,V>: A map entry (key-value pair).
NavigableMap<K,V>: A `SortedMap` extended with navigation methods returning the closest matches for given search targets.
NavigableSet<E>: A `SortedSet` extended with navigation methods reporting closest matches for given search targets.
Observer: A class can implement the `Observer` interface when it wants to be informed of changes in observable objects.
PrimitiveIterator<T,T_CONS>: A base type for primitive specializations of `Iterator`.
PrimitiveIterator.OfDouble: An Iterator specialized for `double` values.
PrimitiveIterator.OfInt: An Iterator specialized for `int` values.
PrimitiveIterator.OfLong: An Iterator specialized for `long` values.
Queue<E>: A collection designed for holding elements prior to processing.
RandomAccess: Marker interface used by `List` implementations to indicate that they support fast (generally constant time) random access.
Set<E>: A collection that contains no duplicate elements.
SortedMap<K,V>: A `Map` that further provides a total ordering on its keys.
SortedSet<E>: A `Set` that further provides a total ordering on its elements.
Spliterator<T>: An object for traversing and partitioning elements of a source.
Spliterator.OfDouble: A Spliterator specialized for `double` values.
Spliterator.OfInt: A Spliterator specialized for `int` values.
Spliterator.OfLong: A Spliterator specialized for `long` values.
Spliterator.OfPrimitive<T,T_CONS,T_SPLITR extends Spliterator.OfPrimitive<T,T_CONS,T_SPLITR>>: A Spliterator specialized for primitive values.

Class Summary
AbstractCollection<E>: This class provides a skeletal implementation of the `Collection` interface, to minimize the effort required to implement this interface.
AbstractList<E>: This class provides a skeletal implementation of the `List` interface to minimize the effort required to implement this interface backed by a "random access" data store (such as an array).
AbstractMap<K,V>: This class provides a skeletal implementation of the `Map` interface, to minimize the effort required to implement this interface.
AbstractMap.SimpleEntry<K,V>: An Entry maintaining a key and a value.
AbstractMap.SimpleImmutableEntry<K,V>: An Entry maintaining an immutable key and value.
AbstractQueue<E>: This class provides skeletal implementations of some `Queue` operations.
AbstractSequentialList<E>: This class provides a skeletal implementation of the `List` interface to minimize the effort required to implement this interface backed by a "sequential access" data store (such as a linked list).
AbstractSet<E>: This class provides a skeletal implementation of the `Set` interface to minimize the effort required to implement this interface.
ArrayDeque<E>: Resizable-array implementation of the `Deque` interface.
ArrayList<E>: Resizable-array implementation of the `List` interface.
Arrays: This class contains various methods for manipulating arrays (such as sorting and searching).
Base64: This class consists exclusively of static methods for obtaining encoders and decoders for the Base64 encoding scheme.
Base64.Decoder: This class implements a decoder for decoding byte data using the Base64 encoding scheme as specified in RFC 4648 and RFC 2045.
Base64.Encoder: This class implements an encoder for encoding byte data using the Base64 encoding scheme as specified in RFC 4648 and RFC 2045.
BitSet: This class implements a vector of bits that grows as needed.
Calendar: The `Calendar` class is an abstract class that provides methods for converting between a specific instant in time and a set of `calendar fields` such as `YEAR`, `MONTH`, `DAY_OF_MONTH`, `HOUR`, and so on, and for manipulating the calendar fields, such as getting the date of the next week.
Calendar.Builder: `Calendar.Builder` is used for creating a `Calendar` from various date-time parameters.
Collections: This class consists exclusively of static methods that operate on or return collections.
Currency: Represents a currency.
Date: The class `Date` represents a specific instant in time, with millisecond precision.
Dictionary<K,V>: The `Dictionary` class is the abstract parent of any class, such as `Hashtable`, which maps keys to values.
DoubleSummaryStatistics: A state object for collecting statistics such as count, min, max, sum, and average.
EnumMap<K extends Enum<K>,V>: A specialized `Map` implementation for use with enum type keys.
EnumSet<E extends Enum<E>>: A specialized `Set` implementation for use with enum types.
EventListenerProxy<T extends EventListener>: An abstract wrapper class for an `EventListener` class which associates a set of additional parameters with the listener.
EventObject: The root class from which all event state objects shall be derived.
FormattableFlags: FomattableFlags are passed to the `Formattable.formatTo()` method and modify the output format for Formattables.
Formatter: An interpreter for printf-style format strings.
GregorianCalendar: `GregorianCalendar` is a concrete subclass of `Calendar` and provides the standard calendar system used by most of the world.
HashMap<K,V>: Hash table based implementation of the `Map` interface.
HashSet<E>: This class implements the `Set` interface, backed by a hash table (actually a `HashMap` instance).
Hashtable<K,V>: This class implements a hash table, which maps keys to values.
IdentityHashMap<K,V>: This class implements the `Map` interface with a hash table, using reference-equality in place of object-equality when comparing keys (and values).
IntSummaryStatistics: A state object for collecting statistics such as count, min, max, sum, and average.
LinkedHashMap<K,V>: Hash table and linked list implementation of the `Map` interface, with predictable iteration order.
LinkedHashSet<E>: Hash table and linked list implementation of the `Set` interface, with predictable iteration order.
LinkedList<E>: Doubly-linked list implementation of the `List` and `Deque` interfaces.
ListResourceBundle: `ListResourceBundle` is an abstract subclass of `ResourceBundle` that manages resources for a locale in a convenient and easy to use list.
Locale: A `Locale` object represents a specific geographical, political, or cultural region.
Locale.Builder: `Builder` is used to build instances of `Locale` from values configured by the setters.
Locale.LanguageRange: This class expresses a Language Range defined in RFC 4647 Matching of Language Tags.
LongSummaryStatistics: A state object for collecting statistics such as count, min, max, sum, and average.
Objects: This class consists of `static` utility methods for operating on objects.
Observable: This class represents an observable object, or "data" in the model-view paradigm.
Optional<T>: A container object which may or may not contain a non-null value.
OptionalDouble: A container object which may or may not contain a `double` value.
OptionalInt: A container object which may or may not contain a `int` value.
OptionalLong: A container object which may or may not contain a `long` value.
PriorityQueue<E>: An unbounded priority queue based on a priority heap.
Properties: The `Properties` class represents a persistent set of properties.
PropertyPermission: This class is for property permissions.
PropertyResourceBundle: `PropertyResourceBundle` is a concrete subclass of `ResourceBundle` that manages resources for a locale using a set of static strings from a property file.
Random: An instance of this class is used to generate a stream of pseudorandom numbers.
ResourceBundle: Resource bundles contain locale-specific objects.
ResourceBundle.Control: `ResourceBundle.Control` defines a set of callback methods that are invoked by the `ResourceBundle.getBundle` factory methods during the bundle loading process.
Scanner: A simple text scanner which can parse primitive types and strings using regular expressions.
ServiceLoader<S>: A simple service-provider loading facility.
SimpleTimeZone: `SimpleTimeZone` is a concrete subclass of `TimeZone` that represents a time zone for use with a Gregorian calendar.
Spliterators: Static classes and methods for operating on or creating instances of `Spliterator` and its primitive specializations `Spliterator.OfInt`, `Spliterator.OfLong`, and `Spliterator.OfDouble`.
Spliterators.AbstractDoubleSpliterator: An abstract `Spliterator.OfDouble` that implements `trySplit` to permit limited parallelism.
Spliterators.AbstractIntSpliterator: An abstract `Spliterator.OfInt` that implements `trySplit` to permit limited parallelism.
Spliterators.AbstractLongSpliterator: An abstract `Spliterator.OfLong` that implements `trySplit` to permit limited parallelism.
Spliterators.AbstractSpliterator<T>: An abstract `Spliterator` that implements `trySplit` to permit limited parallelism.
SplittableRandom: A generator of uniform pseudorandom values applicable for use in (among other contexts) isolated parallel computations that may generate subtasks.
Stack<E>: The `Stack` class represents a last-in-first-out (LIFO) stack of objects.
StringJoiner: `StringJoiner` is used to construct a sequence of characters separated by a delimiter and optionally starting with a supplied prefix and ending with a supplied suffix.
StringTokenizer: The string tokenizer class allows an application to break a string into tokens.
Timer: A facility for threads to schedule tasks for future execution in a background thread.
TimerTask: A task that can be scheduled for one-time or repeated execution by a Timer.
TimeZone: `TimeZone` represents a time zone offset, and also figures out daylight savings.
TreeMap<K,V>: A Red-Black tree based `NavigableMap` implementation.
TreeSet<E>: A `NavigableSet` implementation based on a `TreeMap`.
UUID: A class that represents an immutable universally unique identifier (UUID).
Vector<E>: The `Vector` class implements a growable array of objects.
WeakHashMap<K,V>: Hash table based implementation of the `Map` interface, with weak keys.

Enum Summary
Formatter.BigDecimalLayoutForm: Enum for `BigDecimal` formatting.
Locale.Category: Enum for locale categories.
Locale.FilteringMode: This enum provides constants to select a filtering mode for locale matching.

Exception Summary
ConcurrentModificationException: This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.
DuplicateFormatFlagsException: Unchecked exception thrown when duplicate flags are provided in the format specifier.
EmptyStackException: Thrown by methods in the `Stack` class to indicate that the stack is empty.
FormatFlagsConversionMismatchException: Unchecked exception thrown when a conversion and flag are incompatible.
FormatterClosedException: Unchecked exception thrown when the formatter has been closed.
IllegalFormatCodePointException: Unchecked exception thrown when a character with an invalid Unicode code point as defined by `Character.isValidCodePoint(int)` is passed to the `Formatter`.
IllegalFormatConversionException: Unchecked exception thrown when the argument corresponding to the format specifier is of an incompatible type.
IllegalFormatException: Unchecked exception thrown when a format string contains an illegal syntax or a format specifier that is incompatible with the given arguments.
IllegalFormatFlagsException: Unchecked exception thrown when an illegal combination flags is given.
IllegalFormatPrecisionException: Unchecked exception thrown when the precision is a negative value other than `-1`, the conversion does not support a precision, or the value is otherwise unsupported.
IllegalFormatWidthException: Unchecked exception thrown when the format width is a negative value other than `-1` or is otherwise unsupported.
IllformedLocaleException: Thrown by methods in `Locale` and `Locale.Builder` to indicate that an argument is not a well-formed BCP 47 tag.
InputMismatchException: Thrown by a `Scanner` to indicate that the token retrieved does not match the pattern for the expected type, or that the token is out of range for the expected type.
InvalidPropertiesFormatException: Thrown to indicate that an operation could not complete because the input did not conform to the appropriate XML document type for a collection of properties, as per the `Properties` specification.
MissingFormatArgumentException: Unchecked exception thrown when there is a format specifier which does not have a corresponding argument or if an argument index refers to an argument that does not exist.
MissingFormatWidthException: Unchecked exception thrown when the format width is required.
MissingResourceException: Signals that a resource is missing.
NoSuchElementException: Thrown by various accessor methods to indicate that the element being requested does not exist.
TooManyListenersException: The `TooManyListenersException` Exception is used as part of the Java Event model to annotate and implement a unicast special case of a multicast Event Source.
UnknownFormatConversionException: Unchecked exception thrown when an unknown conversion is given.
UnknownFormatFlagsException: Unchecked exception thrown when an unknown flag is given.

Error Summary
ServiceConfigurationError: Error thrown when something goes wrong while loading a service provider.

## Package java.util Description

Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).

## Package Specification

- Collections Framework Overview
- Collections Framework Annotated Outline

## Related Documentation

For overviews, tutorials, examples, guides, and tool documentation, please see:

- Collections Framework Tutorial
- Collections Framework Design FAQ

Since:
  JDK1.0