Java 8 stream map to list of keys sorted by values

Question

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?

Solution 1

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

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

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 4

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 5

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);

View more solutions

Comments

[Alexis C.]

... or .sorted(Map.Entry.comparingByValue())

[dabadaba]

After chaining sorted how can I get the stream back to a map? Well technically I know I couldn't revert it to a map because its sets are not ordered. But I would want to print the map like this .forEach((k,v) -> System.out.println(k + ": " + v). How can I achieve this using that stream?

[Jesper]

@dabadaba after the .sorted(...) you have a stream of entries, so you could append .forEach(e -> System.out.println(e.getKey() + ": " + e.getValue()));

[dabadaba]

@Jesper got it, thanks! And last but not least, how can I get the reversed order? I am using .sorted(Map.Entry.comparingByValue())

[Jesper]

@dabadaba you can use .reversed() but because of limitations in type inference you need to specify type arguments: .sorted(Map.Entry.<Type, Long>comparingByValue().reversed())

[dabadaba]

@Jesper I actually figured out doing it this way: .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()‌​))

[Cugomastik]

what does this (e1, e2) -> e2 do?