Java中HashMap遍历的两种方式及其效率比较

第一种:
  Map map = new HashMap();
  Iterator iter = map.entrySet().iterator();
  while (iter.hasNext()) {
  Map.Entry entry = (Map.Entry) iter.next();
  Object key = entry.getKey();
  Object val = entry.getValue();
  }
  效率高,以后一定要使用此种方式!
第二种:
  Map map = new HashMap();
  Iterator iter = map.keySet().iterator();
  while (iter.hasNext()) {
  Object key = iter.next();
  Object val = map.get(key);
  }
  效率低,以后尽量少使用!

HashMap的遍历有两种常用的方法,那就是使用keyset及entryset来进行遍历,但两者的遍历速度是有差别的,下面请看实例:
  public class HashMapTest {
  public static void main(String[] args) ...{
  HashMap hashmap = new HashMap();
  for (int i = 0; i < 1000; i ) ...{
  hashmap.put("" i, "thanks");
  }
  long bs = Calendar.getInstance().getTimeInMillis();
  Iterator iterator = hashmap.keySet().iterator();
  while (iterator.hasNext()) ...{
  System.out.print(hashmap.get(iterator.next()));
  }
  System.out.println();
  System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
  listHashMap();
  }
  public static void listHashMap() ...{
  java.util.HashMap hashmap = new java.util.HashMap();
  for (int i = 0; i < 1000; i ) ...{
  hashmap.put("" i, "thanks");
  }
  long bs = Calendar.getInstance().getTimeInMillis();
  java.util.Iterator it = hashmap.entrySet().iterator();
  while (it.hasNext()) ...{
  java.util.Map.Entry entry = (java.util.Map.Entry) it.next();
  // entry.getKey() 返回与此项对应的键
  
// entry.getValue() 返回与此项对应的值
  System.out.print(entry.getValue());
  }
  System.out.println();
  System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
  }
  }
  对于keySet其实是遍历了2次,一次是转为iterator,一次就从hashmap中取出key所对于的value。而entryset只是遍历了第一次,他把key和value都放到了entry中,所以就快了。

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