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).
Following are the Important Classes in Java.util package :

• AbstractCollection: This class provides a skeletal implementation of the Collection interface, to minimize the effort required to implement this interface.
• 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).
• 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: This class provides skeletal implementations of some Queue operations.
• 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).
• AbstractSet: This class provides a skeletal implementation of the Set interface to minimize the effort required to implement this interface.
• ArrayDeque: Resizable-array implementation of the Deque interface.
• ArrayList: Resizable-array implementation of the List interface.
• Arrays: This class contains various methods for manipulating arrays (such as sorting and searching).
• 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.
• 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.
• EnumMap,V>: A specialized Map implementation for use with enum type keys.
• EnumSet: A specialized Set implementation for use with enum types.
• EventListenerProxy: 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: 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).
• LinkedHashMap<K,V>: Hash table and linked list implementation of the Map interface, with predictable iteration order.
• LinkedHashSet: Hash table and linked list implementation of the Set interface, with predictable iteration order.
• LinkedList: 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 – Set 1, Set 2: 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.
• 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.
• PriorityQueue: 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: 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.
• Stack: The Stack class represents a last-in-first-out (LIFO) stack of objects.
• 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: A NavigableSet implementation based on a TreeMap.
• UUID: A class that represents an immutable universally unique identifier (UUID).
• Vector: The Vector class implements a growable array of objects.
• WeakHashMap<K,V>: Hash table based implementation of the Map interface, with weak keys.