How to collect Strings read from file into a custom class using collect from Streams

Question

I have a custom class State

public class State {
    private double sum;
    private int count;
    public void inc(double d){
        sum+=d;
        count++;
    }
    public void add(State s){
        this.sum+=s.sum;
        this.count+=s.count;
    }
    public double average(){
        return sum/count;
    }
}

And I have a tester class for that class:

import java.io.*;
import java.util.Arrays;
import java.util.function.*;
import java.util.stream.*;
import java.nio.file.*;

public class StateTest {
    public static void main(String[] a) throws IOException{
        BiConsumer<State,Double> acc = (s,d)->s.inc(d);
        BiConsumer<State,State> comb = (s,s1)->s.add(s1);
        Stream<String> ss = Files.lines(Path.of(a[0]));
        //double avg = ss.mapToDouble(Double::valueOf).filter(b -> b <= 42.0f && b >= -42.0f).collect(//Dont know what to write here).average()
    }
}

for which I want to do the following:

Read out lines of doubles from a text file, convert them into Doubles, and collect them using .collect(Supplier, Accumulator, Combiner) method.

I am having trouble writing the supplier for State class. I don't also clearly get the meaning of this kind of .collect() and what does it mean to supply? I have to collect it into the State and then call .average() of the State function to get the average of the double.