hollow/src/main/java/com/netflix/hollow/core/index/HollowSparseIntegerSet.java
public long size() {
        SparseBitSet current;
        long size;
        do {
            current = sparseBitSetVolatile;
            size = current.estimateBitsUsed();
        } while (current != sparseBitSetVolatile);
        return size;
    }
public long size() {
        SparseBitSet current;
        long size;
        do {
            current = sparseBitSetVolatile;
            size = current.estimateBitsUsed();
        } while (current != sparseBitSetVolatile);
        return size;
    }
Estimate the total number of bits used to represent the integer set.

@return Calculates the total number of bits used by longs in underlying data structure.