Java 8 stream map to list of keys sorted by values
JavaCollectionsLambdaJava 8Java StreamJava Problem Overview
I have map Map<Type, Long> countByType
and I want to have a list which has sorted (min to max) keys by their corresponding values. My try is:
countByType.entrySet().stream().sorted().collect(Collectors.toList());
however this just gives me a list of entries, how can I get a list of types, without losing the order?
Java Solutions
Solution 1 - Java
You say you want to sort by value, but you don't have that in your code. Pass a lambda (or method reference) to sorted
to tell it how you want to sort.
And you want to get the keys; use map
to transform entries to keys.
List<Type> types = countByType.entrySet().stream()
.sorted(Comparator.comparing(Map.Entry::getValue))
.map(Map.Entry::getKey)
.collect(Collectors.toList());
Solution 2 - Java
You have to sort with a custom comparator based on the value of the entry. Then select all the keys before collecting
countByType.entrySet()
.stream()
.sorted((e1, e2) -> e1.getValue().compareTo(e2.getValue())) // custom Comparator
.map(e -> e.getKey())
.collect(Collectors.toList());
Solution 3 - Java
You can sort a map by value as below, more example here
//Sort a Map by their Value.
Map<Integer, String> random = new HashMap<Integer, String>();
random.put(1,"z");
random.put(6,"k");
random.put(5,"a");
random.put(3,"f");
random.put(9,"c");
Map<Integer, String> sortedMap =
random.entrySet().stream()
.sorted(Map.Entry.comparingByValue())
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
(e1, e2) -> e2, LinkedHashMap::new));
System.out.println("Sorted Map: " + Arrays.toString(sortedMap.entrySet().toArray()));
Solution 4 - Java
Map<Integer, String> map = new HashMap<>();
map.put(1, "B");
map.put(2, "C");
map.put(3, "D");
map.put(4, "A");
List<String> list = map.values()
.stream()
.sorted()
.collect(Collectors.toList());
Output: [A, B, C, D]
Solution 5 - Java
You can use this as an example of your problem
Map<Integer, String> map = new HashMap<>();
map.put(10, "apple");
map.put(20, "orange");
map.put(30, "banana");
map.put(40, "watermelon");
map.put(50, "dragonfruit");
// split a map into 2 List
List<Integer> resultSortedKey = new ArrayList<>();
List<String> resultValues = map.entrySet().stream()
//sort a Map by key and stored in resultSortedKey
.sorted(Map.Entry.<Integer, String>comparingByKey().reversed())
.peek(e -> resultSortedKey.add(e.getKey()))
.map(x -> x.getValue())
// filter banana and return it to resultValues
.filter(x -> !"banana".equalsIgnoreCase(x))
.collect(Collectors.toList());
resultSortedKey.forEach(System.out::println);
resultValues.forEach(System.out::println);
Solution 6 - Java
Here is the simple solution with StreamEx
EntryStream.of(countByType).sortedBy(e -> e.getValue()).keys().toList();