Java programming with threads

People need help with an activity:

  

Consider three number vectors.   Write a program to print out the largest Twins in each vector.

     

Which are the largest prime numbers of twins considering all the elements.   (In all vectors)

     

NOTE: The solution should optimize the runtime.        You are not allowed to use the Collections API.

I'm having a hard time in the Threads implementation part where I have to allocate the optimization to the runtime.

public class NumeroPrimoGemeo {

    public List<Integer> retornaNumeroGemeos(Integer comeco, Integer fim){
        int i = comeco;
        while(i <= fim){
            if(verificaNumeroPrimo(i)){
                int x = i + 2;
                if(verificaNumeroPrimo(x)){
                    List<Integer> integers = new ArrayList<>();
                    integers.add(i);
                    integers.add(x);
                    return integers;
                }
            }
            i++;
        } throw new RuntimeException("Não há nenhum numero primo gemeos ");
    }

Can someone make a code and explain me with comments?