在Java中高效地对多个数组列表执行操作

Question

I have ArrayLists of doubles within an ArrayList. So it is something like this

ArrayList<ArrayList<Double>> x = new ArrayList<ArrayList<Double>>();

I have 19 ArrayLists within my x . I have a method called distance(ArrayList x, ArrayList y) which computes the distance between ArrayLists x and y and returns a double . I want to compute the distance between every single vector and all others.

My method would be to do the following

for(int i=0 ; i<(x.size()-1) ; i++)
    {
        for(int j=(i+1) ; j<x.size() ; j++)
        {
            System.out.println(distance( x.get(i) , x.get(j) ));
        }
    }

However, it does not look as efficient. The efficiency is O(N^2). Efficiency is VERY important to me. I need help to figure out what is the MOST efficient way of doing this operation.

Answer 1

If the performance hit is from the algorithm of distance(x,y) and you can't optimize it anyhow, why don't you start by trying to cache the result of distance for a given pair of (x,y)? If you do, please consider also the case where distance(x,y) == distance(y,x) .

Moreover, pre-calculate the x.size() and store it might save you some processing.