一、概念

组合模式（Composite Pattern）：将对象组合成树形结构以表示“部分-整体”的层次结构。组合模式使得用户对单个对象和组合对象的使用具有一致性。

使用场景：组合结构不常用，需要部分与整体的层次关系为树形结构，并且部分与整体的对象是能让客户端能统一对待、不需区分的对象，例如：文件管理系统，在遍历查找的时候，文件和文件夹对我们来说不做区分，可以做到逻辑统一。

二、实现

这里借鉴参考文章公司和职员的例子，公司的组织结构包含部门和员工两种数据类型。其中，部门又可以包含子部门和员工。然后通过构建树形结构来遍历计算公司总的工资支出成本。

1、定义公司及员工的父类HumanResource，用来统一处理薪资计算。

public abstract class HumanResource { 
    protected long id; 
    protected double salary; 
    public HumanResource(long id) { 
        this.id = id; 
    } 
    
    public long getId() {
       return id; 
    } 

    public abstract double calculateSalary();
}

2、员工和部门

#Employee 
public class Employee extends HumanResource { 
    public Employee(long id, double salary) {
       super(id); this.salary = salary;
    } 

    @Override 
    public double calculateSalary() {
       return salary; 
    }
}

#Department 
public class Department extends HumanResource { 
    private List<HumanResource> subNodes = new ArrayList<>();
    public Department(long id) { 
        super(id); 
    } 

    @Override 
    public double calculateSalary() {
       double totalSalary = 0;
       if (subNodes.size() <= 0) {
            return 0;
        }

       for (HumanResource hr : subNodes) { 
            totalSalary += hr.calculateSalary(); 
       } 
       this.salary = totalSalary; 
       return totalSalary;
    } 
    
    public void addSubNode(HumanResource hr) {
       subNodes.add(hr);
    }
}

3、我们建立一个树形部门架构，如下图：

测试类

public class Client {
    public static void main(String[] args) {
        Department department = new Department(0);
        Department departmentA = new Department(1);
        Department departmentA1 = new Department(2);
        Department departmentB = new Department(3);
        Department departmentB1 = new Department(4);

        Employee employee0 = new Employee(0,50000);
        Employee employee1 = new Employee(1,8000);
        Employee employee2 = new Employee(2,8000);
        Employee employee3 = new Employee(3,4000);
        Employee employee4 = new Employee(4,4000);

        department.addSubNode(departmentA);
        department.addSubNode(departmentB);
        department.addSubNode(employee0);

        departmentA.addSubNode(employee1);
        departmentA.addSubNode(departmentA1);
        departmentB.addSubNode(employee2);
        departmentB.addSubNode(departmentB1);

        departmentA1.addSubNode(employee3);
        departmentB1.addSubNode(employee4);

        double salary = department.calculateSalary();
        System.out.println("公司工资开支：" + salary);
    }
}

4、运行结果

参考文章：
极客时间《设计模式》（王争）