Java stream api using reference of original object within chain

Question

I am trying to create a Map from a list, whereas value of Map would be a result of some transformation, this is how my code looks like.

empIdList.stream()
         .map(id-> getDepartment(id))
         .collect(Collectors.toMap(id, department:String -> department)

in this above example I am looking to use id as key and department as value. can you please help me achieving my expected results.

Gautham M · Accepted Answer · 2021-05-18 11:25:54Z

6

You could avoid the map function and directly invoke collect

empIdList.stream()    
         .collect(Collectors.toMap(Function.identity(), this::getDepartment);

If the list does not have unique ids, then it would result in an IllegalStateException so:

Either pass a Set of ids instead of a List if possible.
OR use distinct() in between stream() and collect().
OR provide a dummy merging function as the third argument to toMap: Collectors.toMap(Function.identity(), this::getDepartment, (a,b) -> a)

edited May 18, 2021 at 11:25

answered May 18, 2021 at 10:07

Gautham M

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2 Comments

Eritrean Over a year ago

To be on the safe side, make sure that the list does not contain duplicate ids by using distinct() or by providing a merging function when collecting to map.

Gautham M Over a year ago

Manish, as @Eritrean pointed out, take care of the possibility of receiving a non-unique id list. I have updated the answer with the relevant info.

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