Агрегация в java

import java.io.*; 
import java.util.*; 

// student class 
class Student  
{ 
    String name; 
    int id ; 
    String dept; 

    Student(String name, int id, String dept)  
    { 

        this.name = name; 
        this.id = id; 
        this.dept = dept; 

    } 
} 

/* Department class contains list of student 
Objects. It is associated with student 
class through its Object(s). */
class Department  
{ 

    String name; 
    private List students; 
    Department(String name, List students)  
    { 

        this.name = name; 
        this.students = students; 

    } 

    public List getStudents()  
    { 
        return students; 
    } 
} 

/* Institute class contains list of Department 
Objects. It is asoociated with Department 
class through its Object(s).*/
class Institute  
{ 

    String instituteName; 
    private List departments; 

    Institute(String instituteName, List departments) 
    { 
        this.instituteName = instituteName; 
        this.departments = departments; 
    } 

    // count total students of all departments 
    // in a given institute  
    public int getTotalStudentsInInstitute() 
    { 
        int noOfStudents = 0; 
        List students;  
        for(Department dept : departments) 
        { 
            students = dept.getStudents(); 
            for(Student s : students) 
            { 
                noOfStudents++; 
            } 
        } 
        return noOfStudents; 
    } 

}  

// main method 
class Agg 
{ 
    public static void main (String[] args)  
    { 
        Student s1 = new Student("Mia", 1, "CSE"); 
        Student s2 = new Student("Priya", 2, "CSE"); 
        Student s3 = new Student("John", 1, "EE"); 
        Student s4 = new Student("Rahul", 2, "EE"); 

        // making a List of  
        // CSE Students. 
        List  cse_students = new ArrayList(); 
        cse_students.add(s1); 
        cse_students.add(s2); 

        // making a List of  
        // EE Students 
        List  ee_students = new ArrayList(); 
        ee_students.add(s3); 
        ee_students.add(s4); 

        Department CSE = new Department("CSE", cse_students); 
        Department EE = new Department("EE", ee_students); 

        List  departments = new ArrayList(); 
        departments.add(CSE); 
        departments.add(EE); 

        // creating an instance of Institute. 
        Institute institute = new Institute("BITS", departments); 

        System.out.print("Total students in institute: "); 
        System.out.print(institute.getTotalStudentsInInstitute()); 
    } 
}