Question

I’m new to java world from C++ background. I’d like to port some C++ code to Java.
The code uses Sparse vectors:

struct Feature{
int index;
double value;
};

typedef std::vector<Feature> featvec_t;

As I understood, if one makes an object, there will be some overhead on memory usage.
So naive implementation of Feature will overhead signifiantly when there will be 10-100 millions of Features in a set of featvec_t.

How to represent this structure memory efficiently in Java?

Answer 1

If memory is really your bottleneck, try storing your data in two separate arrays:
int[] index and double[] value.

But in most cases with such big structures performance (time) will be the main issue. Depending on operations mostly performed on your data (insert, delete, get, etc.) you need to choose appropriate data structure to store objects of class Feature.
Start your explorations with java.util.Collection interface, its subinterfaces (List, Set, etc) and their implementations provided in java.util package.