HashMap 的合並密鑰

Question

我正在從數據庫中獲取數據並制作 map，如下所示：

   List<MyBO> data = callDatabase();

   //here I am grouping the channels by run date
    Map<String, List<String>> groupDatesByChannel = data .parallelStream().collect(
                Collectors.groupingBy(MyBO::getChannelName,
                        Collectors.mapping(MyBO::getRunDate, Collectors.toList())));

現在的問題是，對於其中一個通道，運行日期是每小時一次，即通道上的批處理每小時運行一次，因此從數據庫中獲取的特定源運行日期的形式是 yyyyMMddHH，而不是 yyyyMMdd。

我的代碼適用於運行日期為 yyyMMdd 格式的頻道，我上面的 map 示例（groupDatesByChannel）如下：

Channel_2[20200101 , 20200103 , 20200107],
Channel_5[20200103 ]
Channel_7[20200102 , 20200104 ]

我想為每小時運行日期的頻道制作此映射。

Channel_1[**2020010100,2020010101...2020010123,20200101**, 20200102, 20200104 ],
Channel_2[20200101 , 20200103 , 20200107],
Channel_5[20200103 ]
Channel_7[20200102 , 20200104 ]   

至

    Channel_1[**20200101**, 20200102, 20200104 ],
    Channel_2[20200101 , 20200103 , 20200107],
    Channel_5[20200103 ]
    Channel_7[20200102 , 20200104 ]

即我想合並所有相同的日期，無論是在 yyyyMMddHH 還是 yyyyMMdd fromat。

Answer 1

以下代碼經過測試並且工作正常。

public static Map<String, List<String>> mergeData(Map<String, List<String>> originalData){

    Map<String, List<String>> mergedData = new HashMap<String, List<String>>();

    for(String key : originalData.keySet()){
        // Cleaning the key string. Removing *'s
        String trimmedKey = key.replaceAll("\\*","");
        // Trimming the key string. Trimming the hour digits, so that key will only contain date value
        String dateOnlyKey = trimmedKey.substring(0,8);
        // If mergedData already has the key, add values to the list.
        if(mergedData.containsKey(dateOnlyKey)){
            // Adding directly to the list may create duplicate entries. So, appending all data to HashSet and writing back to list
            // If having duplicates is ok, next two lines can be ommitted and third line would be mergedData.get(dateOnlyKey).addAll(originalData.get(key));
            Set<String> channels = new HashSet<String>(mergedData.get(dateOnlyKey));
            channels.addAll(originalData.get(key));
            mergedData.put(dateOnlyKey, new ArrayList<String>(channels));
        }
        // If mergedData doesn't have the key, add the new entry
        else{
            mergedData.put(dateOnlyKey, originalData.get(key));
        }
    }
    System.out.println(mergedData.entrySet());
    return mergedData;

}

邏輯很簡單

1. 新建 HashMap
2. 從原始數據中讀取每個鍵值對
3. 修剪鍵以僅包含日期值
4. 使用 trimmedKey 將值插入新的 HashMap

您可以運行代碼並自行檢查。 對我來說工作得很好。

Answer 2

實現這一目標的最簡單方法是僅將 DateTime 的前 8 個字符用於分組。

Map<String, List<String>> groupDatesByChannel = data .parallelStream().collect(
            Collectors.groupingBy(myBO -> myBO.getChannelName().substring(0,8),
                    Collectors.mapping(MyBO::getRunDate, Collectors.toList())));