Java 中聚合和组合之间的实现差异

组成

final class Car {

  private final Engine engine;

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

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

集合体

final class Car {

  private Engine engine;

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

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

在构图的情况下，发动机完全由汽车封装。外部世界无法获得对引擎的引用。发动机与汽车一起生死。通过聚合，汽车也通过发动机执行其功能，但发动机并不总是汽车的内部部分。引擎可以更换，甚至完全移除。不仅如此，外界仍然可以参考发动机，并对其进行修补，无论它是否在车内。

我会用一个很好的UML例子。

以一所有1到20个不同系的大学为例，每个系有1到5名教授。大学与其部门之间存在组合联系。一个系与其教授之间存在聚合联系。

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，我们不应该杀死教授，即使他们各自的部门消失了。

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);
     }
}

围绕这一点。

编辑：根据要求提供示例

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());

    }


}

大学课程

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}";
    }
}

教授班

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，雇用删除等。与OP无关