performance - Java foreach efficiency

Question

I have something like this:

Map<String, String> myMap = ...;

for(String key : myMap.keySet()) {
   System.out.println(key);
   System.out.println(myMap.get(key)); 
}

So is myMap.keySet() called once in the foreach loop? I think it is, but want your opinion.

I would like to know if using foreach in this way (myMap.keySet()) has a performance impact or it is equivalent to this:

Set<String> keySet = myMap.keySet();
for (String key : keySet) {
   ...
}

Answer 1

If you want to be absolutely certain, then compile it both ways and decompile it and compare. I did this with the following source:

public void test() {
  Map<String, String> myMap = new HashMap<String, String>();

  for (String key : myMap.keySet()) {
    System.out.println(key);
    System.out.println(myMap.get(key));
  }

  Set<String> keySet = myMap.keySet();
  for (String key : keySet) {
    System.out.println(key);
    System.out.println(myMap.get(key));
  }
}

and when I decompiled the class file with Jad, I get:

public void test()
{
    Map myMap = new HashMap();
    String key;
    for(Iterator iterator = myMap.keySet().iterator(); iterator.hasNext(); System.out.println((String)myMap.get(key)))
    {
        key = (String)iterator.next();
        System.out.println(key);
    }

    Set keySet = myMap.keySet();
    String key;
    for(Iterator iterator1 = keySet.iterator(); iterator1.hasNext(); System.out.println((String)myMap.get(key)))
    {
        key = (String)iterator1.next();
        System.out.println(key);
    }
}

So there's your answer. It is called once with either for-loop form.