21

C H A P T E R

The Package

java.util

T

HE

java.util

The P

 package contains v

ackage java.util

arious utility classes and interfaces.

Notable among these utilities is the

Enumeration

 interface. An object that

implements this interface will generate a series of items, delivering them on

demand, one by one. Container classes such as

Dictionary

 and

Vector

 provide

one or more methods that return an

Enumeration

.

A

BitSet

 contains an indexed collection of bits that may be used to represent

a set of nonnegative integers.

The class

Date

 provides a convenient way to represent and manipulate time

and date information. Dates may be constructed from a year, month, day of

month, hour, minute, and second, and those six components, as well as the day of

the week, may be extracted from a date. Time zones and daylight saving time are

properly accounted for.

The

abstract

 class

Dictionary

 represents a collection of key value pairs

and allows a value to be fetched given the key. The class

Hashtable

 is one con 

crete implementation of

Dictionary

. The class

Properties

 extends

Hashtable

by allowing one table to provide default values for another and by providing stan 

dard means for reading entries from files and writing entries to files.

The class

Observable

 provides a mechanism for notifying other objects,

called  observers,  whenever an

Observable

 object is changed. An observer

object may be any object that implements the

Observer

 interface. (This notifica 

tion mechanism is distinct from that provided by the

wait

 and

notify

 methods of

class

Object

 ( 20.1) and is not connected with the thread scheduling mecha 

nism.)

The class

Random

 provides an extensive set of methods for pseudorandomly

generating numeric values of various primitive types and with various distribu 

tions. Each instance of class

Random

 is an independent pseudorandom generator.

A

StringTokenizer

 provides an easy way to divide strings into tokens. The

set of characters that delimit tokens is programmable. The tokenizing method is

much simpler than the one used by the class

java.io.StreamTokenizer

. For

example, a

StringTokenizer

 does not distinguish among identifiers, numbers,

and quoted strings; moreover, it does not recognize and skip comments.

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