Java 中聚合和组合之间的实现差异

Question

我知道聚合和组合之间的概念差异。有人能用实例告诉我它们之间在Java中的实现区别吗？

Answer 1

一个简单的合成程序

public class Person {
    private double salary;
    private String name;
    private Birthday bday;

    public Person(int y,int m,int d,String name){
        bday=new Birthday(y, m, d);
        this.name=name;
    }


    public double getSalary() {
        return salary;
    }

    public String getName() {
        return name;
    }

    public Birthday getBday() {
        return bday;
    }

    ///////////////////////////////inner class///////////////////////
    private class Birthday{
        int year,month,day;

        public Birthday(int y,int m,int d){
            year=y;
            month=m;
            day=d;
        }

        public String toString(){
           return String.format("%s-%s-%s", year,month,day);

        }
    }

    //////////////////////////////////////////////////////////////////

}
public class CompositionTst {

    public static void main(String[] args) {
        // TODO code application logic here
        Person person=new Person(2001, 11, 29, "Thilina");
        System.out.println("Name : "+person.getName());
        System.out.println("Birthday : "+person.getBday());

        //The below object cannot be created. A bithday cannot exixts without a Person 
        //Birthday bday=new Birthday(1988,11,10);

    }
}

Answer 2

我会使用一个很好的 UML 示例。

以一所大学为例，它有1至20个不同的系，每个系有1至5名教授。一所大学和它的院系之间有一个组成联系。一个系和它的教授之间有一种聚合联系。

组成只是一个强大的聚合，如果大学被摧毁，那么部门也应该被摧毁。但是，即使教授们各自的系消失了，我们也不应该杀死他们。

在java中:

public class University {

     private List<Department> departments;

     public void destroy(){
         //it's composition, when I destroy a university I also destroy the departments. they cant live outside my university instance
         if(departments!=null)
             for(Department d : departments) d.destroy();
         departments.clean();
         departments = null;
     }
}

public class Department {

     private List<Professor> professors;
     private University university;

     Department(University univ){
         this.university = univ;
         //check here univ not null throw whatever depending on your needs
     }

     public void destroy(){
         //It's aggregation here, we just tell the professor they are fired but they can still keep living
         for(Professor p:professors)
             p.fire(this);
         professors.clean();
         professors = null;
     }
}

public class Professor {

     private String name;
     private List<Department> attachedDepartments;

     public void destroy(){

     }

     public void fire(Department d){
         attachedDepartments.remove(d);
     }
}

围绕这个的东西。

编辑:要求的示例

public class Test
{
    public static void main(String[] args)
    {
        University university = new University();
        //the department only exists in the university
        Department dep = university.createDepartment();
        // the professor exists outside the university
        Professor prof = new Professor("Raoul");
        System.out.println(university.toString());
        System.out.println(prof.toString());

        dep.assign(prof);
        System.out.println(university.toString());
        System.out.println(prof.toString());
        dep.destroy();

        System.out.println(university.toString());
        System.out.println(prof.toString());

    }


}

大学班级

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class University {

    private List<Department> departments = new ArrayList<>();

    public Department createDepartment() {
        final Department dep = new Department(this, "Math");
        departments.add(dep);
        return dep;
    }

    public void destroy() {
        System.out.println("Destroying university");
        //it's composition, when I destroy a university I also destroy the departments. they cant live outside my university instance
        if (departments != null)
            departments.forEach(Department::destroy);
        departments = null;
    }

    @Override
    public String toString() {
        return "University{\n" +
                "departments=\n" + departments.stream().map(Department::toString).collect(Collectors.joining("\n")) +
                "\n}";
    }
}

部门类

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class Department {

    private final String name;
    private List<Professor> professors = new ArrayList<>();
    private final University university;

    public Department(University univ, String name) {
        this.university = univ;
        this.name = name;
        //check here univ not null throw whatever depending on your needs
    }

    public void assign(Professor p) {
        //maybe use a Set here
        System.out.println("Department hiring " + p.getName());
        professors.add(p);
        p.join(this);
    }

    public void fire(Professor p) {
        //maybe use a Set here
        System.out.println("Department firing " + p.getName());
        professors.remove(p);
        p.quit(this);
    }

    public void destroy() {
        //It's aggregation here, we just tell the professor they are fired but they can still keep living
        System.out.println("Destroying department");
        professors.forEach(professor -> professor.quit(this));
        professors = null;
    }

    @Override
    public String toString() {
        return professors == null
                ? "Department " + name + " doesn't exists anymore"
                : "Department " + name + "{\n" +
                "professors=" + professors.stream().map(Professor::toString).collect(Collectors.joining("\n")) +
                "\n}";
    }
}

教授课

import java.util.ArrayList;
import java.util.List;

public class Professor {

    private final String name;
    private final List<Department> attachedDepartments = new ArrayList<>();

    public Professor(String name) {
        this.name = name;
    }

    public void destroy() {

    }

    public void join(Department d) {
        attachedDepartments.add(d);
    }

    public void quit(Department d) {
        attachedDepartments.remove(d);
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Professor " + name + " working for " + attachedDepartments.size() + " department(s)\n";
    }
}

实现是有争议的，因为它取决于你需要如何处理创建，雇佣删除等。与行动无关

Answer 3

组成

final class Car {

  private final Engine engine;

  Car(EngineSpecs specs) {
    engine = new Engine(specs);
  }

  void move() {
    engine.work();
  }
}

聚合

final class Car {

  private Engine engine;

  void setEngine(Engine engine) {
    this.engine = engine;
  }

  void move() {
    if (engine != null)
      engine.work();
  }
}

在合成的情况下，发动机完全由汽车封装。外部世界无法获得对引擎的引用。发动机与汽车生死攸关。通过聚合，汽车也通过引擎执行其功能，但引擎并不总是汽车的内部部件。发动机可能被更换，甚至完全拆下。不仅如此，外部世界仍然可以参考发动机，并对其进行修补，无论其是否在汽车中。