CHAPTER 21
The java.util
package contains various 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 concrete implementation of Dictionary
. The class Properties
extends Hashtable
by allowing one table to provide default values for another and by providing standard 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 notification 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 mechanism.)
The class Random
provides an extensive set of methods for pseudorandomly generating numeric values of various primitive types and with various distributions. 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.
The classes Vector
and Stack
are simple container classes that provide extensions to the capabilities of Java arrays. A Vector
, unlike a Java array, can change its size, and many convenient methods are provided for adding, removing, and searching for items. A Stack
is a Vector
with additional operations such as push
and pop
.
The hierarchy of classes defined in package java.util
is as follows. (Classes whose names are shown here in boldface
are in package java.util
; the others are in package java.lang
and are shown here to clarify subclass relationships.)
Object§20.1
interfaceEnumeration §21.1
BitSet §21.2
Date §21.3
Dictionary §21.4
Hashtable §21.5
Properties §21.6
Observable §21.7
interfaceObserver §21.8
Random §21.9
StringTokenizer §21.10
Vector §21.11
Stack §21.12
Throwable§20.22 Exception RuntimeException
EmptyStackException §21.13
NoSuchElementException §21.14
java.util.Enumeration
Enumeration
interface will generate a series of
elements, one at a time. Successive calls to the nextElement
method will return
successive elements of the series.
public interfaceEnumeration
{ public booleanhasMoreElements
(); public ObjectnextElement
() throws NoSuchElementException; }
21.1.1 public boolean
hasMoreElements
()
The result is true
if and only if this enumeration object has at least one more element
to provide.
21.1.2 public Object
nextElement
()
throws NoSuchElementException
If this enumeration object has at least one more element to provide, such an element
is returned; otherwise, a NoSuchElementException
is thrown.
As an example, the following code prints every key in the hashtable ht
and its length. The method keys
returns an enumeration that will deliver all the keys, and we suppose that the keys are, in this case, known to be strings:
Enumeration e = ht.keys(); while (e.hasMoreElements()) { String key = (String)e.nextElement(); System.out.println(key + " " + key.length()); }
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