如何遍历HashMap

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

java Map 遍历速度最优解

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！

---

BEST:

• public static void printMap(Map mp) {
  Iterator it = mp.entrySet().iterator();
  while (it.hasNext()) {
  Map.Entry pairs = (Map.Entry)it.next();
  System.out.println(pairs.getKey() + " = " + pairs.getValue());
  it.remove(); // avoids a ConcurrentModificationException
  }
  }

If you're only interested in the keys, you can iterate through the keySet() of the map:

• Map<String, Object> map = ...;
• for (String key : map.keySet()) {
  // ...
  }

If you only need the values, use values():

• for (Object value : map.values()) {
  // ...
  }

Finally, if you want both the key and value, use entrySet():

• for (Map.Entry<String, Object> entry : map.entrySet()) {
  String key = entry.getKey();
  Object value = entry.getValue();
  // ...
  }

One caveat: if you want to remove items mid-iteration, you'll need to do so via an Iterator (see BEST, before ). However, changing item values is OK .

来自为知笔记(Wiz)