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.