HashMap 的合并密钥

Question

我正在从数据库中获取数据并制作 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())));

现在的问题是，对于其中一个通道，运行日期是每小时一次，即通道上的批处理每小时运行一次，因此从数据库中获取的特定源运行日期的形式是 yyyyMMddHH，而不是 yyyyMMdd。

我的代码适用于运行日期为 yyyMMdd 格式的频道，我上面的 map 示例（groupDatesByChannel）如下：

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

我想为每小时运行日期的频道制作此映射。

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 ]

即我想合并所有相同的日期，无论是在 yyyyMMddHH 还是 yyyyMMdd fromat。

Answer 1

以下代码经过测试并且工作正常。

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;

}

逻辑很简单

1. 新建 HashMap
2. 从原始数据中读取每个键值对
3. 修剪键以仅包含日期值
4. 使用 trimmedKey 将值插入新的 HashMap

您可以运行代码并自行检查。 对我来说工作得很好。

Answer 2

实现这一目标的最简单方法是仅将 DateTime 的前 8 个字符用于分组。

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