Using Java streams to convert collection to map: how to put predefined Enum as value

Question

I have a collection of Long, and for a reason I need to create a map from this collection, which has the elements of the collection as keys, and 1 predefined Enum as value (all keys have the same value).

I am trying to achieve this with Streams, like below:

private Map<Long, Marker> mapMarker(Collection<Long> ids, Marker marker) {
    return ids.stream().collect(Collectors.toMap(Function.identity(), marker));
}

Compiler failed with this error:

no instance(s) of type variable(s) T, U exist so that Marker conforms to Function<? super T, ? extends U>

Could someone please explain to me why would it fails? Is there anyway to get the expected result with Streams?

Naman · Accepted Answer · 2018-12-04 05:19:20Z

3

If Marker is the enum you want to map against ll the keys in ids, you can do it as:

return ids.stream().collect(Collectors.toMap(Function.identity(), id -> marker));

You were quite close, just that id -> marker is a Function as expected for Collectors.toMap

answered Dec 4, 2018 at 5:19

Naman

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nbrooks · Accepted Answer · 2018-12-04 05:18:15Z

2

The parameters to Collectors.toMap should be functions that convert the input to your desired output. You can use a placeholder variable (such as i) to represent your input. For example, for the identity function, take input i and return i. For mapping every item to marker, take input i and return marker:

private Map<Long, Marker> mapMarker(Collection<Long> ids, Marker marker) {
    return ids
            .stream()
            .collect(Collectors.toMap(i -> i, i -> marker));
}

answered Dec 4, 2018 at 5:18

nbrooks

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6 Comments

Naman Over a year ago

quite close, just that i->i is identity :)

ernest_k Over a year ago

@nullpointer does that mean that using Function.identity() has some practical advantage over using the lambda i -> i? (other than readability, that is...)

nbrooks Over a year ago

They're not exactly the same, but close enough. I find this more readable, but also found it helpful to illustrate how a "Function" worked in this case. Mostly a matter of preference though.

Jai Over a year ago

Seriously though, I don't see there is any way that is clearer than i -> i. For someone who isn't a native English speaker, Function.identify() will cause them to read the Javadoc to be very sure it does the right thing.

Holger Over a year ago

@Jai everyone is free to use whatever they think is more readable, but still, even if you’re not a native English speaker, it’s recommended to know the meaning of certain words (after all, the Javadoc is in English too), just like the keywords of the Java language, which are also English words. The term identity is too important to miss it, e.g. it also relates to identity value, but not to confuse with an object’s identity…

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