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I have a map kind of grouping values by key Map<String, List<Integer>>, i want to revert in order to map each value to the corresponding key
Example: I want to transform the code below
Map<String, List<Integer>> mapOfIntList = new HashMap<String, List<Integer>>();
mapOfIntList.put("UNIT", Arrays.asList(1, 2, 3, 8, 7, 0, 8, 6));
mapOfIntList.put("TEN", Arrays.asList(24, 90, 63, 87));
mapOfIntList.put("HUNDRED", Arrays.asList(645, 457, 306, 762));
mapOfIntList.put("THOUSAND", Arrays.asList(1234, 3456, 5340, 9876));
to another Map(Integer, String) where i can find : (1, "UNIT"), (2, "UNIT")...(24, "TEN"), (90, "TEN")...(645, "HUNDRED")...(3456, "THOUSAND")...
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Method 1: ( Using reverse() method) This method is used to reverse the elements or mappings order in the LinkedHashMap. Parameters: myList is the List provided to the Collections. reverse(myList) method. Returns: It does not return anything but modifies the list internally.
util. HashMap. get() method of HashMap class is used to retrieve or fetch the value mapped by a particular key mentioned in the parameter. It returns NULL when the map contains no such mapping for the key.
Example: Get key for a given value in HashMap Here, the entrySet() method returns a set view of all the entries. Inside the if statement we check if the value from the entry is the same as the given value. And, for matching value, we get the corresponding key.
A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value. It models the mathematical function abstraction.
You can use
Map<Integer, String> mapNumberToType = mapOfIntList.entrySet().stream()
.collect(HashMap::new, (m,e)->e.getValue().forEach(v->m.put(v,e.getKey())), Map::putAll);
You may recognize the similarity to the forEach based code of this answer within the second function passed to collect (the accumulator) function. For a sequential execution, they do basically the same, but this Stream solution supports parallel processing. That’s why it needs the other two functions, to support creating local containers and to merge them.
See also the Mutable reduction section of the documentation.
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