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.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/collection/SimpleArrayMap.java