Java中聚合和组合的区别

聚合和组合描述对象之间相互通信时的关系类型，这可能在低级设计中用于描述对象之间的关联。在本文中，我们将讨论Java编程语言中聚合和组合之间的区别。

聚合

它是一种特殊的协会形式:

• 它代表Has-A的关系。
• 这是一种单向关联，即单向关系。例如，一个系可以有学生，但反过来是不可能的，因此是单向的。
• 在聚合中，两个条目都可以单独存在，这意味着结束一个实体不会影响另一个实体。

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());
    }
}

输出:

Total students in institute: 4

输出解释: 在这个例子中，有一个研究所，它有许多部门，如CSE和EE。每个部门都没有。的学生。因此，我们创建了一个Institute类，它有一个Object或no的引用。Department类的object(即List of Objects)。这意味着Institute类通过它的Object与Department类相关联。Department类也引用了Student类的一个或多个对象(即对象列表)，这意味着它通过它的对象与Student类相关联。它表示Has-A关系。在上面的例子中:Student has a name。学生有一个ID。优等生系优等生系优等生如下图所示。

我们什么时候使用聚合?

代码重用最好通过聚合来实现。

组合

组合是一种限制形式的聚合，其中两个实体高度依赖彼此。

• 它代表了关系的一部分。
• 在组合中，两个实体相互依赖。
• 当两个实体之间存在组合时，组合的对象不能离开另一个实体而存在。

示例:

// Java program to illustrate
// the concept of Composition
  
// Importing required classes
import java.io.*;
import java.util.*;
  
// Class 1
// Book
class Book {
  
    // Attributes of book
    public String title;
    public String author;
  
    // Constructor of Book class
    Book(String title, String author)
    {
  
        // This keyword refers to current instance itself
        this.title = title;
        this.author = author;
    }
}
  
// Class 2
class Library {
  
    // Reference to refer to list of books
    private final List<Book> books;
  
    // Library class contains list of books
    Library(List<Book> books)
    {
  
        // Referring to same book as
        // this keyword refers to same instance itself
        this.books = books;
    }
  
    // Method
    // To get total number of books in library
    public List<Book> getTotalBooksInLibrary()
    {
  
        return books;
    }
}
  
// Class 3
// Main class
class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating objects of Book class inside main()
        // method Custom inputs
        Book b1
            = new Book("EffectiveJ Java", "Joshua Bloch");
        Book b2
            = new Book("Thinking in Java", "Bruce Eckel");
        Book b3 = new Book("Java: The Complete Reference",
                        "Herbert Schildt");
  
        // Creating the list which contains number of books
        List<Book> books = new ArrayList<Book>();
  
        // Adding books
        // using add() method
        books.add(b1);
        books.add(b2);
        books.add(b3);
  
        Library library = new Library(books);
  
        // Calling method to get total books in library
        // and storing it in list of user0defined type -
        // Books
        List<Book> bks = library.getTotalBooksInLibrary();
  
        // Iterating over books using for each loop
        for (Book bk : bks) {
  
            // Printing the title and author name of book on
            // console
            System.out.println("Title : " + bk.title
                            + " and "
                            + " Author : " + bk.author);
        }
    }
}

输出:

Title : EffectiveJ Java and  Author : Joshua Bloch
Title : Thinking in Java and  Author : Bruce Eckel
Title : Java: The Complete Reference and  Author : Herbert Schildt

输出解释: 在上面的例子中，一个图书馆可以有许多关于相同或不同主题的书。所以，如果图书馆被毁了，那么这个图书馆里的所有书都会被毁。也就是说，书不能没有图书馆。这就是构成的原因。书是图书馆的一部分。

Java中聚合和组合的区别