How to convert enum with attribute to map using java8 stream

Question

this answer doesn't math: How to store enum to map using Java 8 stream API

I have an enum:

public enum SheetRows{
    totalActive("Total active");

    String value;

    SheetRows(String value){
        this.value = value;
    }

    public String getValueForTable() {
        return value;
    }
}

How to convert this enum to HashMap<SheetRows, String>?

I try to use:

HashMap<SheetRows, String> cellsMap = Arrays.asList(SheetRows.values()).stream()
            .collect(Collectors.toMap(k -> k, v -> v.getValueForTable()));

but this code isn't compile.

That's because toMap is written to return a Map, not a HashMap. Is that all? — ernest_k
– ernest_k, Commented Apr 7, 2021 at 9:50

Yassin Hajaj · Accepted Answer · 2021-04-07 09:55:55Z

2

The following should work, the problem is that you're trying to assign a Map to HashMap

Map<SheetRows, String> map = EnumSet.allOf(SheetRows.class)
        .stream()
        .collect(Collectors.toMap(Function.identity(), SheetRows::getValueForTable));

System.out.println("map = " + map); // map = {totalActive=Total active}

If you really need to return a HashMap, you can also use the overload that takes a supplier and a mergeFunction like hereunder.

The mergeFunction will never be called since your enums are unique, so just choose a random one.

HashMap<SheetRows, String> map = EnumSet.allOf(SheetRows.class)
        .stream()
        .collect(Collectors.toMap(Function.identity(), SheetRows::getValueForTable, (o1, o2) -> o1, HashMap::new));

System.out.println("map = " + map); // map = {totalActive=Total active}

edited Apr 7, 2021 at 9:55

answered Apr 7, 2021 at 9:50

Yassin Hajaj

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3 Comments

Sergey Afinogenov Over a year ago

I thought @Yassin Hajaj that by default toMap method generates a HashMap, in my Java version 1.8 it calls toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new) in this case it could be casted using (HashMap) ?

Yassin Hajaj Over a year ago

Yes it could indeed @SergeyAfinogenov, but there is no guarantee that in the future, the JDK will keep sending a HashMap, this is an implementation detail, and the only thing that matters is the contract / spec

Yassin Hajaj Over a year ago

@SergeyAfinogenov See this question

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