jdk/src/java.base/share/classes/java/util/Dictionary.java
author mchung
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/*
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 * Copyright (c) 1995, 2004, Oracle and/or its affiliates. All rights reserved.
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
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 * This code is free software; you can redistribute it and/or modify it
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 * under the terms of the GNU General Public License version 2 only, as
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 * published by the Free Software Foundation.  Oracle designates this
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 * particular file as subject to the "Classpath" exception as provided
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 * by Oracle in the LICENSE file that accompanied this code.
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 *
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 * This code is distributed in the hope that it will be useful, but WITHOUT
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * version 2 for more details (a copy is included in the LICENSE file that
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 * accompanied this code).
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 *
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 * You should have received a copy of the GNU General Public License version
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 * 2 along with this work; if not, write to the Free Software Foundation,
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 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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 *
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 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 * or visit www.oracle.com if you need additional information or have any
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 * questions.
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 */
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package java.util;
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/**
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 * The <code>Dictionary</code> class is the abstract parent of any
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 * class, such as <code>Hashtable</code>, which maps keys to values.
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 * Every key and every value is an object. In any one <tt>Dictionary</tt>
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 * object, every key is associated with at most one value. Given a
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 * <tt>Dictionary</tt> and a key, the associated element can be looked up.
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 * Any non-<code>null</code> object can be used as a key and as a value.
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 * <p>
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 * As a rule, the <code>equals</code> method should be used by
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 * implementations of this class to decide if two keys are the same.
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 * <p>
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 * <strong>NOTE: This class is obsolete.  New implementations should
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 * implement the Map interface, rather than extending this class.</strong>
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 *
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 * @author  unascribed
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 * @see     java.util.Map
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 * @see     java.lang.Object#equals(java.lang.Object)
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 * @see     java.lang.Object#hashCode()
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 * @see     java.util.Hashtable
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 * @since   1.0
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 */
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public abstract
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class Dictionary<K,V> {
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    /**
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     * Sole constructor.  (For invocation by subclass constructors, typically
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     * implicit.)
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     */
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    public Dictionary() {
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    }
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    /**
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     * Returns the number of entries (distinct keys) in this dictionary.
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     *
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     * @return  the number of keys in this dictionary.
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     */
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    abstract public int size();
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    /**
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     * Tests if this dictionary maps no keys to value. The general contract
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     * for the <tt>isEmpty</tt> method is that the result is true if and only
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     * if this dictionary contains no entries.
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     *
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     * @return  <code>true</code> if this dictionary maps no keys to values;
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     *          <code>false</code> otherwise.
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     */
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    abstract public boolean isEmpty();
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    /**
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     * Returns an enumeration of the keys in this dictionary. The general
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     * contract for the keys method is that an <tt>Enumeration</tt> object
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     * is returned that will generate all the keys for which this dictionary
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     * contains entries.
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     *
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     * @return  an enumeration of the keys in this dictionary.
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     * @see     java.util.Dictionary#elements()
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     * @see     java.util.Enumeration
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     */
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    abstract public Enumeration<K> keys();
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    /**
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     * Returns an enumeration of the values in this dictionary. The general
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     * contract for the <tt>elements</tt> method is that an
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     * <tt>Enumeration</tt> is returned that will generate all the elements
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     * contained in entries in this dictionary.
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     *
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     * @return  an enumeration of the values in this dictionary.
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     * @see     java.util.Dictionary#keys()
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     * @see     java.util.Enumeration
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     */
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    abstract public Enumeration<V> elements();
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    /**
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     * Returns the value to which the key is mapped in this dictionary.
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     * The general contract for the <tt>isEmpty</tt> method is that if this
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     * dictionary contains an entry for the specified key, the associated
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     * value is returned; otherwise, <tt>null</tt> is returned.
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     *
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     * @return  the value to which the key is mapped in this dictionary;
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     * @param   key   a key in this dictionary.
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     *          <code>null</code> if the key is not mapped to any value in
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     *          this dictionary.
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     * @exception NullPointerException if the <tt>key</tt> is <tt>null</tt>.
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     * @see     java.util.Dictionary#put(java.lang.Object, java.lang.Object)
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     */
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    abstract public V get(Object key);
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    /**
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     * Maps the specified <code>key</code> to the specified
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     * <code>value</code> in this dictionary. Neither the key nor the
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     * value can be <code>null</code>.
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     * <p>
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     * If this dictionary already contains an entry for the specified
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     * <tt>key</tt>, the value already in this dictionary for that
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     * <tt>key</tt> is returned, after modifying the entry to contain the
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     *  new element. <p>If this dictionary does not already have an entry
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     *  for the specified <tt>key</tt>, an entry is created for the
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     *  specified <tt>key</tt> and <tt>value</tt>, and <tt>null</tt> is
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     *  returned.
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     * <p>
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     * The <code>value</code> can be retrieved by calling the
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     * <code>get</code> method with a <code>key</code> that is equal to
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     * the original <code>key</code>.
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     *
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     * @param      key     the hashtable key.
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     * @param      value   the value.
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     * @return     the previous value to which the <code>key</code> was mapped
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     *             in this dictionary, or <code>null</code> if the key did not
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     *             have a previous mapping.
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     * @exception  NullPointerException  if the <code>key</code> or
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     *               <code>value</code> is <code>null</code>.
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     * @see        java.lang.Object#equals(java.lang.Object)
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     * @see        java.util.Dictionary#get(java.lang.Object)
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     */
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    abstract public V put(K key, V value);
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    /**
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     * Removes the <code>key</code> (and its corresponding
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     * <code>value</code>) from this dictionary. This method does nothing
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     * if the <code>key</code> is not in this dictionary.
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     *
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     * @param   key   the key that needs to be removed.
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     * @return  the value to which the <code>key</code> had been mapped in this
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     *          dictionary, or <code>null</code> if the key did not have a
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     *          mapping.
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     * @exception NullPointerException if <tt>key</tt> is <tt>null</tt>.
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     */
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    abstract public V remove(Object key);
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}