Added in API level 1

java.util

Contains the collections framework, some internationalization support classes, a service loader, properties, random number generation, string parsing and scanning classes, base64 encoding and decoding, a bit array, and several miscellaneous utility classes. This package also contains legacy collection classes and legacy date and time classes.

"Java Collections Framework"

For an overview, API outline, and design rationale, please see:

For a tutorial and programming guide with examples of use of the collections framework, please see:

Interfaces

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, where the user has precise control over where in the list each element is inserted. 
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 This interface was deprecated in API level 33. This interface has been deprecated. See the Observable class for further information. 
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. 
SequencedCollection<E> A collection that has a well-defined encounter order, that supports operations at both ends, and that is reversible. 
SequencedMap<K, V> A Map that has a well-defined encounter order, that supports operations at both ends, and that is reversible. 
SequencedSet<E> A collection that is both a SequencedCollection and a Set. 
ServiceLoader.Provider<S> Represents a service provider located by ServiceLoader. 
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 OfPrimitive<T, T_CONS, T_SPLITR>> A Spliterator specialized for primitive values. 

Classes

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 unmodifiable Entry maintaining a key and a 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 FormattableFlags 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. 
HexFormat HexFormat converts between bytes and chars and hex-encoded strings which may include additional formatting markup such as prefixes, suffixes, and delimiters. 
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 well-defined encounter order. 

LinkedHashSet<E>

Hash table and linked list implementation of the Set interface, with well-defined encounter 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, or checking certain conditions before operation. 
Observable This class was deprecated in API level 33. This class and the Observer interface have been deprecated. The event model supported by Observer and Observable is quite limited, the order of notifications delivered by Observable is unspecified, and state changes are not in one-for-one correspondence with notifications. For a richer event model, consider using the java.beans package. For reliable and ordered messaging among threads, consider using one of the concurrent data structures in the java.util.concurrent package. For reactive streams style programming, see the Flow API. 
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 an 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 Legacy security code; do not use. 
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; its period is only 248. 
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 facility to load implementations of a service. 
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 (with period 264) 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. 

Enums

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. 
Locale.IsoCountryCode Enum for specifying the type defined in ISO 3166. 

Exceptions

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 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. 

Errors

ServiceConfigurationError Error thrown when something goes wrong while locating, loading, or instantiating a service provider.