public K keyAt(int index) {
        return (K)mArray[index << 1];
    }
Return the key at the given index in the array.
@param index The desired index, must be between 0 and {@link #size()}-1.
@return Returns the key stored at the given index.
core/src/main/java/com/bluelinelabs/logansquare/util/SimpleArrayMap.java