This method accesses the value of a Map entry, using a key that was retrieved from a keySet iterator. It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) look

sonar掃描的嚴重問題對應。

This method accesses the value of a Map entry, using a key that was retrieved from a keySet iterator. It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.

代碼寫這樣就會報上面的嚴重問題

Map<String, Integer> params = new HashMap<String, Integer>();
        params.put("resultID", 1);
        params.put("execID", 2);
        params.put("num", 3);

        Map<String, Object> mapOut = new HashMap<String, Object>();
        // Map<String, Integer> 轉 HashMap<String, Object>
        Set<String> set = params.keySet();
        for (String key : set) {
            Integer value = params.get(key);
            mapOut.put(key,value);
        }

意思是說要避免用.get(key)，可能有點脫褲子放屁的感覺。

修改后

        for (Map.Entry<String, Integer> entry : params.entrySet()) {
            mapOut.put(entry.getKey(),entry.getValue());
        }

參考這篇

https://www.cnblogs.com/dreammyone/articles/9960400.html

還有這篇

https://blog.csdn.net/jdsjlzx/article/details/34487299

 

This method accesses the value of a Map entry, using a key that was retrieved from a keySet iterator. It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.
解釋：
很多人都這樣遍歷Map，沒錯，但是效率很低，先一個一個的把key遍歷，然后在根據key去查找value，這不是多此一舉么，為什么不遍歷entry（桶）然后直接從entry得到value呢？它們的執行效率大概為1.5:1（有人實際測試過）。
我們看看HashMap.get方法的源代碼： 1. public V get(Object key) { 2. if (key == null) 3. return getForNullKey(); 4. int hash = hash(key.hashCode()); 5. for (Entry<K,V> e = table[indexFor(hash, table.length)]; 6. e != null; 7. e = e.next) { 8. Object k; 9. if (e.hash == hash && ((k = e.key) == key || key.equals(k))) 10. return e.value; 11. } 12. return null; 13. }
從這里可以看出查找value的原理，先計算出hashcode，然后散列表里取出entry，不管是計算hashcode，還是執行循環for以及執行equals方法，都是CPU密集運算，非常耗費CPU資源，如果對一個比較大的map進行遍歷，會出現CPU迅速飈高的現象，直接影響機器的響應速度，在並發的情況下，簡直就是一場災難。
解決方法： 1. for (Map.Entry<String, JMenu> entry : menuList.entrySet()) { 2. mb.add(entry.getValue());
}
for(Map.Entry<String, List<BlackListDO>> tempEntiy: companyBlackItemsMap.entrySet()) {
String key = tempEntiy.getKey();
List<BlackListDO> eachCompanyBlackItems = tempEntiy.getValue();