[英]Invert Map | Values ---> Keys
我希望能夠反轉具有多個鍵可以指向相同值的給定 HashMap。
HashMap<String, String> cities = new HashMap<String, String>();
cities.put("Manchester", "UK");
cities.put("London", "UK");
static HashMap<String, String> inverseMap(HashMap map) {
// something that has "UK" point to both "Manchester" and "London"
// if it can be done without using any special Java 8 feature, that would be great
}
我不確定從哪里開始。
像這樣做。 基本上,它使用合並函數來連接重復鍵的值。
HashMap<String, String> cities = new HashMap<String, String>();
cities.put("Manchester", "UK");
cities.put("London", "UK");
cities.put("New York", "US");
cities.put("Chicago", "US");
Map<String,String> inverted = new HashMap<>();
for (String key : cities.keySet()) {
String newKey = cities.get(key);
String value = inverted.get(newKey);
if (value == null) {
inverted.put(newKey, key);
} else {
value = value + ", " + key;
inverted.put(newKey, value);
}
}
for (Entry<String,String> e : inverted.entrySet()) {
System.out.println(e.getKey() + " -> " + e.getValue());
}
它打印
UK -> Manchester, London
US -> New York, Chicago
由於您沒有指定如何處理重復鍵。 我也可以將它存儲在Map<String,List<String>>
由於多個鍵可以包含相同的值,因此您必須能夠在逆映射中為每個鍵存儲多個值。 我建議使用 Set 作為這個值。
這也應該適用於沒有依賴關系的 Java 7。
static HashMap<String, Set<String>> inverseMap(HashMap<String,String> map) {
HashMap<String,Set<String>> inversed=new HashMap<>();
for(Map.Entry<String,String> entry:map.entrySet()){
if(!inversed.containsKey(entry.getValue())){
inversed.put(entry.getValue(),new HashSet<>());
}
inversed.get(entry.getValue()).add(entry.getKey());
}
return inversed;
}
{Manchester=UK,London=UK}
將變成{UK={Manchester,London}}
(順序可能不同)。
這是你要找的嗎?
Map<String, String> cities = new HashMap<>();
cities.put("Manchester", "UK");
cities.put("London", "UK");
Map<String, List<String>> reverseMap = new HashMap<>();
for (Entry<String, String> entry : cities.entrySet()) {
List<String> list = reverseMap.get(entry.getValue());
if (list == null) {
list = new ArrayList<>();
reverseMap.put(entry.getValue(), list);
}
list.add(entry.getKey());
}
System.out.println(reverseMap);
你可以看看MultiMap。 它允許將單個鍵映射到多個值。
這是谷歌番石榴https://guava.dev/releases/19.0/api/docs/com/google/common/collect/Multimap.html
ListMultimap<String, String> multimap = ArrayListMultimap.create();
for (President pres : US_PRESIDENTS_IN_ORDER) {
multimap.put(pres.firstName(), pres.lastName());
}
for (String firstName : multimap.keySet()) {
List<String> lastNames = multimap.get(firstName);
out.println(firstName + ": " + lastNames);
}
... produces output such as:
Zachary: [Taylor]
John: [Adams, Adams, Tyler, Kennedy] // Remember, Quincy!
George: [Washington, Bush, Bush]
Grover: [Cleveland, Cleveland] // Two, non-consecutive terms, rep'ing NJ!
在 Apache Commons Collections https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/MultiValuedMap.html
MultiValuedMap<K, String> map = new MultiValuedHashMap<K, String>();
map.put(key, "A");
map.put(key, "B");
map.put(key, "C");
Collection<String> coll = map.get(key);
coll will be a collection containing "A", "B", "C".
您可以遍歷原始地圖的條目集並使用值(國家/地區代碼)作為鍵並將每個鍵(城市)添加到列表中:
static HashMap<String, List<String>> inverseMap(HashMap<String, String> map) {
HashMap<String, List<String>> countryToCity = new HashMap<>();
for(Map.Entry<String,String> entry: map.entrySet()){
countryToCity.computeIfAbsent(entry.getValue(), k -> new ArrayList<>()).add(entry.getKey());
}
return countryToCity;
}
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