How to sort generic list in java8

Question

I have a map Map<K, List<V>> inputMap I want to sort list before storing to list

List<String> outputList = inputMap.keySet()
 .stream()
 .filter(Objects::nonNull)
 .map(key -> (key + "==" + inputMap.get(key).sort(c);))
 .sorted()
 .collect(Collectors.toList());

the most important, you must implements your comparator c correctly. and the code above even can't be compiled due to List.sort not return anything. — holi-java
– holi-java, Commented Apr 29, 2017 at 17:52
So you want to sort each List<V> and then join them into a bigger list? — Nick Ziebert
– Nick Ziebert, Commented Apr 29, 2017 at 17:53
@NickZiebert what he want is: {b:[4,3],null:[5,6], a:[2,1]} will output: ["a == [1, 2]", "b ==[3, 4]"] — holi-java
– holi-java, Commented Apr 29, 2017 at 17:55

holi-java · Accepted Answer · 2017-04-29 17:47:45Z

3

How about this?

List<String> outputList = inputMap.keySet().stream()
            .filter(Objects::nonNull)
            .map(key -> key + "==" 
                            + inputMap.get(key).stream().sorted(c)
                                               .collect(Collectors.toList()))
            .sorted()
            .collect(Collectors.toList());

OR

List<String> outputList = inputMap.keySet().stream()
            .filter(Objects::nonNull)
            .map(key -> key + "=="
                            + inputMap.get(key).stream().sorted(c)
                                        .map(Object::toString)
                                        .collect(Collectors.joining(", ","[","]")))
            .sorted()
            .collect(Collectors.toList());

edited Apr 29, 2017 at 17:47

answered Apr 29, 2017 at 17:40

holi-java

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3 Comments

Ritesh Chouhan Over a year ago

Thanks, second solution worked for me, but I changed my map to Map<K, List<String>>. but Can you help for generic type sorted(c) comparator part

holi-java Over a year ago

@RiteshChouhan Hi, but I still remind you that the first solution can also work fine.

Ritesh Chouhan Over a year ago

Yeah its working, I have verified that also, Thanks :)

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