在进入（Iterator.hasNext（））循环之前，Map.isEmpty（）是否有意义？

Question

In the following code snippet, does the check for dic.isEmpty() result in any performance improvement?

    for (Map<String, String> dic : dics) {
      if (!dic.isEmpty()) { 
        Iterator<Map.Entry<String, String>> it = dic.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<String, String> pair = it.next();
            Log.d("Substitute", pair.getKey() + " => " + pair.getValue());
        }
      }
    }

After all, if the map/dic is empty, the while() loop will not be entered, so it looks like the check for dic.isEmpty() is redundant - unless there is some other justification for it?

Answer 1

It is redundant. I'd say, leave it out. This smells premature micro-optimization.

Since the empty map should not be treated as a special case in this scenario, the isEmpty just clutters the code imo.

Answer 2

It is an attempt to prematurely optimize the code.

Your Map is either always going to be empty or not empty:

1. If it is empty, then returning back a "no item" iterator is likely to return back the same static "empty iterator" (Collections.EmptyIterator) so the impact on memory is negligable.

2. If it is full, then the check is just going to slow down the access to the needed iterator.

The main issue is that it could incur race conditions, if another thread adds items between the check for empty and the grab of the iterator. That said, it's likely that the cost of returning a null iterator is negligible (many tuned collections return back a singleton null iterator), so the extra check is overkill.