Grouping Object with certain field using Java Collectors.toMap

Question

I want to group Person objects base on gender with Collectors.toMap() function as describe below, but this does not work as I expected.

class Person{
   private Integer id;
   private String gender;
   private String name;
   public String getGender(){
       return gender;
   }
   public Person(Integer id, String gender, String name){
       this.id=id;
       this.gender=gender;
       this.name=name;    
   }
}


public static void main(String[]args){
   List<Person> persons=new ArrayList<>();
   persons.add(new Person(1,"Man","John"));
   persons.add(new Person(2,"Man","Steve"));
   persons.add(new Person(3,"Women","Linda"));
   persons.add(new Person(4,"Man","Bill"));
   persons.add(new Person(5,"Women","Siti"));
   persons.add(new Person(6,"Man","Buzzer"));

   Map<String,List<Person>> filtered=persons
            .stream()
            .collect(Collectors.toMap(Person:getGender:Function.identity()));

}

Error hint

incompatible types: inference variable U has incompatible bounds equality constraints: List lower bounds: Person,T#2,T#1 where U,T#1,K,T#2 are type-variables: U extends Object declared in method <T#1,K,U>toMap(Function<? super T#1,? > > extends K>,Function<? super T#1,? extends U>) T#1 extends Object declared in method <T#1,K,U>toMap(Function<? super T#1,? > extends K>,Function<? super T#1,? extends U>) K extends Object declared in method <T#1,K,U>toMap(Function<? super T#1,? > extends K>,Function<? super T#1,? extends U>) T#2 extends Object declared in method <T#2>identity()

type of <R,A>collect(Collector<? super T,A,R>) is erroneous where R,A,T are type-variables: R extends Object declared in method <R,A>collect(Collector<? super T,A,R>) A extends Object declared in method <R,A>collect(Collector<? super T,A,R>)

Can someone explain this error?

Onome Sotu · Accepted Answer · 2021-02-20 01:09:03Z

1

Try this:

final var filtered = persons.stream().collect(Collectors.groupingBy(Person::getGender));

answered Feb 20, 2021 at 1:09

Onome Sotu

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1 Comment

Centra Telemedia Over a year ago

this work but i wonder when add forEach((gender,p)->{ System.out.println(gender};); this work

magicmn · Accepted Answer · 2021-02-20 01:13:04Z

0

Collectors.toMap() isn't that good for what you want to do. Instead try out Collectors.groupingBy().

Map<String,List<Person>> filtered = persons.stream().collect(Collectors.groupingBy(Person::getGender));

answered Feb 20, 2021 at 1:13

magicmn

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2 Comments

Centra Telemedia Over a year ago

@magicom, so Collectors.groupingBy best what for?

magicmn Over a year ago

Well groupingBy is as the name suggest best for grouping stuff. So you have one key and multiple values. toMap is much simpler and just accepts two functions to create your key and value.

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