Java.util Package

 

Java.util Package

It 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).

The java.util package is a built-in package in the Java programming language that provides a collection of utility classes for performing various tasks in Java. Some of the important classes in this package are:

1. ArrayList: An implementation of the List interface that allows for dynamic resizing of the list.

2. LinkedList: An implementation of the List and Queue interfaces that provides a doubly linked list data structure.

3. HashMap: An implementation of the Map interface that provides a key-value mapping.

4. TreeMap: An implementation of the SortedMap interface that provides a key-value mapping sorted by the keys.

5. HashSet: An implementation of the Set interface that provides an unordered collection of unique elements.

6. TreeSet: An implementation of the SortedSet interface that provides an ordered collection of unique elements.

7. Date: A class for representing a specific moment in time.

8. Calendar: A class for manipulating dates and times.

9. Random: A class for generating random numbers.

10. Scanner: A class for parsing input from various sources, such as keyboard and files.

These classes and many others in the java.util package are commonly used in Java programming for a wide range of applications, including data structures, input/output, and date/time manipulation.

Following are the Important Classes in Java.util package :

1. AbstractCollection: This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface.
2. AbstractList: 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).
3. AbstractMap<K,V>: This class provides a skeletal implementation of the Map interface, to minimize the effort required to implement this interface.
4. AbstractMap.SimpleEntry<K,V>: An Entry maintaining a key and a value.
5. AbstractMap.SimpleImmutableEntry<K,V>: An Entry maintaining an immutable key and value.
6. AbstractQueue: This class provides skeletal implementations of some Queue operations.
7. AbstractSequentialList: 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).
8. AbstractSet: This class provides a skeletal implementation of the Set interface to minimize the effort required to implement this interface.
9. ArrayDeque: Resizable-array implementation of the Deque interface.
10. ArrayList: Resizable-array implementation of the List interface.
11. Arrays: This class contains various methods for manipulating arrays (such as sorting and searching).
12. BitSet: This class implements a vector of bits that grows as needed.
13. 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.
14. Collections: This class consists exclusively of static methods that operate on or return collections.
15. Currency: Represents a currency.
16. Date: The class Date represents a specific instant in time, with millisecond precision.
17. Dictionary<K,V>: The Dictionary class is the abstract parent of any class, such as Hashtable, which maps keys to values.
18. EnumMap,V>: A specialized Map implementation for use with enum type keys.
19. EnumSet: A specialized Set implementation for use with enum types.
20. EventListenerProxy: An abstract wrapper class for an EventListener class which associates a set of additional parameters with the listener.
21. EventObject: The root class from which all event state objects shall be derived.
22. FormattableFlags: FomattableFlags are passed to the Formattable.formatTo() method and modify the output format for Formattables.
23. Formatter: An interpreter for printf-style format strings.
24. GregorianCalendar: GregorianCalendar is a concrete subclass of Calendar and provides the standard calendar system used by most of the world.
25. HashMap<K,V>: Hash table based implementation of the Map interface.
26. HashSet: This class implements the Set interface, backed by a hash table (actually a HashMap instance).
27. Hashtable<K,V>: This class implements a hash table, which maps keys to values.
28. 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).
29. LinkedHashMap<K,V>: Hash table and linked list implementation of the Map interface, with predictable iteration order.
30. LinkedHashSet: Hash table and linked list implementation of the Set interface, with predictable iteration order.
31. LinkedList: Doubly-linked list implementation of the List and Deque interfaces.
32. ListResourceBundle: ListResourceBundle is an abstract subclass of ResourceBundle that manages resources for a locale in a convenient and easy to use list.
33. Locale – Set 1, Set 2: A Locale object represents a specific geographical, political, or cultural region.
34. Locale.Builder: Builder is used to build instances of Locale from values configured by the setters.
35. Objects: This class consists of static utility methods for operating on objects.
36. Observable: This class represents an observable object, or “data” in the model-view paradigm.
37. PriorityQueue: An unbounded priority queue based on a priority heap.
38. Properties: The Properties class represents a persistent set of properties.
39. PropertyPermission: This class is for property permissions.
40. PropertyResourceBundle: PropertyResourceBundle is a concrete subclass of ResourceBundle that manages resources for a locale using a set of static strings from a property file.
41. Random: An instance of this class is used to generate a stream of pseudorandom numbers.
42. ResourceBundle: Resource bundles contain locale-specific objects.
43. ResourceBundle.Control: ResourceBundle.Control defines a set of callback methods that are invoked by the ResourceBundle.getBundle factory methods during the bundle loading process.
44. Scanner: A simple text scanner which can parse primitive types and strings using regular expressions.
45. ServiceLoader: A simple service-provider loading facility.
46. SimpleTimeZone: SimpleTimeZone is a concrete subclass of TimeZone that represents a time zone for use with a Gregorian calendar.
47. Stack: The Stack class represents a last-in-first-out (LIFO) stack of objects.
48. StringTokenizer: The string tokenizer class allows an application to break a string into tokens.
49. Timer: A facility for threads to schedule tasks for future execution in a background thread.
50. TimerTask: A task that can be scheduled for one-time or repeated execution by a Timer.
51. TimeZone: TimeZone represents a time zone offset, and also figures out daylight savings.
52. TreeMap<K,V>: A Red-Black tree based NavigableMap implementation.
53. TreeSet: A NavigableSet implementation based on a TreeMap.
54. UUID: A class that represents an immutable universally unique identifier (UUID).
55. Vector: The Vector class implements a growable array of objects.
56. WeakHashMap<K,V>: Hash table based implementation of the Map interface, with weak keys.

The basic utility classes required by the programmer are provided by this #.

Class	Description
AbstractCollection<E>	The class is used for providing a rough implementation of the Collection interface. The class is declared as: public abstract class AbstractCollection<E> extends Object implements Collection<E> The class methods are extended from the Object class.
AbstractList<E>	The class is used for providing a rough implementation of the List interface. The class is declared as: public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> The class methods are extended from the AbstractCollection<E> class.
AbstractMap<K,V>	The class is used for providing a rough implementation of the List interface. The class is declared as: public abstract class AbstractMap<K,V> extends Object implements Map<K,V> The class methods are extended from the Object class.
AbstractMap.SimpleEntry<K,V>	The class instance is used for maintaining the value and key. The class is declared as: public static class AbstractMap.SimpleEntry<K,V> extends Object implements Map.Entry<K,V>, Serializable The class methods are extended from the Object class.
AbstractMap.SimpleImmutableEntry<K,V>	The class instance is used for maintaining the value and key which is immutable. The class is declared as: public static class AbstractMap.SimpleImmutableEntry<K,V> extends Object implements Map.Entry<K,V>, Serializable The class methods are extended from the Object class.
AbstractQueue<E>	The class is used for providing a rough implementation of the Queue interface. The class is declared as: public abstract class AbstractQueue<E> extends AbstractCollection<E> implements Queue<E> The class methods are extended from the AbstractCollection class.
AbstractSequentialList<E>	The class is used for providing a rough implementation of the List interface. The class is declared as: public abstract class AbstractSequentialList<E> extends AbstractList<E> The class methods are extended from the AbstractList<E> class.
AbstractSet<E>	The class is used for providing a rough implementation of the Set interface. The class is declared as: public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> The class methods are extended from the AbstractCollection<E> class.
ArrayDeque<E>	The class is used for providing a rough implementation of the Deque interface. The class is declared as: public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable The class methods are extended from the AbstractCollection<E> class.
ArrayList<E>	The class is used for providing a rough implementation of the List interface. The class is declared as: public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable The class methods are extended from the AbstractList<E> class.
Arrays	The class contains various methods which can be used for the performing large number of operations on arrays. The class is declared as: public class Array extends Object The class methods are extended from the Objetct class.
BitSet	A vector consisting of bits is created by this class. The class is declared as: public class BitSet extends Object implements Cloneable, Serializable The class methods are extended from the Object class.
Calendar	It is an abstract class for providing the date time related conversions. The class is declared as: public class BitSet extends Object implements Cloneable, Serializable The class methods are extended from the Object class.
Collections	The class consists of various methods which is used for performing of various operations on collections in java. The class is declared as: public class Collections extends Object The class methods are extended from the Object class.
Currency	The class is used for the representation of currency in various dominations.The class is declared as: public final class Currency extends Object implements Serializable The class methods are extended from the Object class.
Date	The class is used for representation of date. The class is declared as: public class Date extends Object implements Serializable, Cloneable, Comparable The class methods are extended from the Object class.
Dictionary<K,V>	The following abstract class is a parent of many. It is used for the mapping of key into values. The class is declared as: public abstract class Dictionary<K,V> extends Object The class methods are extended from the Object class.
EnumMap,V>	The class is a special implementation of Map class designed for enum values. The class is declared as: public class EnumMap,V> extends AbstractMap<K,V> implements Serializable, Cloneable The class methods are extended from the AbstractMap<K,V> class.
EnumSet	The class is used for providing a rough implementation of the Set interface for the enum values. The class is declared as: public abstract class EnumSet extends AbstractSet<E> implements Cloneable, Serializable The class methods are extended from the AbstractSet<E> class.
EventListenerProxy	It is the wrapper class for the Event Listener. The class is declared as: public abstract class EventListenerProxy extends Object implements EventListener The class methods are extended from the Object class.
EventObject	All the objects of Event states are derived from this class. The class is declared as: public class EventObject extends Object implements Serializable The class methods are extended from the Object class.
GregorianCalendar	It is the child class of Calendar class. The class is declared as: public class GregorianCalendar extends Calendar The class methods are extended from the Calendar class.
HashMap<K,V>	The class is an implementation of the map interface which basically is designed for hash tables. The class is declared as: public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable The class methods are extended from the AbstractMap<K,V> class.
HashSet<E>	The class is an implementation of the set interface which basically is designed for hash tables. The class is declared as: public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable The class methods are extended from the AbstractSet<E> class.
Hashtable<K,V>	Hash table is implemented by this class. The class is declared as: public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable The class methods are extended from the Dictionary<K,V> class.
IdentityHashMap<K,V>	The class is an implementation of the map interface for hash tables. The class is declared as: public class IdentityHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Serializable, Cloneable The class methods are extended from the AbstractMap class.
LinkedHashMap<K,V>	The class is an implementation of the map interface for hash tables with linked list. The class is declared as: public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V> The class methods are extended from the HashMap<K,V> class.
LinkedHashSet<E>	The class is an implementation of the set interface for hash tables with linked list. The class is declared as: public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, Serializable The class methods are extended from the HashSet<E> class.
LinkedList<E>	Interfaces such as List and Deque are implemented by using doubly linked list. The class is declared as: public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, Serializable The class methods are extended from the AbstractSequentialList<E>class.
ListResourceBundle	The class is the successor of ResourceBundle which is designed to make the resources available to the locale. The class is declared as: public abstract class ListResourceBundle extends ResourceBundle The class methods are extended from the ResourceBundle class.
Locale	It is used to reflect a region on space of any type. The class is declared as: public final class Locale extends Object implements Cloneable, Serializable The class methods are extended from the Object class.
Locale.Builder	Instances of the class are used to make the Locale class objects. The class is declared as: public static final class Locale.Builder extends Object The class methods are extended from the Object class.
Objects	The various methods of the class are used to manipulate the number of operations to be performed on objects. The class is declared as: public class Objects extends Object The class methods are extended from the Object class.
Observable	The objects that we can see, monitor or observe are represented by this class. The class is declared as: public class Observable extends Object The class methods are extended from the Object class.
PriorityQueue<E>	The class represents a priority queue. It is a data structure in which each element has a assigned priority with it. The class is declared as: public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable The class methods are extended from the AbstractQueue<E> class.
Properties	A wide set of properties are defined by this class. The class is declared as: public class Properties extends Hashtable<Object,Object> The class methods are extended from the Hashtable<Object,Object> class.
PropertyPermission	Various permissions related to the property of any entity in the program is handled by this class. The class is declared as: public final class PropertyPermission extends BasicPermission The class methods are extended from the BasicPermission class.
PropertyResourceBundle	It is a child class of the ResoruceBundle class. The class manages the resource allocation to the locale with the specific set of properties. The class is declared as: public class PropertyResourceBundle extends ResourceBundle The class methods are extended from the ResourceBundle class.
Random	The class instances are used to design a set of random numbers. The class is declared as: public class Random extends Object implements Serializable The class methods are extended from the Object class.
ResourceBundle	The class contains various objects which are specific to locale. The class is declared as: public abstract class ResourceBundle extends Object The class methods are extended from the Object class.
Scanner	It is a very important utility class which is used to add user defined values to the class. The class is declared as: public final class Scanner extends Object implements Iterator, Closeable The class methods are extended from the Object class.
ServiceLoader<S>	It is a basic service providing class. The class is declared as: public final class ServiceLoader<S> extends Object implements Iterable<S> The class methods are extended from the Object class.
SimpleTimeZone	The class instances represent time zone to be used with the Georgians Calendar. The class is declared as: public class SimpleTimeZone extends TimeZone The class methods are extended from the TimeZone class.
Stack<E>	The class represents a data type based on the LIFO principle. The class is declared as: public class Stack<E> extends Vector<E> The class methods are extended from the Vector<E> class.
StringTokenizer	It is used for creation of various tokens from a string. The class is declared as: public class StringTokenizer extends Object implements Enumeration<Object> The class methods are extended from the Object class.
Timer	The class provides facility of scheduling of tasks. The class is declared as: public class Timer extends Object The class methods are extended from the Object class.
TimerTask	The instances of the class represent various tasks which are scheduled using the timer. The class is declared as: public abstract class TimerTask extends Object implements Runnable The class methods are extended from the Object class.
TimeZone	The instances of the class represent the offset of the time zone. The class is declared as: public abstract class TimeZone extends Object implements Serializable, Cloneable The class methods are extended from the Object class.
TreeMap<K,V>	It is a map implementation which is based on the tree structure. The class is declared as: public class TreeMap<K,V> extends AbstractMap<K,V> implements NavigableMap<K,V>, Cloneable, Serializable The class methods are extended from the AbstractMap class.
UUID	The instances of the class represent UUID. The class is declared as: public final class UUID extends Object implements Serializable, Comparable The class methods are extended from the Object class.
Vector<E>	The class represents an array of object which can further also be extended. The class is declared as: public class Vector<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable The class methods are extended from the AbstractList<E> class.
WeakHashMap<K,V>	The class implements the map interface based on the hash table. The class is declared as: public class WeakHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V> The class methods are extended from the extends AbstractMap<K,V> class.