Java: How to sort custom type ArrayList

Question

I have a custom type Position(x,y,z),now I create a ArrayList, i want to sort this array ordered by the value of z, from small to b

Answer 1

I use this (just an example I cut and past but same idea) for descending sort:

@Override
public int compare(Member m1, Member m2) {

    double fit1 = m1.getFitness() ;
    double fit2 = m2.getFitness() ;
    if (fit2>fit1)
            return 1;
    else if (fit2