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#acl YoonKyungKoo:read,write All:read = Iterating HashMap Tip = October 10, 2004 |
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!'''[http://united-airline.boom.ru/ | United Airlines tickets flights] !'''! !'''[http://computer-desk.fromru.com/ | Computer desk] !'''! |
HashMap이나 Hashtable과 같은 클래스를 iterator를 사용하여 각 엔트리들을 찾을 때 Map 인터페이스가 제공하는 keySet(), entrySet(), values() 등을 사용하게 됩니다. 이들 중 어느 것이 더 효율적일까요? 간단한 테스트를 통해 알아볼 수 있습니다. |
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첫번째 방법은 keySet() 을 사용하여 key를 검색한 다음 Map에 대해 get()을 호출하여 value를 구하는 방법입니다. {{{#!java Iterator it = map.keySet().iterator(); StringBuffer buffer = new StringBuffer(); while (it.hasNext()) { Object key = it.next(); buffer.append("key=").append(key).append(",value=").append(map.get(key)); } }}} 두번째 방법은 entrySet()을 사용하여 key와 value를 구하는 방법입니다. {{{#!java Iterator it = map.entrySet().iterator(); StringBuffer buffer = new StringBuffer(); while (it.hasNext()) { Map.Entry entry = (Map.Entry) it.next(); buffer.append("key=").append(entry.getKey()).append(",value=").append(entry.getValue()); } }}} |
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Best links: '''equipment''' | 세번째 방법은 key 정보가 필요없을 때 value만을 차례로 읽어들이는 방법입니다. |
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{{{#!java Iterator it = map.values().iterator(); StringBuffer buffer = new StringBuffer(); for (int i = 0; it.hasNext(); i++) { buffer.append("value=").append(it.next()); } }}} |
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물론 키 정보만 필요할 때에는 keySet()을 사용하면 되겠지요. 위의 세 가지 방법 중 가장 효율이 좋은 것은 물론 key 값에 대한 access를 하지 않는 values()의 경우입니다. 사실 이 경우는 다른 두 가지와 직접적인 비교는 의미가 없겠지요. 아무래도 다른 용도에 사용되는 것이니까요. |
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Best links: '''executive recruiting''' |
많이 사용되는 것은 첫번째와 두번째인데 결론적으로 말하자면, key와 value를 모두 찾아야 하는 Map의 순차는 entrySet()을 사용하는 것이 더 효율적이라는 것입니다. keySet()의 경우에는 key를 가져온 다음 다시 Map.get(key)를 호출하여 해싱 알고리즘을 매번 거쳐야 하므로 아무래도 엔트리에 key, value 쌍이 다 들어있는 entrySet() 방식에 비해 프로세싱이 더 들 수밖에 없습니다. 내부적으로 lock을 사용하는 Hashtable 같은 경우에는 entrySet() 방식이 keySet()과 get()을 함께 쓰는 방식보다 더 확연한 차이를 보여주게 됩니다. |
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물론 key만 필요하거나 value만 필요할 때에는 해당하는 keySet()이나 values()를 사용하는 것이 효율적일 것입니다. |
Iterating HashMap Tip
October 10, 2004
HashMap이나 Hashtable과 같은 클래스를 iterator를 사용하여 각 엔트리들을 찾을 때 Map 인터페이스가 제공하는 keySet(), entrySet(), values() 등을 사용하게 됩니다. 이들 중 어느 것이 더 효율적일까요? 간단한 테스트를 통해 알아볼 수 있습니다.
첫번째 방법은 keySet() 을 사용하여 key를 검색한 다음 Map에 대해 get()을 호출하여 value를 구하는 방법입니다.
1 Iterator it = map.keySet().iterator();
2 StringBuffer buffer = new StringBuffer();
3 while (it.hasNext()) {
4 Object key = it.next();
5 buffer.append("key=").append(key).append(",value=").append(map.get(key));
6 }
두번째 방법은 entrySet()을 사용하여 key와 value를 구하는 방법입니다.
1 Iterator it = map.entrySet().iterator();
2 StringBuffer buffer = new StringBuffer();
3 while (it.hasNext()) {
4 Map.Entry entry = (Map.Entry) it.next();
5 buffer.append("key=").append(entry.getKey()).append(",value=").append(entry.getValue());
6 }
세번째 방법은 key 정보가 필요없을 때 value만을 차례로 읽어들이는 방법입니다.
1 Iterator it = map.values().iterator();
2 StringBuffer buffer = new StringBuffer();
3 for (int i = 0; it.hasNext(); i++) {
4 buffer.append("value=").append(it.next());
5 }
물론 키 정보만 필요할 때에는 keySet()을 사용하면 되겠지요. 위의 세 가지 방법 중 가장 효율이 좋은 것은 물론 key 값에 대한 access를 하지 않는 values()의 경우입니다. 사실 이 경우는 다른 두 가지와 직접적인 비교는 의미가 없겠지요. 아무래도 다른 용도에 사용되는 것이니까요.
많이 사용되는 것은 첫번째와 두번째인데 결론적으로 말하자면, key와 value를 모두 찾아야 하는 Map의 순차는 entrySet()을 사용하는 것이 더 효율적이라는 것입니다. keySet()의 경우에는 key를 가져온 다음 다시 Map.get(key)를 호출하여 해싱 알고리즘을 매번 거쳐야 하므로 아무래도 엔트리에 key, value 쌍이 다 들어있는 entrySet() 방식에 비해 프로세싱이 더 들 수밖에 없습니다. 내부적으로 lock을 사용하는 Hashtable 같은 경우에는 entrySet() 방식이 keySet()과 get()을 함께 쓰는 방식보다 더 확연한 차이를 보여주게 됩니다.
물론 key만 필요하거나 value만 필요할 때에는 해당하는 keySet()이나 values()를 사용하는 것이 효율적일 것입니다.