CODEWITHSUNDEEP
javajava-8java-streamgrouping
jisuskraist
jisuskraist

Reputation: 451

Java Streams list to map index of ocurrence as key

Let's say I have a list of Strings 

["bird", "bird", "dog", "dog", "bird","bog"]

And I want them to be collected as a Map in the form of

{"bird": [0, 1, 4], "dog": [2, 3, 5]}

being the lists values the indexes where the word is on the input list.

Is there a way to do this with Java Streams?

Upvotes: 3

Views: 518

Answers (2)

Harshil
Harshil

Reputation: 1300

One Little twist to above problem.

Given a list of Objects, create a map where Key is 'x' field and value is list of Objects with same value for 'x' field

@Value
@RequiredArgsConstructor
class Employee{
    String Firstname;
    String LastName;
    String Dept;
    Integer Salary;
}

public class EmployeeStreamTest {

    public static void main(String[] args) {
        Employee e1 = new Employee("ABC", "Raval", "Fin", 100);
        Employee e2 = new Employee("BCD", "Raval", "Fin", 100);
        Employee e3 = new Employee("CDE", "Raval", "Fin", 100);
        Employee e4 = new Employee("WXY", "", "Fin", 100);
        Employee e5 = new Employee("XYZ", "", "Fin", 100);

        Stream<Employee> es = Stream.of(e1,e2,e3,e4,e5);

        //given a list of employees , create a map where Key is last name and value is list of employees with same last name
        Map<String, List<Employee>> lmse = es.collect(Collectors.groupingBy(emp -> emp.getLastName()));
        System.out.println(lmse);

    }

}

Output:

{=[Employee(Firstname=WXY, LastName=, Dept=Fin, Salary=100), Employee(Firstname=XYZ, LastName=, Dept=Fin, Salary=100)], Raval=[Employee(Firstname=ABC, LastName=Raval, Dept=Fin, Salary=100), Employee(Firstname=BCD, LastName=Raval, Dept=Fin, Salary=100), Employee(Firstname=CDE, LastName=Raval, Dept=Fin, Salary=100)]}

Thank you @Jacob for above answer.

Upvotes: 0

Jacob G.
Jacob G.

Reputation: 29680

Yes, you can use an IntStream along with Collectors.groupingBy:

List<String> list = List.of("bird", "bird", "dog", "dog", "bird", "bog");

IntStream.range(0, list.size())
         .boxed()
         .collect(Collectors.groupingBy(list::get));

Output:

{bird=[0, 1, 4], bog=[5], dog=[2, 3]}

Upvotes: 16

Related Questions