Performance issue - clear and reuse a collection OR throw it and get a new one [duplicate]

Question

Say we try to implement a merge sort algorithm, given an Array of Arrays to merge what is a better approach, this:

public void merge(ArrayList<ArrayList<E>> a) {
    ArrayList<ArrayList<E>> tmp = new ArrayList<ArrayList<E>>() ;
    while (a.size()>1) {
        for (int i=1; i<a.size();i+=2) {
            tmp.add(merge(a.get(i-1),a.get(i)));
        }
        if (a.size()%2==1) tmp.add(a.get(a.size()-1));
        a = tmp;
        tmp = new ArrayList<ArrayList<E>>() ;
    }
}

or this :

public void merge(ArrayList<ArrayList<E>> a) {
    ArrayList<ArrayList<E>> tmp = new ArrayList<ArrayList<E>>(),tmp2  ;
    while (a.size()>1) {
        for (int i=1; i<a.size();i+=2) {
            tmp.add(merge(a.get(i-1),a.get(i)));
        }
        if (a.size()%2==1) tmp.add(a.get(a.size()-1));
        tmp2 = a;
        a = tmp;
        tmp = tmp2;
        tmp.clear();
    }
}

to make it clearer, what i was doing is to merge each couple of neighbors in a and put the resulting merged arrays in an external Array of Arrays tmp, after merging all couples, one approach is to clear a and then move tmp to a, and then move the cleared a to tmp. second approach is to "throw" old tmp and get a new tmp instead of reusing the old one.

Answer 1

As a general rule, don't spend energy trying to reuse old collections; it just makes your code harder to read (and frequently doesn't give you any actual benefit). Only try optimizations like these if you already have your code working, and you have hard numbers that say the speed of your algorithm is improved.