HashMap遍历的两种方式

 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!

  例:

  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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