我有一個地圖列表如下：List<Map<String,Object>> someObjectsList = new ArrayList<Map<String,Object>>();我在每個 HashMap 中存儲以下數(shù)據(jù)key value2017-07-21 2017-07-21-07.33.28.4293402017-07-24 2017-07-24-01.23.33.5913402017-07-24 2017-07-24-01.23.33.4923402017-07-21 2017-07-21-07.33.28.429540我想遍歷HashMaps 的列表并檢查鍵是否與任何HashMap值的前 10 個字符匹配，然后我想以以下格式存儲這些鍵和值。即通過使用遙測“逗號”。最終目的是將 HashMap 的唯一鍵及其相對值（如果鍵與任何HashMap值的前 10 個字符匹配）分組到一個新的 HashMap 中。key value2017-07-21 2017-07-21-07.33.28.429340,2017-07-21-07.33.28.4295402017-07-24 2017-07-24-01.23.33.591340,2017-07-24-01.23.33.492340我正在嘗試使用以下 java 代碼StringJoiner，但沒有得到預(yù)期的結(jié)果。關(guān)于如何在這里構(gòu)建邏輯的任何線索？import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;import java.util.StringJoiner;public class SampleOne { public static void main(String[] args) { // TODO Auto-generated method stub List<Map<String, Object>> someObjectsList = new ArrayList<Map<String, Object>>(); Map<String, Object> mapOne = new HashMap<String, Object>(); mapOne.put("2017-07-21", "2017-07-21-07.33.28.429340"); Map<String, Object> mapTwo = new HashMap<String, Object>(); mapTwo.put("2017-07-24", "2017-07-24-01.23.33.591340"); Map<String, Object> mapThree = new HashMap<String, Object>(); mapThree.put("2017-07-24", "2017-07-24-01.23.33.492340"); Map<String, Object> mapFour = new HashMap<String, Object>(); mapFour.put("2017-07-21", "2017-07-21-07.33.28.429540"); someObjectsList.add(mapOne); someObjectsList.add(mapTwo); someObjectsList.add(mapThree); someObjectsList.add(mapFour);
4 回答
慕田峪9158850
TA貢獻1794條經(jīng)驗 獲得超7個贊
使用.merge函數(shù)：
Map<String, Object> finalMap = new HashMap<String, Object>();
for (Map map : someObjectsList) {
for (Object key : map.keySet()) {
String value = ((String) map.get(key));
finalMap.merge((String) key, value, (k, v) -> k + "," + v);
}
}
輸出：
{2017-07-21=2017-07-21-07.33.28.429340,2017-07-21-07.33.28.429540, 2017-07-24=2017-07-24-01.23.33.591340,2017-07-24-01.23.33.492340}
同樣可以通過以下一行實現(xiàn)：
someObjectsList.stream()
.flatMap(i -> i.entrySet().stream())
.collect(Collectors.toMap(Entry::getKey, Entry::getValue,
(k, v) -> k + "," + v));
慕沐林林
TA貢獻2016條經(jīng)驗 獲得超9個贊
你有什么理由Object過度使用String并避免安全檢查？也就是說，它不是“前 10 個字符”，你想看看是否value以key句號開頭（你所有的鍵都是 10 個字符）。所以在那種情況下你可以這樣做if (value.startsWith(key)) { ... }。如果 stringjoiner 未滿，請不要忘記您的換行符。最后，您不需要 a List， aMap可以一次容納多個密鑰。另一種方法：
//LinkedHashMap will preserve our insertion order
Map<String, String> map = new LinkedHashMap<>();
map.put("2017-07-21", "2017-07-21-07.33.28.429340");
map.put("2017-07-24", "2017-07-24-01.23.33.591340");
//note duplicates are overwritten, but no value change here
map.put("2017-07-24", "2017-07-24-01.23.33.492340");
map.put("2017-07-21", "2017-07-21-07.33.28.429540");
// You can also use Java 8 streams for the concatenation
// but I left it simple
List<String> matches = map.entrySet()
.filter(e -> e.getValue().startsWith(e.getKey())
.collect(Collectors.toList());
String concatenated = String.join("\n", matches);
如果你想生成沒有流的字符串，它看起來像這樣（同樣，#entrySet為了簡單起見不使用，但在這里會更有效率）：
List<String> matches = new ArrayList<>();
StringJoiner joiner = new StringJoiner("\n");
for (String key : map.keySet()) {
String value = map.get(key);
if (value.startsWith(key)) {
joiner.add(value);
}
}
//joiner#toString will give the expected result
慕無忌1623718
TA貢獻1744條經(jīng)驗 獲得超4個贊
您正在尋找處理 a 的分組行為List。你可以利用的優(yōu)勢java流自從Java-8. 在任何情況下，您都需要一個新Map的來存儲值以便打印它們。:
someObjectsList.stream()
.flatMap(i -> i.entrySet().stream()) // flatmapping to entries
.collect(Collectors.groupingBy(Entry::getKey)) // grouping them using the key
如果您想使用 for 循環(huán)。在這種情況下，它會更難，因為每個列表項中可能會出現(xiàn)更多條目：
final Map<String, List<Object>> map = new HashMap<>();
for (Map<String, Object> m: someObjectsList) { // iterate List<Map>
for (Entry<String, Object> entry: m.entrySet()) { // iterate entries of each Map
List<Object> list;
final String key = entry.getKey(); // key of the entry
final Object value = entry.getValue(); // value of the entry
if (map.containsKey(key)) { // if the key exists
list = map.get(key); // ... use it
} else {
list = new ArrayList<>(); // ... or else create a new one
}
list.add(value); // add the new value
map.put(key, list); // and add/update the entry
}
}
在這兩種情況下打印出來Map<String, List<Object>> map將產(chǎn)生以下輸出：
2017-07-21=[2017-07-21-07.33.28.429340, 2017-07-21-07.33.28.429540],
2017-07-24=[2017-07-24-01.23.33.591340, 2017-07-24-01.23.33.492340]
繁花如伊
TA貢獻2012條經(jīng)驗 獲得超12個贊
在您的代碼中，您在每張地圖上使用不同的 StringJoiner。因此，它正在創(chuàng)建它的一個新實例。
您可以將鑰匙保存在地圖上。示例代碼：（編輯：我沒有刪除您的 StringJoiner 部分。）
public static void main(String[] args) {
// TODO Auto-generated method stub
List<Map<String, Object>> someObjectsList = new ArrayList<Map<String, Object>>();
Map<String, Object> mapOne = new HashMap<String, Object>();
mapOne.put("2017-07-21", "2017-07-21-07.33.28.429340");
Map<String, Object> mapTwo = new HashMap<String, Object>();
mapTwo.put("2017-07-24", "2017-07-24-01.23.33.591340");
Map<String, Object> mapThree = new HashMap<String, Object>();
mapThree.put("2017-07-24", "2017-07-24-01.23.33.492340");
Map<String, Object> mapFour = new HashMap<String, Object>();
mapFour.put("2017-07-21", "2017-07-21-07.33.28.429540");
someObjectsList.add(mapOne);
someObjectsList.add(mapTwo);
someObjectsList.add(mapThree);
someObjectsList.add(mapFour);
Map<String, Object> outputMap = new HashMap<String, Object>();
for (Map map : someObjectsList) {
StringJoiner sj = new StringJoiner(",");
for (Object key : map.keySet()) {
String value = ((String) map.get(key));
String date = value.substring(0, Math.min(value.length(), 10));
//System.out.println(str);
//System.out.println(value);
if(key.equals(date)) {
sj.add(value);
System.out.println(sj.toString());
if(outputMap.containsKey(key)) {
String str = (String) map.get(key);
str = str + "," + value;
outputMap.put((String)key, str);
} else {
outputMap.put((String)key, value);
}
}
}
}
for (String map : outputMap.keySet()) {
System.out.println(map + " " + outputMap.get(map));
}
}
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