CHAPTER 21

The Package java.util

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
	interface Enumeration												§21.1
	BitSet												§21.2
	Date												§21.3
	Dictionary												§21.4
		Hashtable												§21.5
			Properties												§21.6
	Observable												§21.7
	interface Observer												§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

21.1 The Interface java.util.Enumeration

public interface Enumeration {
	public boolean hasMoreElements();
	public Object nextElement() 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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Java Language Specification (HTML generated by Suzette Pelouch on February 24, 1998)
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