public int indexOfKey(@Nullable 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.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/collection/SimpleArrayMap.java