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NOTE: This class is obsolete.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