StreamTest.java

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class StreamTest {

    public static void main(String[] args) {
        Student student1 = new Student();
        student1.dept = "Science";
        student1.studentName = "StudentOne";

        Student student2 = new Student();
        student2.dept = "Science";
        student2.studentName = "StudentTwo";

        Student student3 = new Student();
        student3.dept = "Maths";
        student3.studentName = "StudentThree";

        Clazz clazz = new Clazz();
        clazz.className = "ClassA";
        clazz.students = Arrays.asList(student1, student2, student3);

        School school = new School();
        school.schoolName = "ExampleSchool";
        school.classes = Arrays.asList(clazz);

        school.classes.stream()
                .forEach(StreamTest::processClazz);

    }
    static void processClazz(Clazz clazz) {
        Map<String, List<Student>> studentsByDept =
                clazz.students.stream()
                    .collect(Collectors.groupingBy(Student::getDept));

        Integer count = studentsByDept.entrySet().stream()
                .map(e -> processStudents(e.getValue()))
                .reduce(0 ,(partialCount, integer) -> partialCount + integer);

        System.out.println("Count : " + count);
    }

    static Integer processStudents(List<Student> students) {
        return  students.size();
    }
}

class School {
    String schoolName;
    List<Clazz> classes;
}

class Clazz {
    String className;
    List<Student> students;
}

class Student {
    String dept;
    String studentName;

    public String getDept() {
        return dept;
    }

    public void setDept(String dept) {
        this.dept = dept;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }
}