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| 1 | +/* |
| 2 | + * Copyright (c) 2023, 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 jdk.internal.util; |
| 27 | + |
| 28 | +import java.util.Map; |
| 29 | +import java.util.Objects; |
| 30 | + |
| 31 | +/** |
| 32 | + * An immutable container for a key and a value, both of which are nullable. |
| 33 | + * |
| 34 | + * <p>This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a> |
| 35 | + * class; programmers should treat instances that are |
| 36 | + * {@linkplain #equals(Object) equal} as interchangeable and should not |
| 37 | + * use instances for synchronization, or unpredictable behavior may |
| 38 | + * occur. For example, in a future release, synchronization may fail. |
| 39 | + * |
| 40 | + * @apiNote |
| 41 | + * This class is not exported. Instances are created by various Map implementations |
| 42 | + * when they need a Map.Entry that isn't connected to the Map. |
| 43 | + * |
| 44 | + * <p>This class differs from AbstractMap.SimpleImmutableEntry in that it is not |
| 45 | + * serializable and that it is final. This class differs from java.util.KeyValueHolder |
| 46 | + * in that the key and value are nullable. |
| 47 | + * |
| 48 | + * <p>In principle this class could be a variation on KeyValueHolder. However, |
| 49 | + * making that class selectively support nullable keys and values is quite intricate. |
| 50 | + * Various specifications (such as Map.ofEntries and Map.entry) specify non-nullability |
| 51 | + * of the key and the value. Map.Entry.copyOf also requires non-null keys and values; |
| 52 | + * but it simply passes through KeyValueHolder instances, assuming their keys and values |
| 53 | + * are non-nullable. If a KVH with nullable keys and values were introduced, some way |
| 54 | + * to distinguish it would be necessary. This could be done by introducing a subclass |
| 55 | + * (requiring KVH to be made non-final) or by introducing some kind of "mode" field |
| 56 | + * (potentially increasing the size of every KVH instance, though another field could |
| 57 | + * probably fit into the object's padding in most JVMs.) More critically, a mode field |
| 58 | + * would have to be checked in all the right places to get the right behavior. |
| 59 | + * |
| 60 | + * <p>A longer range possibility is to selectively relax the restrictions against nulls in |
| 61 | + * Map.entry and Map.Entry.copyOf. This would also require some intricate specification |
| 62 | + * changes and corresponding implementation changes (e.g., the implementations backing |
| 63 | + * Map.of might still need to reject nulls, and so would Map.ofEntries) but allowing |
| 64 | + * a Map.Entry itself to contain nulls seems beneficial in general. If this is done, |
| 65 | + * merging KeyValueHolder and NullableKeyValueHolder should be reconsidered. |
| 66 | + * |
| 67 | + * @param <K> the key type |
| 68 | + * @param <V> the value type |
| 69 | + */ |
| 70 | +@jdk.internal.ValueBased |
| 71 | +public final class NullableKeyValueHolder<K,V> implements Map.Entry<K,V> { |
| 72 | + final K key; |
| 73 | + final V value; |
| 74 | + |
| 75 | + /** |
| 76 | + * Constructs a NullableKeyValueHolder. |
| 77 | + * |
| 78 | + * @param k the key, may be null |
| 79 | + * @param v the value, may be null |
| 80 | + */ |
| 81 | + public NullableKeyValueHolder(K k, V v) { |
| 82 | + key = k; |
| 83 | + value = v; |
| 84 | + } |
| 85 | + |
| 86 | + /** |
| 87 | + * Constructs a NullableKeyValueHolder from a Map.Entry. No need for an |
| 88 | + * idempotent copy at this time. |
| 89 | + * |
| 90 | + * @param entry the entry, must not be null |
| 91 | + */ |
| 92 | + public NullableKeyValueHolder(Map.Entry<K,V> entry) { |
| 93 | + Objects.requireNonNull(entry); |
| 94 | + key = entry.getKey(); |
| 95 | + value = entry.getValue(); |
| 96 | + } |
| 97 | + |
| 98 | + /** |
| 99 | + * Gets the key from this holder. |
| 100 | + * |
| 101 | + * @return the key, may be null |
| 102 | + */ |
| 103 | + @Override |
| 104 | + public K getKey() { |
| 105 | + return key; |
| 106 | + } |
| 107 | + |
| 108 | + /** |
| 109 | + * Gets the value from this holder. |
| 110 | + * |
| 111 | + * @return the value, may be null |
| 112 | + */ |
| 113 | + @Override |
| 114 | + public V getValue() { |
| 115 | + return value; |
| 116 | + } |
| 117 | + |
| 118 | + /** |
| 119 | + * Throws {@link UnsupportedOperationException}. |
| 120 | + * |
| 121 | + * @param value ignored |
| 122 | + * @return never returns normally |
| 123 | + */ |
| 124 | + @Override |
| 125 | + public V setValue(V value) { |
| 126 | + throw new UnsupportedOperationException("not supported"); |
| 127 | + } |
| 128 | + |
| 129 | + /** |
| 130 | + * Compares the specified object with this entry for equality. |
| 131 | + * Returns {@code true} if the given object is also a map entry and |
| 132 | + * the two entries' keys and values are equal. |
| 133 | + */ |
| 134 | + @Override |
| 135 | + public boolean equals(Object o) { |
| 136 | + return o instanceof Map.Entry<?, ?> e |
| 137 | + && Objects.equals(key, e.getKey()) |
| 138 | + && Objects.equals(value, e.getValue()); |
| 139 | + } |
| 140 | + |
| 141 | + private int hash(Object obj) { |
| 142 | + return (obj == null) ? 0 : obj.hashCode(); |
| 143 | + } |
| 144 | + |
| 145 | + /** |
| 146 | + * Returns the hash code value for this map entry. The hash code |
| 147 | + * is {@code key.hashCode() ^ value.hashCode()}. |
| 148 | + */ |
| 149 | + @Override |
| 150 | + public int hashCode() { |
| 151 | + return hash(key) ^ hash(value); |
| 152 | + } |
| 153 | + |
| 154 | + /** |
| 155 | + * Returns a String representation of this map entry. This |
| 156 | + * implementation returns the string representation of this |
| 157 | + * entry's key followed by the equals character ("{@code =}") |
| 158 | + * followed by the string representation of this entry's value. |
| 159 | + * |
| 160 | + * @return a String representation of this map entry |
| 161 | + */ |
| 162 | + @Override |
| 163 | + public String toString() { |
| 164 | + return key + "=" + value; |
| 165 | + } |
| 166 | +} |
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