第一種:
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,以後一定要使用此種方式!
第二種:
Map map = new HashMap();
Iterator iter = map.keySet().iterator();
while (iter.hasNext()) {
Object key = iter.next();
Object val = map.get(key);
}
效率低,以後儘量少使用!
例:
HashMap的遍歷有兩種常用的方法,那就是使用keyset及entryset來進行遍歷,但兩者的遍歷速度是有差別的,下面請看實例:
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() 返回與此項對應的鍵
// entry.getValue() 返回與此項對應的值
System.out.print(entry.getValue());
}
System.out.println();
System.out.println(Calendar.getInstance().getTimeInMillis() - bs);
}
}
對於keySet其實是遍歷了2次,一次是轉爲iterator,一次就從hashmap中取出key所對於的value。而entryset只是遍歷了第一次,他把key和value都放到了entry中,所以就快了。
到底keyset快還是entryset快,這個例子輸出太多影響效果,改一下例子。
Java代碼
1.public class HashMapTest {
2. private static int MAXKEY = 1234567;
3. public static void main(String[] args) {
4. HashMap hashmap = new HashMap();
5. for (int i = 0; i < HashMapTest.MAXKEY; i++) {
6. hashmap.put(i, "thanks");
7. }
8. long bs = Calendar.getInstance().getTimeInMillis();
9. Iterator iterator = hashmap.keySet().iterator();
10. while (iterator.hasNext()) {
11. Object key = iterator.next();
12. Object value = hashmap.get(key);
13. }
14. System.out.print(Calendar.getInstance().getTimeInMillis() - bs + ",");
15. listHashMap();
16. }
17.
18. public static void listHashMap() {
19. HashMap hashmap = new java.util.HashMap();
20. for (int i = 0; i < HashMapTest.MAXKEY; i++) {
21. hashmap.put(i, "thanks");
22. }
23. long bs = Calendar.getInstance().getTimeInMillis();
24. Iterator it = hashmap.entrySet().iterator();
25. Map.Entry entry;
26. while (it.hasNext()) {
27. entry = (java.util.Map.Entry) it.next();
28. Object key = entry.getKey();
29. Object value = entry.getValue();
30. //Object value = hashmap.get(key);
31. }
32. System.out.print(Calendar.getInstance().getTimeInMillis() - bs);
33. }
34.}
public class HashMapTest {
private static int MAXKEY = 1234567;
public static void main(String[] args) {
HashMap hashmap = new HashMap();
for (int i = 0; i < HashMapTest.MAXKEY; i++) {
hashmap.put(i, "thanks");
}
long bs = Calendar.getInstance().getTimeInMillis();
Iterator iterator = hashmap.keySet().iterator();
while (iterator.hasNext()) {
Object key = iterator.next();
Object value = hashmap.get(key);
}
System.out.print(Calendar.getInstance().getTimeInMillis() - bs + ",");
listHashMap();
}
public static void listHashMap() {
HashMap hashmap = new java.util.HashMap();
for (int i = 0; i < HashMapTest.MAXKEY; i++) {
hashmap.put(i, "thanks");
}
long bs = Calendar.getInstance().getTimeInMillis();
Iterator it = hashmap.entrySet().iterator();
Map.Entry entry;
while (it.hasNext()) {
entry = (java.util.Map.Entry) it.next();
Object key = entry.getKey();
Object value = entry.getValue();
//Object value = hashmap.get(key);
}
System.out.print(Calendar.getInstance().getTimeInMillis() - bs);
}
}
/*
* 先來測測,i爲1234567時,輸出 63,47 47,47 63,47 測了三次
* 改一下代碼,listHashMap方法的while循環中,Object value = hashmap.get(key)
* 再測,63,79 47,78 63,79 三次
*/
keySet比entrySet慢嗎?
keySet第一次遍歷獲得所有key,entrySet第一次遍歷獲得所有key和value,這一步顯然entrySet會慢,看下一步。
iterator之後,keySet的next()是key,entrySet的next()是Map.Entry(包含了key和value)。
改變代碼之後的測試表明,entrySet的getValue()比Map的get()確實快(雖然不知道它們內部是怎樣的原理實現,雖然有人說Map的get()消耗很小)。當然,沒有理由使用更改後的代碼,別誤導了。
於是,我想entrySet()貌似是快一些,把第一次遍歷的獲得同時得到key和value多花的時間搶回來了。
不過要按key排序的話,似乎keySet更方便一些。
Java代碼
1.Object[] key_arr = hashmap.keySet().toArray();
2.Arrays.sort(key_arr);
3.for (Object key : key_arr) {
4. Object value = hashmap.get(key);
5.}