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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN""http://www.w3.org/TR/REC-html40/frameset.dtd"><!--NewPage--><HTML><HEAD><!-- Generated by javadoc on Thu Apr 27 23:36:56 PDT 2000 --><TITLE>Java 2 Platform SE v1.3: Class Hashtable</TITLE><LINK REL ="stylesheet" TYPE="text/css" HREF="../../stylesheet.css" TITLE="Style"></HEAD><BODY BGCOLOR="white"><!-- ========== START OF NAVBAR ========== --><A NAME="navbar_top"><!-- --></A><TABLE BORDER="0" WIDTH="100%" CELLPADDING="1" CELLSPACING="0"><TR><TD COLSPAN=2 BGCOLOR="#EEEEFF" CLASS="NavBarCell1"><A NAME="navbar_top_firstrow"><!-- --></A><TABLE BORDER="0" CELLPADDING="0" CELLSPACING="3"> <TR ALIGN="center" VALIGN="top"> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../overview-summary.html"><FONT CLASS="NavBarFont1"><B>Overview</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="package-summary.html"><FONT CLASS="NavBarFont1"><B>Package</B></FONT></A> </TD> <TD BGCOLOR="#FFFFFF" CLASS="NavBarCell1Rev"> <FONT CLASS="NavBarFont1Rev"><B>Class</B></FONT> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="class-use/Hashtable.html"><FONT CLASS="NavBarFont1"><B>Use</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="package-tree.html"><FONT CLASS="NavBarFont1"><B>Tree</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../deprecated-list.html"><FONT CLASS="NavBarFont1"><B>Deprecated</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../index-files/index-1.html"><FONT CLASS="NavBarFont1"><B>Index</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../help-doc.html"><FONT CLASS="NavBarFont1"><B>Help</B></FONT></A> </TD> </TR></TABLE></TD><TD ALIGN="right" VALIGN="top" ROWSPAN=3><EM><b>Java<sup><font size=-2>TM</font></sup> 2 Platform<br>Std. Ed. v1.3</b></EM></TD></TR><TR><TD BGCOLOR="white" CLASS="NavBarCell2"><FONT SIZE="-2"> <A HREF="../../java/util/HashSet.html"><B>PREV CLASS</B></A> <A HREF="../../java/util/LinkedList.html"><B>NEXT CLASS</B></A></FONT></TD><TD BGCOLOR="white" CLASS="NavBarCell2"><FONT SIZE="-2"> <A HREF="../../index.html" TARGET="_top"><B>FRAMES</B></A> <A HREF="Hashtable.html" TARGET="_top"><B>NO FRAMES</B></A></FONT></TD></TR><TR><TD VALIGN="top" CLASS="NavBarCell3"><FONT SIZE="-2"> SUMMARY: INNER | FIELD | <A HREF="#constructor_summary">CONSTR</A> | <A HREF="#method_summary">METHOD</A></FONT></TD><TD VALIGN="top" CLASS="NavBarCell3"><FONT SIZE="-2">DETAIL: FIELD | <A HREF="#constructor_detail">CONSTR</A> | <A HREF="#method_detail">METHOD</A></FONT></TD></TR></TABLE><!-- =========== END OF NAVBAR =========== --><HR><!-- ======== START OF CLASS DATA ======== --><H2><FONT SIZE="-1">java.util</FONT><BR>Class Hashtable</H2><PRE><A HREF="../../java/lang/Object.html">java.lang.Object</A> | +--<A HREF="../../java/util/Dictionary.html">java.util.Dictionary</A> | +--<B>java.util.Hashtable</B></PRE><DL><DT><B>All Implemented Interfaces:</B> <DD><A HREF="../../java/lang/Cloneable.html">Cloneable</A>, <A HREF="../../java/util/Map.html">Map</A>, <A HREF="../../java/io/Serializable.html">Serializable</A></DD></DL><DL><DT><B>Direct Known Subclasses:</B> <DD><A HREF="../../java/util/Properties.html">Properties</A>, <A HREF="../../javax/swing/UIDefaults.html">UIDefaults</A></DD></DL><HR><DL><DT>public class <B>Hashtable</B><DT>extends <A HREF="../../java/util/Dictionary.html">Dictionary</A><DT>implements <A HREF="../../java/util/Map.html">Map</A>, <A HREF="../../java/lang/Cloneable.html">Cloneable</A>, <A HREF="../../java/io/Serializable.html">Serializable</A></DL><P>This class implements a hashtable, which maps keys to values. Any non-<code>null</code> object can be used as a key or as a value. <p> To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the <code>hashCode</code> method and the <code>equals</code> method. <p> An instance of <code>Hashtable</code> has two parameters that affect its performance: <i>initial capacity</i> and <i>load factor</i>. The <i>capacity</i> is the number of <i>buckets</i> in the hash table, and the <i>initial capacity</i> is simply the capacity at the time the hash table is created. Note that the hash table is <i>open</i>: in the case a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The <i>load factor</i> is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hashtable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the <code>rehash</code> method.<p> Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most <tt>Hashtable</tt> operations, including <tt>get</tt> and <tt>put</tt>).<p> The initial capacity controls a tradeoff between wasted space and the need for <code>rehash</code> operations, which are time-consuming. No <code>rehash</code> operations will <i>ever</i> occur if the initial capacity is greater than the maximum number of entries the <tt>Hashtable</tt> will contain divided by its load factor. However, setting the initial capacity too high can waste space.<p> If many entries are to be made into a <code>Hashtable</code>, creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table. <p> This example creates a hashtable of numbers. It uses the names of the numbers as keys: <p><blockquote><pre> Hashtable numbers = new Hashtable(); numbers.put("one", new Integer(1)); numbers.put("two", new Integer(2)); numbers.put("three", new Integer(3)); </pre></blockquote> <p> To retrieve a number, use the following code: <p><blockquote><pre> Integer n = (Integer)numbers.get("two"); if (n != null) { System.out.println("two = " + n); } </pre></blockquote> <p> As of the Java 2 platform v1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, Hashtable is synchronized.<p> The Iterators returned by the iterator and listIterator methods of the Collections returned by all of Hashtable's "collection view methods" are <em>fail-fast</em>: if the Hashtable is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and values methods are <em>not</em> fail-fast.<P><DL><DT><B>Since: </B><DD>JDK1.0</DD><DT><B>See Also: </B><DD><A HREF="../../java/lang/Object.html#equals(java.lang.Object)"><CODE>Object.equals(java.lang.Object)</CODE></A>, <A HREF="../../java/lang/Object.html#hashCode()"><CODE>Object.hashCode()</CODE></A>, <A HREF="../../java/util/Hashtable.html#rehash()"><CODE>rehash()</CODE></A>, <A HREF="../../java/util/Collection.html"><CODE>Collection</CODE></A>, <A HREF="../../java/util/Map.html"><CODE>Map</CODE></A>, <A HREF="../../java/util/HashMap.html"><CODE>HashMap</CODE></A>, <A HREF="../../java/util/TreeMap.html"><CODE>TreeMap</CODE></A>, <A HREF="../../serialized-form.html#java.util.Hashtable">Serialized Form</A></DL><HR><P><!-- ======== INNER CLASS SUMMARY ======== --><A NAME="inner_classes_inherited_from_class_java.util.Map"><!-- --></A><TABLE BORDER="1" CELLPADDING="3" CELLSPACING="0" WIDTH="100%"><TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor"><TD><B>Inner classes inherited from class java.util.<A HREF="../../java/util/Map.html">Map</A></B></TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><A HREF="../../java/util/Map.Entry.html">Map.Entry</A></CODE></TD></TR></TABLE> <!-- =========== FIELD SUMMARY =========== --><!-- ======== CONSTRUCTOR SUMMARY ======== --><A NAME="constructor_summary"><!-- --></A><TABLE BORDER="1" CELLPADDING="3" CELLSPACING="0" WIDTH="100%"><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TD COLSPAN=2><FONT SIZE="+2"><B>Constructor Summary</B></FONT></TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><B><A HREF="../../java/util/Hashtable.html#Hashtable()">Hashtable</A></B>()</CODE><BR> Constructs a new, empty hashtable with a default capacity and load factor, which is <tt>0.75</tt>.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><B><A HREF="../../java/util/Hashtable.html#Hashtable(int)">Hashtable</A></B>(int initialCapacity)</CODE><BR> Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is <tt>0.75</tt>.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><B><A HREF="../../java/util/Hashtable.html#Hashtable(int, float)">Hashtable</A></B>(int initialCapacity, float loadFactor)</CODE><BR> Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><B><A HREF="../../java/util/Hashtable.html#Hashtable(java.util.Map)">Hashtable</A></B>(<A HREF="../../java/util/Map.html">Map</A> t)</CODE><BR> Constructs a new hashtable with the same mappings as the given Map.</TD></TR></TABLE> <!-- ========== METHOD SUMMARY =========== --><A NAME="method_summary"><!-- --></A><TABLE BORDER="1" CELLPADDING="3" CELLSPACING="0" WIDTH="100%"><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TD COLSPAN=2><FONT SIZE="+2"><B>Method Summary</B></FONT></TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> void</CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#clear()">clear</A></B>()</CODE><BR> Clears this hashtable so that it contains no keys.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> <A HREF="../../java/lang/Object.html">Object</A></CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#clone()">clone</A></B>()</CODE><BR> Creates a shallow copy of this hashtable.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> boolean</CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#contains(java.lang.Object)">contains</A></B>(<A HREF="../../java/lang/Object.html">Object</A> value)</CODE><BR> Tests if some key maps into the specified value in this hashtable.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> boolean</CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#containsKey(java.lang.Object)">containsKey</A></B>(<A HREF="../../java/lang/Object.html">Object</A> key)</CODE><BR> Tests if the specified object is a key in this hashtable.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> boolean</CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#containsValue(java.lang.Object)">containsValue</A></B>(<A HREF="../../java/lang/Object.html">Object</A> value)</CODE><BR> Returns true if this Hashtable maps one or more keys to this value.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> <A HREF="../../java/util/Enumeration.html">Enumeration</A></CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#elements()">elements</A></B>()</CODE><BR> Returns an enumeration of the values in this hashtable.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> <A HREF="../../java/util/Set.html">Set</A></CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#entrySet()">entrySet</A></B>()</CODE><BR> Returns a Set view of the entries contained in this Hashtable.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> boolean</CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#equals(java.lang.Object)">equals</A></B>(<A HREF="../../java/lang/Object.html">Object</A> o)</CODE><BR> Compares the specified Object with this Map for equality, as per the definition in the Map interface.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE> <A HREF="../../java/lang/Object.html">Object</A></CODE></FONT></TD><TD><CODE><B><A HREF="../../java/util/Hashtable.html#get(java.lang.Object)">get</A></B>(<A HREF="../../java/lang/Object.html">Object</A> key)</CODE><BR> Returns the value to which the specified key is mapped in this hashtable.</TD></TR>⌨️ 快捷键说明
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