java 8 - Why do I get compilation error when doing flatmap() call? -


let's have following object:

public class daylydata {  private date date; private list<integer> numerslist;  // standard getters/setters  public map<integer, date> getinttodate() {     map<integer, date> resultmap = new hashmap<>();     for(integer number : getnumerslist()) {         resultmap.put(number, getdate());     }     return resultmap; }

now, let have list of daylydata: list<daylydata> resultlist.

what result of following:

resultlist.stream().flatmap(entity -> entity.getinttodate());

if assign result of stream<map<integer, date>>, getting type mismatch: cannot convert map<integer,date> stream<? extends map<integer,date>>.

thanks in advance.

the flatmap method special case of map , used flattening nested stream, optional, , other monadic tools.

in case, not providing function returns stream can't used flatmap param.

your function work fine standard map(), though:

resultlist.stream()   .map(entity -> entity.getinttodate()); // no compilation errors

you make example work wrapping result in stream instance not give advantage on example above - makes sense educational purposes:

resultlist.stream()    .flatmap(entity -> stream.of(entity.getinttodate())); // no compilation error

it's "daily" not "dayly".


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