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12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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24  */
25 
26 package java.util;
27 
28 import jdk.internal.vm.annotation.Stable;
29 
30 /**
31  * An immutable container for a key and a value, suitable for use
32  * in creating and populating {@code Map} instances.
33  *
34  * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
35  * class; use of identity-sensitive operations (including reference equality
36  * ({@code ==}), identity hash code, or synchronization) on instances of
37  * {@code KeyValueHolder} may have unpredictable results and should be avoided.
38  *
39  * @apiNote
40  * This class is not public. Instances can be created using the
41  * {@link Map#entry Map.entry(k, v)} factory method, which is public.
42  *
43  * <p>This class differs from AbstractMap.SimpleImmutableEntry in the following ways:
44  * it is not serializable, it is final, and its key and value must be non-null.
45  *
46  * @param <K> the key type
47  * @param <V> the value type
48  *
49  * @see Map#ofEntries Map.ofEntries()
50  * @since 9
51  */
52 final class KeyValueHolder<K,V> implements Map.Entry<K,V> {
53     @Stable
54     final K key;
55     @Stable
56     final V value;
57 
KeyValueHolder(K k, V v)58     KeyValueHolder(K k, V v) {
59         key = Objects.requireNonNull(k);
60         value = Objects.requireNonNull(v);
61     }
62 
63     /**
64      * Gets the key from this holder.
65      *
66      * @return the key
67      */
68     @Override
getKey()69     public K getKey() {
70         return key;
71     }
72 
73     /**
74      * Gets the value from this holder.
75      *
76      * @return the value
77      */
78     @Override
getValue()79     public V getValue() {
80         return value;
81     }
82 
83     /**
84      * Throws {@link UnsupportedOperationException}.
85      *
86      * @param value ignored
87      * @return never returns normally
88      */
89     @Override
setValue(V value)90     public V setValue(V value) {
91         throw new UnsupportedOperationException("not supported");
92     }
93 
94     /**
95      * Compares the specified object with this entry for equality.
96      * Returns {@code true} if the given object is also a map entry and
97      * the two entries' keys and values are equal. Note that key and
98      * value are non-null, so equals() can be called safely on them.
99      */
100     @Override
equals(Object o)101     public boolean equals(Object o) {
102         if (!(o instanceof Map.Entry))
103             return false;
104         Map.Entry<?,?> e = (Map.Entry<?,?>)o;
105         return key.equals(e.getKey()) && value.equals(e.getValue());
106     }
107 
108     /**
109      * Returns the hash code value for this map entry. The hash code
110      * is {@code key.hashCode() ^ value.hashCode()}. Note that key and
111      * value are non-null, so hashCode() can be called safely on them.
112      */
113     @Override
hashCode()114     public int hashCode() {
115         return key.hashCode() ^ value.hashCode();
116     }
117 
118     /**
119      * Returns a String representation of this map entry.  This
120      * implementation returns the string representation of this
121      * entry's key followed by the equals character ("{@code =}")
122      * followed by the string representation of this entry's value.
123      *
124      * @return a String representation of this map entry
125      */
126     @Override
toString()127     public String toString() {
128         return key + "=" + value;
129     }
130 }
131