src/java.base/share/classes/java/util/AbstractMap.java - master - githubmirror/jdk22

src/java.base/share/classes/java/util/AbstractMap.java
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8315850: Improve AbstractMap anonymous Iterator classes
12 окт 2023, 10:51
12 окт 2023, 10:51d95b548
/*
 * Copyright (c) 1997, 2023, 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;

import java.util.stream.Stream;
import java.util.function.Consumer;
import java.util.function.IntFunction;
import java.util.function.Predicate;

/**
 * This class provides a skeletal implementation of the {@code Map}
 * interface, to minimize the effort required to implement this interface.
 *
 * <p>To implement an unmodifiable map, the programmer needs only to extend this
 * class and provide an implementation for the {@code entrySet} method, which
 * returns a set-view of the map's mappings.  Typically, the returned set
 * will, in turn, be implemented atop {@code AbstractSet}.  This set should
 * not support the {@code add} or {@code remove} methods, and its iterator
 * should not support the {@code remove} method.
 *
 * <p>To implement a modifiable map, the programmer must additionally override
 * this class's {@code put} method (which otherwise throws an
 * {@code UnsupportedOperationException}), and the iterator returned by
 * {@code entrySet().iterator()} must additionally implement its
 * {@code remove} method.
 *
 * <p>The programmer should generally provide a void (no argument) and map
 * constructor, as per the recommendation in the {@code Map} interface
 * specification.
 *
 * <p>The documentation for each non-abstract method in this class describes its
 * implementation in detail.  Each of these methods may be overridden if the
 * map being implemented admits a more efficient implementation.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see Map
 * @see Collection
 * @since 1.2
 */

public abstract class AbstractMap<K,V> implements Map<K,V> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractMap() {
    }

    // Query Operations

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation returns {@code entrySet().size()}.
     */
    public int size() {
        return entrySet().size();
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation returns {@code size() == 0}.
     */
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation iterates over {@code entrySet()} searching
     * for an entry with the specified value.  If such an entry is found,
     * {@code true} is returned.  If the iteration terminates without
     * finding such an entry, {@code false} is returned.  Note that this
     * implementation requires linear time in the size of the map.
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean containsValue(Object value) {
        Iterator<Entry<K,V>> i = entrySet().iterator();
        if (value==null) {
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getValue()==null)
                    return true;
            }
        } else {
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (value.equals(e.getValue()))
                    return true;
            }
        }
        return false;
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation iterates over {@code entrySet()} searching
     * for an entry with the specified key.  If such an entry is found,
     * {@code true} is returned.  If the iteration terminates without
     * finding such an entry, {@code false} is returned.  Note that this
     * implementation requires linear time in the size of the map; many
     * implementations will override this method.
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean containsKey(Object key) {
        Iterator<Map.Entry<K,V>> i = entrySet().iterator();
        if (key==null) {
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getKey()==null)
                    return true;
            }
        } else {
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (key.equals(e.getKey()))
                    return true;
            }
        }
        return false;
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation iterates over {@code entrySet()} searching
     * for an entry with the specified key.  If such an entry is found,
     * the entry's value is returned.  If the iteration terminates without
     * finding such an entry, {@code null} is returned.  Note that this
     * implementation requires linear time in the size of the map; many
     * implementations will override this method.
     *
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     */
    public V get(Object key) {
        Iterator<Entry<K,V>> i = entrySet().iterator();
        if (key==null) {
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getKey()==null)
                    return e.getValue();
            }
        } else {
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                if (key.equals(e.getKey()))
                    return e.getValue();
            }
        }
        return null;
    }


    // Modification Operations

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public V put(K key, V value) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation iterates over {@code entrySet()} searching for an
     * entry with the specified key.  If such an entry is found, its value is
     * obtained with its {@code getValue} operation, the entry is removed
     * from the collection (and the backing map) with the iterator's
     * {@code remove} operation, and the saved value is returned.  If the
     * iteration terminates without finding such an entry, {@code null} is
     * returned.  Note that this implementation requires linear time in the
     * size of the map; many implementations will override this method.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} if the {@code entrySet}
     * iterator does not support the {@code remove} method and this map
     * contains a mapping for the specified key.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     */
    public V remove(Object key) {
        Iterator<Entry<K,V>> i = entrySet().iterator();
        Entry<K,V> correctEntry = null;
        if (key==null) {
            while (correctEntry==null && i.hasNext()) {
                Entry<K,V> e = i.next();
                if (e.getKey()==null)
                    correctEntry = e;
            }
        } else {
            while (correctEntry==null && i.hasNext()) {
                Entry<K,V> e = i.next();
                if (key.equals(e.getKey()))
                    correctEntry = e;
            }
        }

        V oldValue = null;
        if (correctEntry !=null) {
            oldValue = correctEntry.getValue();
            i.remove();
        }
        return oldValue;
    }


    // Bulk Operations

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation iterates over the specified map's
     * {@code entrySet()} collection, and calls this map's {@code put}
     * operation once for each entry returned by the iteration.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} if this map does not support
     * the {@code put} operation and the specified map is nonempty.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation calls {@code entrySet().clear()}.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} if the {@code entrySet}
     * does not support the {@code clear} operation.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     */
    public void clear() {
        entrySet().clear();
    }


    // Views

    /**
     * Each of these fields are initialized to contain an instance of the
     * appropriate view the first time this view is requested.  The views are
     * stateless, so there's no reason to create more than one of each.
     *
     * <p>Since there is no synchronization performed while accessing these fields,
     * it is expected that java.util.Map view classes using these fields have
     * no non-final fields (or any fields at all except for outer-this). Adhering
     * to this rule would make the races on these fields benign.
     *
     * <p>It is also imperative that implementations read the field only once,
     * as in:
     *
     * <pre> {@code
     * public Set<K> keySet() {
     *   Set<K> ks = keySet;  // single racy read
     *   if (ks == null) {
     *     ks = new KeySet();
     *     keySet = ks;
     *   }
     *   return ks;
     * }
     *}</pre>
     */
    transient Set<K>        keySet;
    transient Collection<V> values;

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation returns a set that subclasses {@link AbstractSet}.
     * The subclass's iterator method returns a "wrapper object" over this
     * map's {@code entrySet()} iterator.  The {@code size} method
     * delegates to this map's {@code size} method and the
     * {@code contains} method delegates to this map's
     * {@code containsKey} method.
     *
     * <p>The set is created the first time this method is called,
     * and returned in response to all subsequent calls.  No synchronization
     * is performed, so there is a slight chance that multiple calls to this
     * method will not all return the same set.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new AbstractSet<>() {
                public Iterator<K> iterator() {
                    return new KeyIterator();
                }

                public int size() {
                    return AbstractMap.this.size();
                }

                public boolean isEmpty() {
                    return AbstractMap.this.isEmpty();
                }

                public void clear() {
                    AbstractMap.this.clear();
                }

                public boolean contains(Object k) {
                    return AbstractMap.this.containsKey(k);
                }
            };
            keySet = ks;
        }
        return ks;
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * This implementation returns a collection that subclasses {@link
     * AbstractCollection}.  The subclass's iterator method returns a
     * "wrapper object" over this map's {@code entrySet()} iterator.
     * The {@code size} method delegates to this map's {@code size}
     * method and the {@code contains} method delegates to this map's
     * {@code containsValue} method.
     *
     * <p>The collection is created the first time this method is called, and
     * returned in response to all subsequent calls.  No synchronization is
     * performed, so there is a slight chance that multiple calls to this
     * method will not all return the same collection.
     */
    public Collection<V> values() {
        Collection<V> vals = values;
        if (vals == null) {
            vals = new AbstractCollection<>() {
                public Iterator<V> iterator() {
                    return new ValueIterator();
                }

                public int size() {
                    return AbstractMap.this.size();
                }

                public boolean isEmpty() {
                    return AbstractMap.this.isEmpty();
                }

                public void clear() {
                    AbstractMap.this.clear();
                }

                public boolean contains(Object v) {
                    return AbstractMap.this.containsValue(v);
                }
            };
            values = vals;
        }
        return vals;
    }

    public abstract Set<Entry<K,V>> entrySet();


    // Comparison and hashing

    /**
     * Compares the specified object with this map for equality.  Returns
     * {@code true} if the given object is also a map and the two maps
     * represent the same mappings.  More formally, two maps {@code m1} and
     * {@code m2} represent the same mappings if
     * {@code m1.entrySet().equals(m2.entrySet())}.  This ensures that the
     * {@code equals} method works properly across different implementations
     * of the {@code Map} interface.
     *
     * @implSpec
     * This implementation first checks if the specified object is this map;
     * if so it returns {@code true}.  Then, it checks if the specified
     * object is a map whose size is identical to the size of this map; if
     * not, it returns {@code false}.  If so, it iterates over this map's
     * {@code entrySet} collection, and checks that the specified map
     * contains each mapping that this map contains.  If the specified map
     * fails to contain such a mapping, {@code false} is returned.  If the
     * iteration completes, {@code true} is returned.
     *
     * @param o object to be compared for equality with this map
     * @return {@code true} if the specified object is equal to this map
     */
    public boolean equals(Object o) {
        if (o == this)
            return true;

        if (!(o instanceof Map<?, ?> m))
            return false;
        if (m.size() != size())
            return false;

        try {
            for (Entry<K, V> e : entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                if (value == null) {
                    if (!(m.get(key) == null && m.containsKey(key)))
                        return false;
                } else {
                    if (!value.equals(m.get(key)))
                        return false;
                }
            }
        } catch (ClassCastException | NullPointerException unused) {
            return false;
        }

        return true;
    }

    /**
     * Returns the hash code value for this map.  The hash code of a map is
     * defined to be the sum of the hash codes of each entry in the map's
     * {@code entrySet()} view.  This ensures that {@code m1.equals(m2)}
     * implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
     * {@code m1} and {@code m2}, as required by the general contract of
     * {@link Object#hashCode}.
     *
     * @implSpec
     * This implementation iterates over {@code entrySet()}, calling
     * {@link Map.Entry#hashCode hashCode()} on each element (entry) in the
     * set, and adding up the results.
     *
     * @return the hash code value for this map
     * @see Map.Entry#hashCode()
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     */
    public int hashCode() {
        int h = 0;
        for (Entry<K, V> entry : entrySet())
            h += entry.hashCode();
        return h;
    }

    /**
     * Returns a string representation of this map.  The string representation
     * consists of a list of key-value mappings in the order returned by the
     * map's {@code entrySet} view's iterator, enclosed in braces
     * ({@code "{}"}).  Adjacent mappings are separated by the characters
     * {@code ", "} (comma and space).  Each key-value mapping is rendered as
     * the key followed by an equals sign ({@code "="}) followed by the
     * associated value.  Keys and values are converted to strings as by
     * {@link String#valueOf(Object)}.
     *
     * @return a string representation of this map
     */
    public String toString() {
        Iterator<Entry<K,V>> i = entrySet().iterator();
        if (! i.hasNext())
            return "{}";

        StringBuilder sb = new StringBuilder();
        sb.append('{');
        for (;;) {
            Entry<K,V> e = i.next();
            K key = e.getKey();
            V value = e.getValue();
            sb.append(key   == this ? "(this Map)" : key);
            sb.append('=');
            sb.append(value == this ? "(this Map)" : value);
            if (! i.hasNext())
                return sb.append('}').toString();
            sb.append(',').append(' ');
        }
    }

    /**
     * Returns a shallow copy of this {@code AbstractMap} instance: the keys
     * and values themselves are not cloned.
     *
     * @return a shallow copy of this map
     */
    protected Object clone() throws CloneNotSupportedException {
        AbstractMap<?,?> result = (AbstractMap<?,?>)super.clone();
        result.keySet = null;
        result.values = null;
        return result;
    }

    /**
     * Utility method for SimpleEntry and SimpleImmutableEntry.
     * Test for equality, checking for nulls.
     *
     * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
     */
    private static boolean eq(Object o1, Object o2) {
        return o1 == null ? o2 == null : o1.equals(o2);
    }

    // Implementation Note: SimpleEntry and SimpleImmutableEntry
    // are distinct unrelated classes, even though they share
    // some code. Since you can't add or subtract final-ness
    // of a field in a subclass, they can't share representations,
    // and the amount of duplicated code is too small to warrant
    // exposing a common abstract class.


    /**
     * An Entry maintaining a key and a value.  The value may be
     * changed using the {@code setValue} method. Instances of
     * this class are not associated with any map nor with any
     * map's entry-set view.
     *
     * @apiNote
     * This class facilitates the process of building custom map
     * implementations. For example, it may be convenient to return
     * arrays of {@code SimpleEntry} instances in method
     * {@code Map.entrySet().toArray}.
     *
     * @param <K> the type of key
     * @param <V> the type of the value
     *
     * @since 1.6
     */
    public static class SimpleEntry<K,V>
        implements Entry<K,V>, java.io.Serializable
    {
        @java.io.Serial
        private static final long serialVersionUID = -8499721149061103585L;

        @SuppressWarnings("serial") // Conditionally serializable
        private final K key;
        @SuppressWarnings("serial") // Conditionally serializable
        private V value;

        /**
         * Creates an entry representing a mapping from the specified
         * key to the specified value.
         *
         * @param key the key represented by this entry
         * @param value the value represented by this entry
         */
        public SimpleEntry(K key, V value) {
            this.key   = key;
            this.value = value;
        }

        /**
         * Creates an entry representing the same mapping as the
         * specified entry.
         *
         * @param entry the entry to copy
         */
        public SimpleEntry(Entry<? extends K, ? extends V> entry) {
            this.key   = entry.getKey();
            this.value = entry.getValue();
        }

        /**
         * Returns the key corresponding to this entry.
         *
         * @return the key corresponding to this entry
         */
        public K getKey() {
            return key;
        }

        /**
         * Returns the value corresponding to this entry.
         *
         * @return the value corresponding to this entry
         */
        public V getValue() {
            return value;
        }

        /**
         * Replaces the value corresponding to this entry with the specified
         * value.
         *
         * @param value new value to be stored in this entry
         * @return the old value corresponding to the entry
         */
        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        /**
         * 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 represent the same mapping.  More formally, two
         * entries {@code e1} and {@code e2} represent the same mapping
         * if<pre>
         *   (e1.getKey()==null ?
         *    e2.getKey()==null :
         *    e1.getKey().equals(e2.getKey()))
         *   &amp;&amp;
         *   (e1.getValue()==null ?
         *    e2.getValue()==null :
         *    e1.getValue().equals(e2.getValue()))</pre>
         * This ensures that the {@code equals} method works properly across
         * different implementations of the {@code Map.Entry} interface.
         *
         * @param o object to be compared for equality with this map entry
         * @return {@code true} if the specified object is equal to this map
         *         entry
         * @see    #hashCode
         */
        public boolean equals(Object o) {
            return o instanceof Map.Entry<?, ?> e
                    && eq(key, e.getKey())
                    && eq(value, e.getValue());
        }

        /**
         * Returns the hash code value for this map entry.  The hash code
         * of a map entry {@code e} is defined to be: <pre>
         *   (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
         *   (e.getValue()==null ? 0 : e.getValue().hashCode())</pre>
         * This ensures that {@code e1.equals(e2)} implies that
         * {@code e1.hashCode()==e2.hashCode()} for any two Entries
         * {@code e1} and {@code e2}, as required by the general
         * contract of {@link Object#hashCode}.
         *
         * @return the hash code value for this map entry
         * @see    #equals
         */
        public int hashCode() {
            return (key   == null ? 0 :   key.hashCode()) ^
                   (value == null ? 0 : 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
         */
        public String toString() {
            return key + "=" + value;
        }

    }

    /**
     * An unmodifiable Entry maintaining a key and a value.  This class
     * does not support the {@code setValue} method. Instances of
     * this class are not associated with any map nor with any map's
     * entry-set view.
     *
     * @apiNote
     * Instances of this class are not necessarily immutable, as the key
     * and value may be mutable. An instance of <i>this specific class</i>
     * is unmodifiable, because the key and value references cannot be
     * changed. A reference of this <i>type</i> may not be unmodifiable,
     * as a subclass may be modifiable or may provide the appearance of modifiability.
     * <p>
     * This class may be convenient in methods that return thread-safe snapshots of
     * key-value mappings. For alternatives, see the
     * {@link Map#entry Map::entry} and {@link Map.Entry#copyOf Map.Entry::copyOf}
     * methods.
     *
     * @param <K> the type of the keys
     * @param <V> the type of the value
     * @since 1.6
     */
    public static class SimpleImmutableEntry<K,V>
        implements Entry<K,V>, java.io.Serializable
    {
        @java.io.Serial
        private static final long serialVersionUID = 7138329143949025153L;

        @SuppressWarnings("serial") // Not statically typed as Serializable
        private final K key;
        @SuppressWarnings("serial") // Not statically typed as Serializable
        private final V value;

        /**
         * Creates an entry representing a mapping from the specified
         * key to the specified value.
         *
         * @param key the key represented by this entry
         * @param value the value represented by this entry
         */
        public SimpleImmutableEntry(K key, V value) {
            this.key   = key;
            this.value = value;
        }

        /**
         * Creates an entry representing the same mapping as the
         * specified entry.
         *
         * @param entry the entry to copy
         */
        public SimpleImmutableEntry(Entry<? extends K, ? extends V> entry) {
            this.key   = entry.getKey();
            this.value = entry.getValue();
        }

        /**
         * Returns the key corresponding to this entry.
         *
         * @return the key corresponding to this entry
         */
        public K getKey() {
            return key;
        }

        /**
         * Returns the value corresponding to this entry.
         *
         * @return the value corresponding to this entry
         */
        public V getValue() {
            return value;
        }

        /**
         * Replaces the value corresponding to this entry with the specified
         * value (optional operation).  This implementation simply throws
         * {@code UnsupportedOperationException}, as this class implements
         * an unmodifiable map entry.
         *
         * @implSpec
         * The implementation in this class always throws {@code UnsupportedOperationException}.
         *
         * @param value new value to be stored in this entry
         * @return (Does not return)
         * @throws UnsupportedOperationException always
         */
        public V setValue(V value) {
            throw new UnsupportedOperationException();
        }

        /**
         * 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 represent the same mapping.  More formally, two
         * entries {@code e1} and {@code e2} represent the same mapping
         * if<pre>
         *   (e1.getKey()==null ?
         *    e2.getKey()==null :
         *    e1.getKey().equals(e2.getKey()))
         *   &amp;&amp;
         *   (e1.getValue()==null ?
         *    e2.getValue()==null :
         *    e1.getValue().equals(e2.getValue()))</pre>
         * This ensures that the {@code equals} method works properly across
         * different implementations of the {@code Map.Entry} interface.
         *
         * @param o object to be compared for equality with this map entry
         * @return {@code true} if the specified object is equal to this map
         *         entry
         * @see    #hashCode
         */
        public boolean equals(Object o) {
            return o instanceof Map.Entry<?, ?> e
                    && eq(key, e.getKey())
                    && eq(value, e.getValue());
        }

        /**
         * Returns the hash code value for this map entry.  The hash code
         * of a map entry {@code e} is defined to be: <pre>
         *   (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
         *   (e.getValue()==null ? 0 : e.getValue().hashCode())</pre>
         * This ensures that {@code e1.equals(e2)} implies that
         * {@code e1.hashCode()==e2.hashCode()} for any two Entries
         * {@code e1} and {@code e2}, as required by the general
         * contract of {@link Object#hashCode}.
         *
         * @return the hash code value for this map entry
         * @see    #equals
         */
        public int hashCode() {
            return (key   == null ? 0 :   key.hashCode()) ^
                   (value == null ? 0 : 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
         */
        public String toString() {
            return key + "=" + value;
        }
    }

    /**
     * Delegates all Collection methods to the provided non-sequenced map view,
     * except add() and addAll(), which throw UOE. This provides the common
     * implementation of each of the sequenced views of the SequencedMap.
     * Each view implementation is a subclass that provides an instance of the
     * non-sequenced view as a delegate and an implementation of reversed().
     * Each view also inherits the default implementations for the sequenced
     * methods from SequencedCollection or SequencedSet.
     * <p>
     * Ideally this would be a private class within SequencedMap, but private
     * classes aren't permitted within interfaces.
     * <p>
     * The non-sequenced map view is obtained by calling the abstract view()
     * method for each operation.
     *
     * @param <E> the view's element type
     */
    /* non-public */ abstract static class ViewCollection<E> implements Collection<E> {
        UnsupportedOperationException uoe() { return new UnsupportedOperationException(); }
        abstract Collection<E> view();

        public boolean add(E t) { throw uoe(); }
        public boolean addAll(Collection<? extends E> c) { throw uoe(); }
        public void clear() { view().clear(); }
        public boolean contains(Object o) { return view().contains(o); }
        public boolean containsAll(Collection<?> c) { return view().containsAll(c); }
        public void forEach(Consumer<? super E> c) { view().forEach(c); }
        public boolean isEmpty() { return view().isEmpty(); }
        public Iterator<E> iterator() { return view().iterator(); }
        public Stream<E> parallelStream() { return view().parallelStream(); }
        public boolean remove(Object o) { return view().remove(o); }
        public boolean removeAll(Collection<?> c) { return view().removeAll(c); }
        public boolean removeIf(Predicate<? super E> filter) { return view().removeIf(filter); }
        public boolean retainAll(Collection<?> c) { return view().retainAll(c); }
        public int size() { return view().size(); }
        public Spliterator<E> spliterator() { return view().spliterator(); }
        public Stream<E> stream() { return view().stream(); }
        public Object[] toArray() { return view().toArray(); }
        public <T> T[] toArray(IntFunction<T[]> generator) { return view().toArray(generator); }
        public <T> T[] toArray(T[] a) { return view().toArray(a); }
        public String toString() { return view().toString(); }
    }

    // Iterator implementations.

    final class KeyIterator implements Iterator<K> {
        private final Iterator<Entry<K,V>> i = entrySet().iterator();
        public boolean hasNext() { return i.hasNext(); }
        public void remove() { i.remove(); }
        public K next() { return i.next().getKey(); }
    }

    final class ValueIterator implements Iterator<V> {
        private final Iterator<Entry<K,V>> i = entrySet().iterator();
        public boolean hasNext() { return i.hasNext(); }
        public void remove() { i.remove(); }
        public V next() { return i.next().getValue(); }
    }
}