How to change my statement to print frequencies of object member type in hashmap?

Question

I created an arrayList of student objects with various attributes. Among those attributes are favorite color. So in this arraylist of n students, each student has a string favoriteColor member variable.

Set<student> studentUnique = new HashSet<student>(studentList);
for (user key : studentUnique) {
    System.out.println(key + ": " + Collections.frequency(studentList, key));
}

I would like to count the frequency of said colors so for example with a hundred students, maybe output:

red: 50
blue: 20
green: 30

I put my arrayList of students (studentList) into a hashmap but I don't know how to write my frequency statement to get the frequency of students that like their respective colors.

Answer 1

studentUnique.stream()
             .collect(Collectors.groupingBy(
                  Student::getColor, 
                  Collectors.counting()))

Assuming getColor exists.

Answer 2

Essentially what you're doing is called "grouping" based on the favourite color.

Here is another approach:

 Map<String, Integer> result = 
         studentList.stream()
                    .collect(toMap(Student::getFavouriteColor, s -> 1, Math::addExact));

This uses the toMap collector, where the keyMapper is Student::getFavouriteColor ie a function extracting the students favourite color as the map key.

Second, we place the valueMapper function s -> 1 ie a function taking a Student and return 1 as the map value.

Lastly, we provide a "merge" function Math::addExact which is a function used to add two corresponding values given a key clash based on the favourite colour.

So, as a result, we will have a map from String ---> Integer where each entry in the map represents the color and the number of times that color has been chosen as a "favourite color" amongst the students' list.

Further, if you want to print this result in an ascending order based on occurrences, you can sort the result and print as follows:

studentList.stream()
           .collect(toMap(Student::getFavouriteColor, s -> 1, Math::addExact))
           .entrySet()
           .stream()
           .sorted(Map.Entry.comparingByValue())
           .forEachOrdered(e -> System.out.println(e.getKey() + ":" + e.getValue())); 

imports needed:

import java.util.stream.*;
import static java.util.stream.Collectors.*;