HashMap の実装 (Java 1.2.2)

HashMap.java

/*
 * @(#)HashMap.java    1.30 01/11/29
 *
 * Copyright 2002 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;

import java.io.*;

/**
 * Hash table based implementation of the <tt>Map</tt> interface.  This
 * implementation provides all of the optional map operations, and permits
 * <tt>null</tt> values and the <tt>null</tt> key.  (The <tt>HashMap</tt>
 * class is roughly equivalent to <tt>Hashtable</tt>, except that it is
 * unsynchronized and permits nulls.)  This class makes no guarantees as to
 * the order of the map; in particular, it does not guarantee that the order
 * will remain constant over time.<p>
 * <p>
 * This implementation provides constant-time performance for the basic
 * operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
 * disperses the elements properly among the buckets.  Iteration over
 * collection views requires time proportional to the "capacity" of the
 * <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
 * of key-value mappings).  Thus, it's very important not to set the intial
 * capacity too high (or the load factor too low) if iteration performance is
 * important.<p>
 * <p>
 * An instance of <tt>HashMap</tt> has two parameters that affect its
 * performance: <i>initial capacity</i> and <i>load factor</i>.  The
 * <i>capacity</i> is the number of buckets in the hash table, and the initial
 * capacity is simply the capacity at the time the hash table is created.  The
 * <i>load factor</i> is a measure of how full the hash table is allowed to
 * get before its capacity is automatically increased.  When the number of
 * entries in the hash table exceeds the product of the load factor and the
 * current capacity, the capacity is roughly doubled by calling the
 * <tt>rehash</tt> method.<p>
 * <p>
 * As a general rule, te default load factor (.75) offers a good tradeoff
 * between time and space costs.  Higher values decrease the space overhead
 * but increase the lookup cost (reflected in most of the operations of the
 * <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>).  The
 * expected number of entries in the map and its load factor should be taken
 * into account when setting its initial capacity, so as to minimize the
 * number of <tt>rehash</tt> operations.  If the initial capacity is greater
 * than the maximum number of entries divided by the load factor, no
 * <tt>rehash</tt> operations will ever occur.<p>
 * <p>
 * If many mappings are to be stored in a <tt>HashMap</tt> instance, creating
 * it with a sufficiently large capacity will allow the mappings to be stored
 * more efficiently than letting it perform automatic rehashing as needed to
 * grow the table.<p>
 *
 * <b>Note that this implementation is not synchronized.</b> If multiple
 * threads access this map concurrently, and at least one of the threads
 * modifies the map structurally, it <i>must</i> be synchronized externally.
 * (A structural modification is any operation that adds or deletes one or
 * more mappings; merely changing the value associated with a key that an
 * instance already contains is not a structural modification.)  This is
 * typically accomplished by synchronizing on some object that naturally
 * encapsulates the map.  If no such object exists, the map should be
 * "wrapped" using the <tt>Collections.synchronizedMap</tt> method.  This is
 * best done at creation time, to prevent accidental unsynchronized access to
 * the map: <pre> Map m = Collections.synchronizedMap(new HashMap(...));
 * </pre><p>
 * <p>
 * The iterators returned by all of this class's "collection view methods" are
 * <i>fail-fast</i>: if the map is structurally modified at any time after the
 * iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> or <tt>add</tt> methods, the iterator will throw a
 * <tt>ConcurrentModificationException</tt>.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the
 * future.
 *
 * @author Josh Bloch
 * @author Arthur van Hoff
 * @version 1.30, 11/29/01
 * @see Object#hashCode()
 * @see Collection
 * @see Map
 * @see TreeMap
 * @see Hashtable
 * @since JDK1.2
 */

public class HashMap extends AbstractMap implements Map, Cloneable,
        java.io.Serializable {
    /**
     * The hash table data.
     */
    private transient Entry table[];

    /**
     * The total number of mappings in the hash table.
     */
    private transient int count;

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    private transient int modCount = 0;

    /**
     * Constructs a new, empty map with the specified initial
     * capacity and the specified load factor.
     *
     * @param initialCapacity the initial capacity of the HashMap.
     * @param loadFactor      the load factor of the HashMap
     * @throws IllegalArgumentException if the initial capacity is less
     *                                  than zero, or if the load factor is nonpositive.
     */
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Initial Capacity: " +
                    initialCapacity);
        if (loadFactor <= 0)
            throw new IllegalArgumentException("Illegal Load factor: " +
                    loadFactor);
        if (initialCapacity == 0)
            initialCapacity = 1;
        this.loadFactor = loadFactor;
        table = new Entry[initialCapacity];
        threshold = (int) (initialCapacity * loadFactor);
    }

    /**
     * Constructs a new, empty map with the specified initial capacity
     * and default load factor, which is <tt>0.75</tt>.
     *
     * @param initialCapacity the initial capacity of the HashMap.
     * @throws IllegalArgumentException if the initial capacity is less
     *                                  than zero.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    /**
     * Constructs a new, empty map with a default capacity and load
     * factor, which is <tt>0.75</tt>.
     */
    public HashMap() {
        this(101, 0.75f);
    }

    /**
     * Constructs a new map with the same mappings as the given map.  The
     * map is created with a capacity of twice the number of mappings in
     * the given map or 11 (whichever is greater), and a default load factor,
     * which is <tt>0.75</tt>.
     */
    public HashMap(Map t) {
        this(Math.max(2 * t.size(), 11), 0.75f);
        putAll(t);
    }

    /**
     * Returns the number of key-value mappings in this map.
     *
     * @return the number of key-value mappings in this map.
     */
    public int size() {
        return count;
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *
     * @return <tt>true</tt> if this map contains no key-value mappings.
     */
    public boolean isEmpty() {
        return count == 0;
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value.
     *
     * @param value value whose presence in this map is to be tested.
     * @return <tt>true</tt> if this map maps one or more keys to the
     * specified value.
     */
    public boolean containsValue(Object value) {
        Entry tab[] = table;

        if (value == null) {
            for (int i = tab.length; i-- > 0; )
                for (Entry e = tab[i]; e != null; e = e.next)
                    if (e.value == null)
                        return true;
        } else {
            for (int i = tab.length; i-- > 0; )
                for (Entry e = tab[i]; e != null; e = e.next)
                    if (value.equals(e.value))
                        return true;
        }

        return false;
    }

    /**
     * Returns <tt>true</tt> if this map contains a mapping for the specified
     * key.
     *
     * @param key key whose presence in this Map is to be tested.
     * @return <tt>true</tt> if this map contains a mapping for the specified
     * key.
     */
    public boolean containsKey(Object key) {
        Entry tab[] = table;
        if (key != null) {
            int hash = key.hashCode();
            int index = (hash & 0x7FFFFFFF) % tab.length;
            for (Entry e = tab[index]; e != null; e = e.next)
                if (e.hash == hash && key.equals(e.key))
                    return true;
        } else {
            for (Entry e = tab[0]; e != null; e = e.next)
                if (e.key == null)
                    return true;
        }

        return false;
    }

    /**
     * Returns the value to which this map maps the specified key.  Returns
     * <tt>null</tt> if the map contains no mapping for this key.  A return
     * value of <tt>null</tt> does not <i>necessarily</i> indicate that the
     * map contains no mapping for the key; it's also possible that the map
     * explicitly maps the key to <tt>null</tt>.  The <tt>containsKey</tt>
     * operation may be used to distinguish these two cases.
     *
     * @param key key whose associated value is to be returned.
     * @return the value to which this map maps the specified key.
     */
    public Object get(Object key) {
        Entry tab[] = table;

        if (key != null) {
            int hash = key.hashCode();
            int index = (hash & 0x7FFFFFFF) % tab.length;
            for (Entry e = tab[index]; e != null; e = e.next)
                if ((e.hash == hash) && key.equals(e.key))
                    return e.value;
        } else {
            for (Entry e = tab[0]; e != null; e = e.next)
                if (e.key == null)
                    return e.value;
        }

        return null;
    }

    /**
     * Rehashes the contents of this map into a new <tt>HashMap</tt> instance
     * with a larger capacity. This method is called automatically when the
     * number of keys in this map exceeds its capacity and load factor.
     */
    private void rehash() {
        int oldCapacity = table.length;
        Entry oldMap[] = table;

        int newCapacity = oldCapacity * 2 + 1;
        Entry newMap[] = new Entry[newCapacity];

        modCount++;
        threshold = (int) (newCapacity * loadFactor);
        table = newMap;

        for (int i = oldCapacity; i-- > 0; ) {
            for (Entry old = oldMap[i]; old != null; ) {
                Entry e = old;
                old = old.next;

                int index = (e.hash & 0x7FFFFFFF) % newCapacity;
                e.next = newMap[index];
                newMap[index] = e;
            }
        }
    }

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for this key, the old
     * value is replaced.
     *
     * @param key   key with which the specified value is to be associated.
     * @param value value to be associated with the specified key.
     * @return previous value associated with specified key, or <tt>null</tt>
     * if there was no mapping for key.  A <tt>null</tt> return can
     * also indicate that the HashMap previously associated
     * <tt>null</tt> with the specified key.
     */
    public Object put(Object key, Object value) {
        // Makes sure the key is not already in the HashMap.
        Entry tab[] = table;
        int hash = 0;
        int index = 0;

        if (key != null) {
            hash = key.hashCode();
            index = (hash & 0x7FFFFFFF) % tab.length;
            for (Entry e = tab[index]; e != null; e = e.next) {
                if ((e.hash == hash) && key.equals(e.key)) {
                    Object old = e.value;
                    e.value = value;
                    return old;
                }
            }
        } else {
            for (Entry e = tab[0]; e != null; e = e.next) {
                if (e.key == null) {
                    Object old = e.value;
                    e.value = value;
                    return old;
                }
            }
        }

        modCount++;
        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            rehash();

            tab = table;
            index = (hash & 0x7FFFFFFF) % tab.length;
        }

        // Creates the new entry.
        Entry e = new Entry(hash, key, value, tab[index]);
        tab[index] = e;
        count++;
        return null;
    }

    /**
     * Removes the mapping for this key from this map if present.
     *
     * @param key key whose mapping is to be removed from the map.
     * @return previous value associated with specified key, or <tt>null</tt>
     * if there was no mapping for key.  A <tt>null</tt> return can
     * also indicate that the map previously associated <tt>null</tt>
     * with the specified key.
     */
    public Object remove(Object key) {
        Entry tab[] = table;

        if (key != null) {
            int hash = key.hashCode();
            int index = (hash & 0x7FFFFFFF) % tab.length;

            for (Entry e = tab[index], prev = null; e != null;
                 prev = e, e = e.next) {
                if ((e.hash == hash) && key.equals(e.key)) {
                    modCount++;
                    if (prev != null)
                        prev.next = e.next;
                    else
                        tab[index] = e.next;

                    count--;
                    Object oldValue = e.value;
                    e.value = null;
                    return oldValue;
                }
            }
        } else {
            for (Entry e = tab[0], prev = null; e != null;
                 prev = e, e = e.next) {
                if (e.key == null) {
                    modCount++;
                    if (prev != null)
                        prev.next = e.next;
                    else
                        tab[0] = e.next;

                    count--;
                    Object oldValue = e.value;
                    e.value = null;
                    return oldValue;
                }
            }
        }

        return null;
    }

    /**
     * Copies all of the mappings from the specified map to this one.
     * <p>
     * These mappings replace any mappings that this map had for any of the
     * keys currently in the specified Map.
     *
     * @param t Mappings to be stored in this map.
     */
    public void putAll(Map t) {
        Iterator i = t.entrySet().iterator();
        while (i.hasNext()) {
            Map.Entry e = (Map.Entry) i.next();
            put(e.getKey(), e.getValue());
        }
    }

    /**
     * Removes all mappings from this map.
     */
    public void clear() {
        Entry tab[] = table;
        modCount++;
        for (int index = tab.length; --index >= 0; )
            tab[index] = null;
        count = 0;
    }

    /**
     * Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
     * values themselves are not cloned.
     *
     * @return a shallow copy of this map.
     */
    public Object clone() {
        try {
            HashMap t = (HashMap) super.clone();
            t.table = new Entry[table.length];
            for (int i = table.length; i-- > 0; ) {
                t.table[i] = (table[i] != null)
                        ? (Entry) table[i].clone() : null;
            }
            t.keySet = null;
            t.entrySet = null;
            t.values = null;
            t.modCount = 0;
            return t;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        }
    }

    // Views

    private transient Set keySet = null;
    private transient Set entrySet = null;
    private transient Collection values = null;

    /**
     * Returns a set view of the keys contained in this map.  The set is
     * backed by the map, so changes to the map are reflected in the set, and
     * vice-versa.  The set supports element removal, which removes the
     * corresponding mapping from this map, via the <tt>Iterator.remove</tt>,
     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
     * <tt>clear</tt> operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * @return a set view of the keys contained in this map.
     */
    public Set keySet() {
        if (keySet == null) {
            keySet = new AbstractSet() {
                public Iterator iterator() {
                    return new HashIterator(KEYS);
                }

                public int size() {
                    return count;
                }

                public boolean contains(Object o) {
                    return containsKey(o);
                }

                public boolean remove(Object o) {
                    return HashMap.this.remove(o) != null;
                }

                public void clear() {
                    HashMap.this.clear();
                }
            };
        }
        return keySet;
    }

    /**
     * Returns a collection view of the values contained in this map.  The
     * collection is backed by the map, so changes to the map are reflected in
     * the collection, and vice-versa.  The collection supports element
     * removal, which removes the corresponding mapping from this map, via the
     * <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
     * It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * @return a collection view of the values contained in this map.
     */
    public Collection values() {
        if (values == null) {
            values = new AbstractCollection() {
                public Iterator iterator() {
                    return new HashIterator(VALUES);
                }

                public int size() {
                    return count;
                }

                public boolean contains(Object o) {
                    return containsValue(o);
                }

                public void clear() {
                    HashMap.this.clear();
                }
            };
        }
        return values;
    }

    /**
     * Returns a collection view of the mappings contained in this map.  Each
     * element in the returned collection is a <tt>Map.Entry</tt>.  The
     * collection is backed by the map, so changes to the map are reflected in
     * the collection, and vice-versa.  The collection supports element
     * removal, which removes the corresponding mapping from the map, via the
     * <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
     * It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * @return a collection view of the mappings contained in this map.
     * @see Map.Entry
     */
    public Set entrySet() {
        if (entrySet == null) {
            entrySet = new AbstractSet() {
                public Iterator iterator() {
                    return new HashIterator(ENTRIES);
                }

                public boolean contains(Object o) {
                    if (!(o instanceof Map.Entry))
                        return false;
                    Map.Entry entry = (Map.Entry) o;
                    Object key = entry.getKey();
                    Entry tab[] = table;
                    int hash = (key == null ? 0 : key.hashCode());
                    int index = (hash & 0x7FFFFFFF) % tab.length;

                    for (Entry e = tab[index]; e != null; e = e.next)
                        if (e.hash == hash && e.equals(entry))
                            return true;
                    return false;
                }

                public boolean remove(Object o) {
                    if (!(o instanceof Map.Entry))
                        return false;
                    Map.Entry entry = (Map.Entry) o;
                    Object key = entry.getKey();
                    Entry tab[] = table;
                    int hash = (key == null ? 0 : key.hashCode());
                    int index = (hash & 0x7FFFFFFF) % tab.length;

                    for (Entry e = tab[index], prev = null; e != null;
                         prev = e, e = e.next) {
                        if (e.hash == hash && e.equals(entry)) {
                            modCount++;
                            if (prev != null)
                                prev.next = e.next;
                            else
                                tab[index] = e.next;

                            count--;
                            e.value = null;
                            return true;
                        }
                    }
                    return false;
                }

                public int size() {
                    return count;
                }

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

        return entrySet;
    }

    /**
     * HashMap collision list entry.
     */
    private static class Entry implements Map.Entry {
        int hash;
        Object key;
        Object value;
        Entry next;

        Entry(int hash, Object key, Object value, Entry next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        protected Object clone() {
            return new Entry(hash, key, value,
                    (next == null ? null : (Entry) next.clone()));
        }

        // Map.Entry Ops

        public Object getKey() {
            return key;
        }

        public Object getValue() {
            return value;
        }

        public Object setValue(Object value) {
            Object oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry e = (Map.Entry) o;

            return (key == null ? e.getKey() == null : key.equals(e.getKey())) &&
                    (value == null ? e.getValue() == null : value.equals(e.getValue()));
        }

        public int hashCode() {
            return hash ^ (value == null ? 0 : value.hashCode());
        }

        public String toString() {
            return key + "=" + value;
        }
    }

    // Types of Iterators
    private static final int KEYS = 0;
    private static final int VALUES = 1;
    private static final int ENTRIES = 2;

    private class HashIterator implements Iterator {
        Entry[] table = HashMap.this.table;
        int index = table.length;
        Entry entry = null;
        Entry lastReturned = null;
        int type;

        /**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
        private int expectedModCount = modCount;

        HashIterator(int type) {
            this.type = type;
        }

        public boolean hasNext() {
            while (entry == null && index > 0)
                entry = table[--index];

            return entry != null;
        }

        public Object next() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            while (entry == null && index > 0)
                entry = table[--index];

            if (entry != null) {
                Entry e = lastReturned = entry;
                entry = e.next;
                return type == KEYS ? e.key : (type == VALUES ? e.value : e);
            }
            throw new NoSuchElementException();
        }

        public void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            Entry[] tab = HashMap.this.table;
            int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;

            for (Entry e = tab[index], prev = null; e != null;
                 prev = e, e = e.next) {
                if (e == lastReturned) {
                    modCount++;
                    expectedModCount++;
                    if (prev == null)
                        tab[index] = e.next;
                    else
                        prev.next = e.next;
                    count--;
                    lastReturned = null;
                    return;
                }
            }
            throw new ConcurrentModificationException();
        }
    }

    /**
     * Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
     * serialize it).
     *
     * @serialData The <i>capacity</i> of the HashMap (the length of the
     * bucket array) is emitted (int), followed  by the
     * <i>size</i> of the HashMap (the number of key-value
     * mappings), followed by the key (Object) and value (Object)
     * for each key-value mapping represented by the HashMap
     * The key-value mappings are emitted in no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
            throws IOException {
        // Write out the threshold, loadfactor, and any hidden stuff
        s.defaultWriteObject();

        // Write out number of buckets
        s.writeInt(table.length);

        // Write out size (number of Mappings)
        s.writeInt(count);

        // Write out keys and values (alternating)
        for (int index = table.length - 1; index >= 0; index--) {
            Entry entry = table[index];

            while (entry != null) {
                s.writeObject(entry.key);
                s.writeObject(entry.value);
                entry = entry.next;
            }
        }
    }

    private static final long serialVersionUID = 362498820763181265L;

    /**
     * Reconstitute the <tt>HashMap</tt> instance from a stream (i.e.,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
            throws IOException, ClassNotFoundException {
        // Read in the threshold, loadfactor, and any hidden stuff
        s.defaultReadObject();

        // Read in number of buckets and allocate the bucket array;
        int numBuckets = s.readInt();
        table = new Entry[numBuckets];

        // Read in size (number of Mappings)
        int size = s.readInt();

        // Read the keys and values, and put the mappings in the HashMap
        for (int i = 0; i < size; i++) {
            Object key = s.readObject();
            Object value = s.readObject();
            put(key, value);
        }
    }

    int capacity() {
        return table.length;
    }

    float loadFactor() {
        return loadFactor;
    }
}