Java中的EnumMap集合解析

    private final Class<K> keyType;
    private transient K[] keyUniverse;
    private transient Object[] vals;
    private transient int size = 0;
    private static final Object NULL = new Object() {
    public int hashCode() {
        return 0;
    }
    public String toString() {
        return "java.util.EnumMap.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);
}

private void typeCheck(K key) {
    Class<?> keyClass = key.getClass();
    if (keyClass != keyType && keyClass.getSuperclass() != keyType)
        throw new ClassCastException(keyClass + " != " + keyType);
}

private Object maskNull(Object value) {
    return (value == null ? NULL : value);
}
@SuppressWarnings("unchecked")
private V unmaskNull(Object value) {
    return (V)(value == NULL ? null : value);
}

private static final Object NULL = new Object() {
    public int hashCode() {
        return 0;
    }
    public String toString() {
        return "java.util.EnumMap.NULL";
    }
};

public V get(Object key) {
    return (isValidKey(key) ?
            unmaskNull(vals[((Enum<?>)key).ordinal()]) : null);
}

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;
}