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 * Copyright (c) 2015, 2017, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

import jdk.internal.vm.annotation.Stable;

/**
 * An immutable container for a key and a value, suitable for use
 * in creating and populating {@code Map} instances.
 *
 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code KeyValueHolder} may have unpredictable results and should be avoided.
 *
 * @apiNote
 * This class is not public. Instances can be created using the
 * {@link Map#entry Map.entry(k, v)} factory method, which is public.
 *
 * <p>This class differs from AbstractMap.SimpleImmutableEntry in the following ways:
 * it is not serializable, it is final, and its key and value must be non-null.
 *
 * @param <K> the key type
 * @param <V> the value type
 *
 * @see Map#ofEntries Map.ofEntries()
 * @since 9
 */
final class KeyValueHolder<K,V> implements Map.Entry<K,V> {
 @Stable
 final K key;
 @Stable
 final V value;

 KeyValueHolder(K k, V v) {
 key = Objects.requireNonNull(k);
 value = Objects.requireNonNull(v);
 }

 /**
 * Gets the key from this holder.
 *
 * @return the key
 */
 @Override
 public K getKey() {
 return key;
 }

 /**
 * Gets the value from this holder.
 *
 * @return the value
 */
 @Override
 public V getValue() {
 return value;
 }

 /**
 * Throws {@link UnsupportedOperationException}.
 *
 * @param value ignored
 * @return never returns normally
 */
 @Override
 public V setValue(V value) {
 throw new UnsupportedOperationException("not supported");
 }

 /**
 * Compares the specified object with this entry for equality.
 * Returns {@code true} if the given object is also a map entry and
 * the two entries' keys and values are equal. Note that key and
 * value are non-null, so equals() can be called safely on them.
 */
 @Override
 public boolean equals(Object o) {
 if (!(o instanceof Map.Entry))
 return false;
 Map.Entry<?,?> e = (Map.Entry<?,?>)o;
 return key.equals(e.getKey()) && value.equals(e.getValue());
 }

 /**
 * Returns the hash code value for this map entry. The hash code
 * is {@code key.hashCode() ^ value.hashCode()}. Note that key and
 * value are non-null, so hashCode() can be called safely on them.
 */
 @Override
 public int hashCode() {
 return key.hashCode() ^ value.hashCode();
 }

 /**
 * Returns a String representation of this map entry. This
 * implementation returns the string representation of this
 * entry's key followed by the equals character ("{@code =}")
 * followed by the string representation of this entry's value.
 *
 * @return a String representation of this map entry
 */
 @Override
 public String toString() {
 return key + "=" + value;
 }
}