Java Aggregation和Composition的區別用法及代碼示例

// Java program to illustrate the  
// Concept of Aggregation 
  
// Importing required classes 
import java.io.*; 
import java.util.*; 
  
// Class 1 
// Student class 
class Student { 
  
    // Attributes of student 
    String name; 
    int id; 
    String dept; 
  
    // Constructor of student class 
    Student(String name, int id, String dept) 
    { 
  
        // This keyword refers to current instance itself 
        this.name = name; 
        this.id = id; 
        this.dept = dept; 
    } 
} 
  
// Class 2 
// Department class contains list of student objects 
// It is associated with student class through its Objects 
class Department { 
    // Attributes of Department class 
    String name; 
    private List<Student> students; 
    Department(String name, List<Student> students) 
    { 
        // this keyword refers to current instance itself 
        this.name = name; 
        this.students = students; 
    } 
  
    // Method of Department class 
    public List<Student> getStudents() 
    { 
        // Returning list of user defined type 
        // Student type 
        return students; 
    } 
} 
  
// Class 3 
// Institute class contains list of Department 
// Objects. It is associated with Department 
// class through its Objects 
class Institute { 
  
    // Attributes of Institute 
    String instituteName; 
    private List<Department> departments; 
  
    // Constructor of institute class 
    Institute(String instituteName,List<Department> departments) 
    { 
        // This keyword refers to current instance itself 
        this.instituteName = instituteName; 
        this.departments = departments; 
    } 
  
    // Method of Institute class 
    // Counting total students of all departments 
    // in a given institute 
    public int getTotalStudentsInInstitute() 
    { 
        int noOfStudents = 0; 
        List<Student> students; 
  
        for (Department dept : departments) { 
            students = dept.getStudents(); 
  
            for (Student s : students) { 
                noOfStudents++; 
            } 
        } 
  
        return noOfStudents; 
    } 
} 
  
// Class 4 
// main class 
class GFG { 
  
    // main driver method 
    public static void main(String[] args) 
    { 
        // Creating object of Student class inside main() 
        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"); 
  
        // Creating a List of CSE Students 
        List<Student> cse_students = new ArrayList<Student>(); 
  
        // Adding CSE students 
        cse_students.add(s1); 
        cse_students.add(s2); 
  
        // Creating a List of EE Students 
        List<Student> ee_students 
            = new ArrayList<Student>(); 
  
        // Adding EE students 
        ee_students.add(s3); 
        ee_students.add(s4); 
  
        // Creating objects of EE and CSE class inside 
        // main() 
        Department CSE = new Department("CSE", cse_students); 
        Department EE = new Department("EE", ee_students); 
  
        List<Department> departments = new ArrayList<Department>(); 
        departments.add(CSE); 
        departments.add(EE); 
  
        // Lastly creating an instance of Institute 
        Institute institute = new Institute("BITS", departments); 
  
        // Display message for better readability 
        System.out.print("Total students in institute: "); 
  
        // Calling method to get total number of students 
        // in institute and printing on console 
        System.out.print(institute.getTotalStudentsInInstitute()); 
    } 
}