public long keyAt(int index) {
        if (mGarbage) {
            gc();
        }

        return mKeys[index];
    }
Given an index in the range <code>0...size()-1</code>, returns
the key from the <code>index</code>th key-value mapping that this
LongSparseArray stores.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/collection/LongSparseArray.java