Sort map by String values with predicate

Question

I am trying to sort the map by String values with predicted. For example, I have the next values in my map

message,
message_message1_message2,
message1_message2,

I wanna sort my map by _ symbols in values String. The result should be like this:

message_message1_message2,
message1_message2,
message

I am trying this way:

        myMap.entrySet().stream().sorted((e1, e2) -> {
            String[] firstCompareValue = e1.getKey().split("_");
            String[] secondCompareValue = e2.getKey().split("_");
            return Integer.compare(firstCompareValue.length, secondCompareValue.length);
        }).collect(Collectors.toMap(Map.Entry::getKey, Function.identity()));

and I've got next notification

Redundant 'sorted' call: subsequent 'toMap' call doesn't depend on the sort order

What am I doing wrong? How it can be done?

Eran · Accepted Answer · 2020-07-01 10:32:35Z

The Collectors.toMap() variant you are using creates a HashMap (at least that's the current implementation), so it doesn't preserve the ordering produced by your Stream.

If you produce a LinkedHashMap, it would be preserved:

    myMap.entrySet().stream().sorted((e1, e2) -> {
        String[] firstCompareValue = e1.getKey().split("_");
        String[] secondCompareValue = e2.getKey().split("_");
        return Integer.compare(firstCompareValue.length, secondCompareValue.length);
    }).collect(Collectors.toMap(Map.Entry::getKey,
                                Function.identity(),
                                (v1,v2)->v1,
                                LinkedHashMap::new));

BTW, you are creating a Map<String,Map.Entry<String,String>>, which I'm not sure is what you want. If you want a Map<String,String> change Function.identity() to Map.Entry::getValue.

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