AbstractMap.java

/*
 * Copyright (c) 1997, 2018, 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.io.Serializable;

/**
 * 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
 */
// Map的抽象实现
public abstract class AbstractMap<K,V> implements Map<K,V> {
    
    /**
     * 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;        // key的集合
    transient Collection<V> values; // value的集合
    
    
    
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractMap() {
    }
    
    
    
    /*▼ 存值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @implSpec This implementation always throws an
     * {@code UnsupportedOperationException}.
     */
    // 将指定的元素（key-value）存入Map，并返回旧值，允许覆盖
    public V put(K key, V value) {
        throw new UnsupportedOperationException();
    }
    
    /**
     * {@inheritDoc}
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@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.
     */
    // 将指定Map中的元素存入到当前Map（允许覆盖）
    public void putAll(Map<? extends K, ? extends V> m) {
        for(Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            put(e.getKey(), e.getValue());
        }
    }
    
    /*▲ 存值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@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.
     */
    // 根据指定的key获取对应的value，如果不存在，则返回null
    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;
    }
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 移除 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@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.
     */
    // 移除拥有指定key的元素，并返回刚刚移除的元素的值
    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;
    }
    
    
    /**
     * {@inheritDoc}
     *
     * @throws UnsupportedOperationException {@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.
     */
    // 清空当前Map中所有元素
    public void clear() {
        entrySet().clear();
    }
    
    /*▲ 移除 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@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.
     */
    // 判断Map中是否存在指定key的元素
    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}
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@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.
     */
    // 判断Map中是否存在指定value的元素
    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;
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@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.
     */
    // 获取Map中key的集合
    public Set<K> keySet() {
        Set<K> ks = keySet;
        
        if(ks == null) {
            ks = new AbstractSet<K>() {
                public Iterator<K> iterator() {
                    return new Iterator<K>() {
                        private Iterator<Entry<K, V>> i = entrySet().iterator();
                        
                        public boolean hasNext() {
                            return i.hasNext();
                        }
                        
                        public K next() {
                            return i.next().getKey();
                        }
                        
                        public void remove() {
                            i.remove();
                        }
                    };
                }
                
                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.
     */
    // 获取Map中value的集合
    public Collection<V> values() {
        Collection<V> vals = values;
        
        if(vals == null) {
            vals = new AbstractCollection<V>() {
                public Iterator<V> iterator() {
                    return new Iterator<V>() {
                        private Iterator<Entry<K, V>> i = entrySet().iterator();
                        
                        public boolean hasNext() {
                            return i.hasNext();
                        }
                        
                        public V next() {
                            return i.next().getValue();
                        }
                        
                        public void remove() {
                            i.remove();
                        }
                    };
                }
                
                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;
    }
    
    // 获取Map中key-value对的集合
    public abstract Set<Entry<K, V>> entrySet();
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @implSpec This implementation returns {@code entrySet().size()}.
     */
    // 获取当前Map中的元素数量
    public int size() {
        return entrySet().size();
    }
    
    /**
     * {@inheritDoc}
     *
     * @implSpec This implementation returns {@code size() == 0}.
     */
    // 判断当前Map是否为空集
    public boolean isEmpty() {
        return size() == 0;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * 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);
    }
    
    
    
    /**
     * 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.
     *
     * @param o object to be compared for equality with this map
     *
     * @return {@code true} if the specified object is equal to this map
     *
     * @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.
     */
    public boolean equals(Object o) {
        if(o == this) {
            return true;
        }
        
        if(!(o instanceof Map)) {
            return false;
        }
        
        Map<?, ?> m = (Map<?, ?>) o;
        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 unused) {
            return false;
        } catch(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}.
     *
     * @return the hash code value for this map
     *
     * @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.
     * @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;
    }
    
    
    
    
    
    
    /*
     * 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.  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}.
     *
     * @since 1.6
     */
    // 键值对的简单实现
    public static class SimpleEntry<K, V> implements Entry<K, V>, Serializable {
        private static final long serialVersionUID = -8499721149061103585L;
        
        private final K key;
        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) {
            if(!(o instanceof Map.Entry)) {
                return false;
            }
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return 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 Entry maintaining an immutable key and value.  This class
     * does not support method {@code setValue}.  This class may be
     * convenient in methods that return thread-safe snapshots of
     * key-value mappings.
     *
     * @since 1.6
     */
    // 键值对的简单实现(只读版本的)
    public static class SimpleImmutableEntry<K, V> implements Entry<K, V>, Serializable {
        private static final long serialVersionUID = 7138329143949025153L;
        
        private final K key;
        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 <i>immutable</i> map entry.
         *
         * @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) {
            if(!(o instanceof Map.Entry)) {
                return false;
            }
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return 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;
        }
    }
}