public int indexOfKey(Object key) {
        return key == null ? indexOfNull() : indexOf(key, key.hashCode());
    }
Returns the index of a key in the set.

@param key The key to search for.
@return Returns the index of the key if it exists, else a negative integer.
core/src/main/java/com/bluelinelabs/logansquare/util/SimpleArrayMap.java