Java的foreach效率 [英] Java foreach efficiency

问题描述

  Map< String，String> myMap = ...; （字符串键：myMap.keySet（））{
 System.out.println（key）; 
 
 
 System.out.println（myMap.get（key））; 
 
 
 
 $ b $  myMap.keySet（）在 foreach循环中调用一次？ 
我想是的，但是想要你的意见。
 
 
我想知道如果以这种方式使用foreach（ myMap.keySet （））对性能有影响，或者相当于这个： 
 
 
  Set< String> keySet = myMap.keySet（）; 
 for（String key：keySet）{
 ... 
} 
  
 
 
解决方案
如果你想完全确定，那么编译它并反编译并比较。我做了这个与以下来源︰
 
 
  public void test（）{
 Map< String，String> myMap = new HashMap< String，String>（）; （字符串键：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））; 
 
 
 
 
 
 
 
当我用 Jad ，我得到： 
 
 
  public void test（）
 {
 Map myMap = new HashMap（）; 
字符串键; 
 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（）; 
字符串键; 
 for Iterator iterator1 = keySet.iterator（）; iterator1.hasNext（）; System.out.println（（String）myMap.get（key）））
 {
 key =（字符串）iterator1.next（）; 
 System.out.println（key）; 
 
 
 
 
 $ b $ p $所以你有答案。它用for循环形式调用一次。
 
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) {
   ...
}

解决方案
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.

                        
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