How to transform a Map to a SortedMap by lambda expression in JAVA8?

Question

For example, I have a class Student

public class Student{
    private String name;
    private int age;

    public int getAge(){
        return this.age;
    }
}

And a class School:

public class School{
    private Map<String,Student> students=new TreeMap<>();
    //stroe the index of students in the school by key is their names.

    public SortedMap<Integer,Long> countingByAge(){
        return this.students.entrySet().stream().map(s->s.getValue())
               .collect(groupingBy((Student s)->s.getAge(),counting()));
    }
}

The countingByAge method ask for return of a SortedMap<Integer,Long >, the key is the age of student, value is the number of students per distinct age, i.e. I need to count how many students per age.

I have almost finished the method, but I don't know how to transform the Map<Integer,Long> to SortedMap<Integer,Long> without (SortedMap<Integer,Long>) casting.

Answer 1

You can use groupingBy(classifier, mapFactory, downstream) and as mapFactory pass Supplier returning instance of Map implementing SortedMap like TreeMap::new

public SortedMap<Integer, Long> countingByAge(){
    return  students.entrySet()
            .stream()
            .map(Map.Entry::getValue)
            .collect(groupingBy(Student::getAge, TreeMap::new, counting()));
}

BTW, as @Holger mentioned in comment you can simplify

map.entrySet()
.stream()
.map(Map.Entry::getValue)

with

map.values()
.stream()