How to get nth item from a TreeMap?

Question

I have a TreeMap<Date, Integer> and I want to fetch the nth item from that map. What I came up with now is this:

((Integer)myTreeMap.values().toArray()[index]).intValue();

but this feels quite clunky, not to mention what is happening on the heap or with performance?

Is there a concise way to get the nth item from a TreeMap/SortedMap ?

Answer 1

Edit: an alternative is

final Optional<T> first =
        treeMap.values()
               .stream()
               .skip(index - 1)
               .findFirst();

if (first.isPresent()) {
    final T value = first.get();
}

What about

final List<T> values = new ArrayList<>(myTreeMap.values());
final T value = values.get(index);

T is just a generic type in place of your Map value type.
index is your nth element.

As pointed out, that's inefficient for large maps, as internally the ArrayList constructor the toArray method is called

public Object[] toArray() {
    // Estimate size of array; be prepared to see more or fewer elements
    Object[] r = new Object[size()];
    Iterator<E> it = iterator();
    for (int i = 0; i < r.length; i++) {
        if (! it.hasNext()) // fewer elements than expected
            return Arrays.copyOf(r, i);
        r[i] = it.next();
    }
    return it.hasNext() ? finishToArray(r, it) : r;
}