AbstractMap学习记录

package java.util;
import java.util.Map.Entry;

/**
* This class provides a skeletal implementation of the <tt>Map</tt>
* interface, to minimize the effort required to implement this interface.
*什么叫最小化的努力去实现这个接口？
* <p>To implement an unmodifiable map, the programmer needs only to extend this
* class and provide an implementation for the <tt>entrySet</tt> method, which
* returns a set-view of the map's mappings.  Typically, the returned set
* will, in turn, be implemented atop <tt>AbstractSet</tt>.  This set should
* not support the <tt>add</tt> or <tt>remove</tt> methods, and its iterator
* should not support the <tt>remove</tt> method.
*
* <p>To implement a modifiable map, the programmer must additionally override
* this class's <tt>put</tt> method (which otherwise throws an
* <tt>UnsupportedOperationException</tt>), and the iterator returned by
* <tt>entrySet().iterator()</tt> must additionally implement its
* <tt>remove</tt> method.
*
* <p>The programmer should generally provide a void (no argument) and map
* constructor, as per the recommendation in the <tt>Map</tt> 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}/../technotes/guides/collections/index.html">
* 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
* @version %I%, %G%
* @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}
     *
     * <p>This implementation returns <tt>entrySet().size()</tt>.
     */
    public int size() {

//为什么是entrySet()的size呢？
    return entrySet().size();
    }

/**
     * {@inheritDoc}
     *
     * <p>This implementation returns <tt>size() == 0</tt>.
     */
    public boolean isEmpty() {

//为什么isEmpty()是判断的size等于0
    return size() == 0;
    }

/**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over <tt>entrySet()</tt> searching
     * for an entry with the specified value.  If such an entry is found,
     * <tt>true</tt> is returned.  If the iteration terminates without
     * finding such an entry, <tt>false</tt> 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) {

//entrySet()这个是个没有实现的方法
    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}
     *
     * <p>This implementation iterates over <tt>entrySet()</tt> searching
     * for an entry with the specified key.  If such an entry is found,
     * <tt>true</tt> is returned.  If the iteration terminates without
     * finding such an entry, <tt>false</tt> 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}
     *
     * <p>This implementation iterates over <tt>entrySet()</tt> 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, <tt>null</tt> 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}
     *
     * <p>This implementation always throws an
     * <tt>UnsupportedOperationException</tt>.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public V put(K key, V value) {
    throw new UnsupportedOperationException();
    }

/**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over <tt>entrySet()</tt> searching for an
     * entry with the specified key.  If such an entry is found, its value is
     * obtained with its <tt>getValue</tt> operation, the entry is removed
     * from the collection (and the backing map) with the iterator's
     * <tt>remove</tt> operation, and the saved value is returned.  If the
     * iteration terminates without finding such an entry, <tt>null</tt> 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
     * <tt>UnsupportedOperationException</tt> if the <tt>entrySet</tt>
     * iterator does not support the <tt>remove</tt> 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}
     *
     * <p>This implementation iterates over the specified map's
     * <tt>entrySet()</tt> collection, and calls this map's <tt>put</tt>
     * operation once for each entry returned by the iteration.
     *
     * <p>Note that this implementation throws an
     * <tt>UnsupportedOperationException</tt> if this map does not support
     * the <tt>put</tt> 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}
     *
     * <p>This implementation calls <tt>entrySet().clear()</tt>.
     *
     * <p>Note that this implementation throws an
     * <tt>UnsupportedOperationException</tt> if the <tt>entrySet</tt>
     * does not support the <tt>clear</tt> 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.
     */
    transient volatile Set<K>        keySet = null;
    transient volatile Collection<V> values = null;

/**
     * {@inheritDoc}
     *
     * <p>This implementation returns a set that subclasses {@link AbstractSet}.
     * The subclass's iterator method returns a "wrapper object" over this
     * map's <tt>entrySet()</tt> iterator.  The <tt>size</tt> method
     * delegates to this map's <tt>size</tt> method and the
     * <tt>contains</tt> method delegates to this map's
     * <tt>containsKey</tt> 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() {
    if (keySet == null) {
        keySet = 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 contains(Object k) {
            return AbstractMap.this.containsKey(k);
        }
        };
    }
    return keySet;
    }

/**
     * {@inheritDoc}
     *
     * <p>returns a This implementation collection that subclasses {@link
     * AbstractCollection}.  The subclass's iterator method returns a
     * "wrapper object" over this map's <tt>entrySet()</tt> iterator.
     * The <tt>size</tt> method delegates to this map's <tt>size</tt>
     * method and the <tt>contains</tt> method delegates to this map's
     * <tt>containsValue</tt> 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() {
    if (values == null) {
        values = 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 contains(Object v) {
            return AbstractMap.this.containsValue(v);
        }
        };
    }
    return values;
    }

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

// Comparison and hashing

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

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

try {
            Iterator<Entry<K,V>> i = entrySet().iterator();
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
        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
     * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
     * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
     * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
     * {@link Object#hashCode}.
     *
     * <p>This implementation iterates over <tt>entrySet()</tt>, 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;
    Iterator<Entry<K,V>> i = entrySet().iterator();
    while (i.hasNext())
        h += i.next().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 <tt>entrySet</tt> view's iterator, enclosed in braces
     * (<tt>"{}"</tt>).  Adjacent mappings are separated by the characters
     * <tt>", "</tt> (comma and space).  Each key-value mapping is rendered as
     * the key followed by an equals sign (<tt>"="</tt>) 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(", ");
    }
    }

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

/**
     * Utility method for SimpleEntry and SimpleImmutableEntry.
     * Test for equality, checking for nulls.
     */
    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 <tt>setValue</tt> method.  This class
     * facilitates the process of building custom map
     * implementations. For example, it may be convenient to return
     * arrays of <tt>SimpleEntry</tt> instances in method
     * <tt>Map.entrySet().toArray</tt>.
     *
     * @since 1.6
     */
    public static class SimpleEntry<K,V>
    implements Entry<K,V>, java.io.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 ("<tt>=</tt>")
         * 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 <tt>setValue</tt>.  This class may be
     * convenient in methods that return thread-safe snapshots of
     * key-value mappings.
     *
     * @since 1.6
     */
    public static class SimpleImmutableEntry<K,V>
    implements Entry<K,V>, java.io.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
         * <tt>UnsupportedOperationException</tt>, 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 ("<tt>=</tt>")
         * followed by the string representation of this entry's value.
         *
         * @return a String representation of this map entry
         */
    public String toString() {
        return key + "=" + value;
    }

}

}