EnumMap类源码解析

EnumMap
内部通过数组存在元素
key：表示的是枚举类型，这个类型要一样
用value存储枚举具体的存储值
通过ordinal方法，使得有序存储

package java.util;

import java.util.Map.Entry;
import sun.misc.SharedSecrets;

/**
 * @author Josh Bloch
 * @see EnumSet
 * @since 1.5
 */
public class EnumMap<K extends Enum<K>, V> extends AbstractMap<K, V>
    implements java.io.Serializable, Cloneable
{
    /**
     * 类型
     *
     * @serial
     */
    private final Class<K> keyType;

    /**
     * key要是枚举类型
     */
    private transient K[] keyUniverse;

    /**
     *内部数组存储values
     */
    private transient Object[] vals;

    /**
     *map的大小
     */
    private transient int size = 0;

    /**
     * 空时候
     */
    private static final Object NULL = new Object() {
        public int hashCode() {
            return 0;
        }

        public String toString() {
            return "java.util.EnumMap.NULL";
        }
    };

    private Object maskNull(Object value) {
        return (value == null ? NULL : value);
    }

    private V unmaskNull(Object value) {
        return (V) (value == NULL ? null : value);
    }
    // 0 个元素的 Enum 
    private static final Enum[] ZERO_LENGTH_ENUM_ARRAY = new Enum[0];

    /**
     * 指定key类型的构造函数
     *
     * @param keyType the class object of the key type for this enum map
     * @throws NullPointerException if <tt>keyType</tt> is null
     */
    public EnumMap(Class<K> keyType) {
        this.keyType = keyType;
        keyUniverse = getKeyUniverse(keyType);
        vals = new Object[keyUniverse.length];
    }

    /**
     * 创建一个enum map 和指定Enummap有相同的key类型，并用该map进行初始化
     *
     * @param m the enum map from which to initialize this enum map
     * @throws NullPointerException if <tt>m</tt> is null
     */
    public EnumMap(EnumMap<K, ? extends V> m) {
        keyType = m.keyType;
        keyUniverse = m.keyUniverse;
        vals = m.vals.clone();
        size = m.size;
    }

    /**
     * 利用指定map 创建一个enum map
     *
     * @param m the map from which to initialize this enum map
     * @throws IllegalArgumentException if <tt>m</tt> is not an
     *     <tt>EnumMap</tt> instance and contains no mappings
     * @throws NullPointerException if <tt>m</tt> is null
     */
    public EnumMap(Map<K, ? extends V> m) {
        if (m instanceof EnumMap) {
            EnumMap<K, ? extends V> em = (EnumMap<K, ? extends V>) m;
            keyType = em.keyType;
            keyUniverse = em.keyUniverse;
            vals = em.vals.clone();
            size = em.size;
        } else {
            if (m.isEmpty())
                throw new IllegalArgumentException("Specified map is empty");
            keyType = m.keySet().iterator().next().getDeclaringClass();
            keyUniverse = getKeyUniverse(keyType);
            vals = new Object[keyUniverse.length];
            putAll(m);
        }
    }

    // Query Operations

    /**
     * 元素个数
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        return size;
    }

    /** 
     * 是否包含value的元素 先验证是否是有效的value，在遍历
     * @param value the value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to this value
     */
    public boolean containsValue(Object value) {
        value = maskNull(value);

        for (Object val : vals)
            if (value.equals(val))
                return true;

        return false;
    }

    /**
     *是否包含key 的元素，先验证是否是有效的key，再遍历
     * @param key the 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) {
        return isValidKey(key) && vals[((Enum)key).ordinal()] != null;
    }
    // 是否包含该元素 
    private boolean containsMapping(Object key, Object value) {
        return isValidKey(key) &&
            maskNull(value).equals(vals[((Enum)key).ordinal()]);
    }

    /**
     * 获取key对应value 
     */
    public V get(Object key) {
        return (isValidKey(key) ?
                unmaskNull(vals[((Enum)key).ordinal()]) : null);
    }

    // Modification Operations

    /**
     * 加入元素 
     * @throws NullPointerException if the specified key is null
     */
    public V put(K key, V value) {
        typeCheck(key);

        int index = key.ordinal();
        Object oldValue = vals[index];
        vals[index] = maskNull(value);
        if (oldValue == null)
            size++;
        return unmaskNull(oldValue);
    }

    /**
     * 删除元素
     */
    public V remove(Object key) {
        if (!isValidKey(key))
            return null;
        int index = ((Enum)key).ordinal();
        Object oldValue = vals[index];
        vals[index] = null;
        if (oldValue != null)
            size--;
        return unmaskNull(oldValue);
    }

    private boolean removeMapping(Object key, Object value) {
        if (!isValidKey(key))
            return false;
        int index = ((Enum)key).ordinal();
        if (maskNull(value).equals(vals[index])) {
            vals[index] = null;
            size--;
            return true;
        }
        return false;
    }

    /**
     *是否有效的key
     */
    private boolean isValidKey(Object key) {
        if (key == null)
            return false;

        // Cheaper than instanceof Enum followed by getDeclaringClass
        Class keyClass = key.getClass();
        return keyClass == keyType || keyClass.getSuperclass() == keyType;
    }

    // Bulk Operations

    /**
     * map中元素加入到EnumMap
     * @param m the mappings to be stored in this map
     * @throws NullPointerException the specified map is null, or if
     *     one or more keys in the specified map are null
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        if (m instanceof EnumMap) {
            EnumMap<? extends K, ? extends V> em =
                (EnumMap<? extends K, ? extends V>)m;
            if (em.keyType != keyType) {
                if (em.isEmpty())
                    return;
                throw new ClassCastException(em.keyType + " != " + keyType);
            }

            for (int i = 0; i < keyUniverse.length; i++) {
                Object emValue = em.vals[i];
                if (emValue != null) {
                    if (vals[i] == null)
                        size++;
                    vals[i] = emValue;
                }
            }
        } else {
            super.putAll(m);
        }
    }

    /**
     * 清空
     */
    public void clear() {
        Arrays.fill(vals, null);
        size = 0;
    }

    // Views


    private transient Set<Map.Entry<K,V>> entrySet = null;

    /**
     * @return a set view of the keys contained in this enum map
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks != null)
            return ks;
        else
            return keySet = new KeySet();
    }

    private class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsKey(o);
        }
        public boolean remove(Object o) {
            int oldSize = size;
            EnumMap.this.remove(o);
            return size != oldSize;
        }
        public void clear() {
            EnumMap.this.clear();
        }
    }

    /**
     * @return a collection view of the values contained in this map
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs != null)
            return vs;
        else
            return values = new Values();
    }

    private class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }
        public int size() {
            return size;
        }
        public boolean contains(Object o) {
            return containsValue(o);
        }
        public boolean remove(Object o) {
            o = maskNull(o);

            for (int i = 0; i < vals.length; i++) {
                if (o.equals(vals[i])) {
                    vals[i] = null;
                    size--;
                    return true;
                }
            }
            return false;
        }
        public void clear() {
            EnumMap.this.clear();
        }
    }

    /**
     * @return a set view of the mappings contained in this enum map
     */
    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        if (es != null)
            return es;
        else
            return entrySet = new EntrySet();
    }

    private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry entry = (Map.Entry)o;
            return containsMapping(entry.getKey(), entry.getValue());
        }
        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry entry = (Map.Entry)o;
            return removeMapping(entry.getKey(), entry.getValue());
        }
        public int size() {
            return size;
        }
        public void clear() {
            EnumMap.this.clear();
        }
        public Object[] toArray() {
            return fillEntryArray(new Object[size]);
        }
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                a = (T[])java.lang.reflect.Array
                    .newInstance(a.getClass().getComponentType(), size);
            if (a.length > size)
                a[size] = null;
            return (T[]) fillEntryArray(a);
        }
        private Object[] fillEntryArray(Object[] a) {
            int j = 0;
            for (int i = 0; i < vals.length; i++)
                if (vals[i] != null)
                    a[j++] = new AbstractMap.SimpleEntry<>(
                        keyUniverse[i], unmaskNull(vals[i]));
            return a;
        }
    }

    private abstract class EnumMapIterator<T> implements Iterator<T> {
        // Lower bound on index of next element to return
        int index = 0;

        // Index of last returned element, or -1 if none
        int lastReturnedIndex = -1;

        public boolean hasNext() {
            while (index < vals.length && vals[index] == null)
                index++;
            return index != vals.length;
        }

        public void remove() {
            checkLastReturnedIndex();

            if (vals[lastReturnedIndex] != null) {
                vals[lastReturnedIndex] = null;
                size--;
            }
            lastReturnedIndex = -1;
        }

        private void checkLastReturnedIndex() {
            if (lastReturnedIndex < 0)
                throw new IllegalStateException();
        }
    }

    private class KeyIterator extends EnumMapIterator<K> {
        public K next() {
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturnedIndex = index++;
            return keyUniverse[lastReturnedIndex];
        }
    }

    private class ValueIterator extends EnumMapIterator<V> {
        public V next() {
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturnedIndex = index++;
            return unmaskNull(vals[lastReturnedIndex]);
        }
    }

    private class EntryIterator extends EnumMapIterator<Map.Entry<K,V>> {
        private Entry lastReturnedEntry = null;

        public Map.Entry<K,V> next() {
            if (!hasNext())
                throw new NoSuchElementException();
            lastReturnedEntry = new Entry(index++);
            return lastReturnedEntry;
        }

        public void remove() {
            lastReturnedIndex =
                ((null == lastReturnedEntry) ? -1 : lastReturnedEntry.index);
            super.remove();
            lastReturnedEntry.index = lastReturnedIndex;
            lastReturnedEntry = null;
        }

        private class Entry implements Map.Entry<K,V> {
            private int index;

            private Entry(int index) {
                this.index = index;
            }

            public K getKey() {
                checkIndexForEntryUse();
                return keyUniverse[index];
            }

            public V getValue() {
                checkIndexForEntryUse();
                return unmaskNull(vals[index]);
            }

            public V setValue(V value) {
                checkIndexForEntryUse();
                V oldValue = unmaskNull(vals[index]);
                vals[index] = maskNull(value);
                return oldValue;
            }

            public boolean equals(Object o) {
                if (index < 0)
                    return o == this;

                if (!(o instanceof Map.Entry))
                    return false;

                Map.Entry e = (Map.Entry)o;
                V ourValue = unmaskNull(vals[index]);
                Object hisValue = e.getValue();
                return (e.getKey() == keyUniverse[index] &&
                        (ourValue == hisValue ||
                         (ourValue != null && ourValue.equals(hisValue))));
            }

            public int hashCode() {
                if (index < 0)
                    return super.hashCode();

                return entryHashCode(index);
            }

            public String toString() {
                if (index < 0)
                    return super.toString();

                return keyUniverse[index] + "="
                    + unmaskNull(vals[index]);
            }

            private void checkIndexForEntryUse() {
                if (index < 0)
                    throw new IllegalStateException("Entry was removed");
            }
        }
    }

    // Comparison and hashing

    /**
     * equals
     */
    public boolean equals(Object o) {
        if (this == o)
            return true;
        if (o instanceof EnumMap)
            return equals((EnumMap)o);
        if (!(o instanceof Map))
            return false;

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

        for (int i = 0; i < keyUniverse.length; i++) {
            if (null != vals[i]) {
                K key = keyUniverse[i];
                V value = unmaskNull(vals[i]);
                if (null == value) {
                    if (!((null == m.get(key)) && m.containsKey(key)))
                       return false;
                } else {
                   if (!value.equals(m.get(key)))
                      return false;
                }
            }
        }

        return true;
    }

    private boolean equals(EnumMap em) {
        if (em.keyType != keyType)
            return size == 0 && em.size == 0;

        // Key types match, compare each value
        for (int i = 0; i < keyUniverse.length; i++) {
            Object ourValue =    vals[i];
            Object hisValue = em.vals[i];
            if (hisValue != ourValue &&
                (hisValue == null || !hisValue.equals(ourValue)))
                return false;
        }
        return true;
    }

    /**
     * hashCode
     */
    public int hashCode() {
        int h = 0;

        for (int i = 0; i < keyUniverse.length; i++) {
            if (null != vals[i]) {
                h += entryHashCode(i);
            }
        }

        return h;
    }

    private int entryHashCode(int index) {
        return (keyUniverse[index].hashCode() ^ vals[index].hashCode());
    }

    /**
     * clone
     */
    public EnumMap<K, V> clone() {
        EnumMap<K, V> result = null;
        try {
            result = (EnumMap<K, V>) super.clone();
        } catch(CloneNotSupportedException e) {
            throw new AssertionError();
        }
        result.vals = result.vals.clone();
        result.entrySet = null;
        return result;
    }

    /**
     * Throws an exception if e is not of the correct type for this enum set.
     */
    private void typeCheck(K key) {
        Class keyClass = key.getClass();
        if (keyClass != keyType && keyClass.getSuperclass() != keyType)
            throw new ClassCastException(keyClass + " != " + keyType);
    }

    /**
     * Returns all of the values comprising K.
     * The result is uncloned, cached, and shared by all callers.
     */
    private static <K extends Enum<K>> K[] getKeyUniverse(Class<K> keyType) {
        return SharedSecrets.getJavaLangAccess()
                                        .getEnumConstantsShared(keyType);
    }

    private static final long serialVersionUID = 458661240069192865L;

    /**
     * Save the state of the <tt>EnumMap</tt> instance to a stream (i.e.,
     * serialize it).
     *
     * @serialData The <i>size</i> of the enum map (the number of key-value
     *             mappings) is emitted (int), followed by the key (Object)
     *             and value (Object) for each key-value mapping represented
     *             by the enum map.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException
    {
        // Write out the key type and any hidden stuff
        s.defaultWriteObject();

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

        // Write out keys and values (alternating)
        int entriesToBeWritten = size;
        for (int i = 0; entriesToBeWritten > 0; i++) {
            if (null != vals[i]) {
                s.writeObject(keyUniverse[i]);
                s.writeObject(unmaskNull(vals[i]));
                entriesToBeWritten--;
            }
        }
    }

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

        keyUniverse = getKeyUniverse(keyType);
        vals = new Object[keyUniverse.length];

        // 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++) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            put(key, value);
        }
    }
}