jdk源码剖析之EnumMap

EnumMap的实现原理

    EnumMap底层是通过数组实现的，他是通过Enum的ordinal实现对数组的索引，当我们要使用get方法，实际上是对数组vals[index]做索引操作，而相比于hashMap实际上是先进行hash，然后针对entry做next遍历，因此比较下来EnumMap的get方法的效率相对HashMap等要高一些。

重要方法的实现

    1.get方法

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * 
More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key == k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * 
A return value of {@code null} does not necessarily
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     */
    public V get(Object key) {
        return (isValidKey(key) ?
                unmaskNull(vals[((Enum)key).ordinal()]) : null);
    }

    从源码中我们可以看出，get方法是通过对数组取值，时间复杂度为O(1)。

    2.put方法

    /**
     * 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 the key with which the specified value is to be associated
     * @param value the value to be associated with the specified key
     *
     * @return the previous value associated with specified key, or
     *     null if there was no mapping for key.  (A null
     *     return can also indicate that the map previously associated
     *     null with the specified key.)
     * @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);
    }

    put方法的关键就是key.ordinal()，可以看出EnumMap依赖其key是Enum这一属性，通过Enum的序列取到响应的值。

    3.初始化

    /**
     * Creates an empty enum map with the specified key type.
     *
     * @param keyType the class object of the key type for this enum map
     * @throws NullPointerException if keyType is null
     */
    public EnumMap(Class keyType) {
        this.keyType = keyType;
        keyUniverse = getKeyUniverse(keyType);
        vals = new Object[keyUniverse.length];
    }

    EnumMap数组初始化大小为Enum枚举类的大小，不需要增大。