1 /*
   2  * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 
  28 /**
  29  * An immutable container for a key and a value, suitable for use
  30  * in creating and populating {@code Map} instances.
  31  *
  32  * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
  33  * class; use of identity-sensitive operations (including reference equality
  34  * ({@code ==}), identity hash code, or synchronization) on instances of
  35  * {@code KeyValueHolder} may have unpredictable results and should be avoided.
  36  *
  37  * @apiNote
  38  * This class is not public. Instances can be created using the
  39  * {@link Map#entry Map.entry(k, v)} factory method, which is public.
  40  * 
  41  * This class differs from AbstractMap.SimpleImmutableEntry in the following ways:
  42  * it is not serializable, and its key and value must be non-null.
  43  *
  44  * @param <K> the key type
  45  * @param <V> the value type
  46  *
  47  * @see Map#ofEntries Map.ofEntries()
  48  * @since 1.9
  49  */
  50 final class KeyValueHolder<K,V> implements Map.Entry<K,V> {
  51     final K key;
  52     final V value;
  53 
  54     KeyValueHolder(K k, V v) {
  55         key = Objects.requireNonNull(k);
  56         value = Objects.requireNonNull(v);
  57     }
  58 
  59     /**
  60      * Gets the key from this holder.
  61      *
  62      * @return the key
  63      */
  64     @Override
  65     public K getKey() {
  66         return key;
  67     }
  68 
  69     /**
  70      * Gets the value from this holder.
  71      *
  72      * @return the value
  73      */
  74     @Override
  75     public V getValue() {
  76         return value;
  77     }
  78 
  79     /**
  80      * Throws {@link UnsupportedOperationException}.
  81      *
  82      * @param value ignored
  83      * @return never returns normally
  84      */
  85     @Override
  86     public V setValue(V value) {
  87         throw new UnsupportedOperationException("not supported");
  88     }
  89 
  90     /**
  91      * Compares the specified object with this entry for equality.
  92      * Returns {@code true} if the given object is also a map entry and
  93      * the two entries' keys and values are equal. Note that key and
  94      * value are non-null, so equals() can be called safely on them.
  95      */
  96     @Override
  97     public boolean equals(Object o) {
  98         if (!(o instanceof Map.Entry))
  99             return false;
 100         Map.Entry<?,?> e = (Map.Entry<?,?>)o;
 101         return key.equals(e.getKey()) && value.equals(e.getValue());
 102     }
 103 
 104     /**
 105      * Returns the hash code value for this map entry. The hash code
 106      * is {@code key.hashCode() ^ value.hashCode()}. Note that key and
 107      * value are non-null, so hashCode() can be called safely on them.
 108      */
 109     @Override
 110     public int hashCode() {
 111         return key.hashCode() ^ value.hashCode();
 112     }
 113 
 114     /**
 115      * Returns a String representation of this map entry.  This
 116      * implementation returns the string representation of this
 117      * entry's key followed by the equals character ("{@code =}")
 118      * followed by the string representation of this entry's value.
 119      *
 120      * @return a String representation of this map entry
 121      */
 122     @Override
 123     public String toString() {
 124         return key + "=" + value;
 125     }
 126 }