組合模式

定義

組合模式（Composite Pattern），又叫部分整體模式，是用于把一組相似的對象當作一個單一的對象。組合模式依據樹形結構來組合對象，用來表示部分以及整體層次。這種類型的設計模式屬于結構型模式，它建立了對象組的樹形結構。

這種模式建立了一個包含自己對象組的類。該類提供了修改相同對象組的方式。

我們通過下面的執行個體來示範組合模式的用法。執行個體示範了一個組織中員工的層次結構。

實作

我們有一個類 Employee，該類被當作組合模型類。CompositePatternDemo，我們的示範類使用 Employee 類來添加部門層次結構，并列印所有員工。

組合模式的 UML 圖

步驟 1
建立 Employee 類，該類帶有 Employee 對象的清單。
Employee.java
import java.util.ArrayList;
import java.util.List;

public class Employee {
   private String name;
   private String dept;
   private int salary;
   private List<Employee> subordinates;

   //構造函數
   public Employee(String name,String dept, int sal) {
      this.name = name;
      this.dept = dept;
      this.salary = sal;
      subordinates = new ArrayList<Employee>();
   }

   public void add(Employee e) {
      subordinates.add(e);
   }

   public void remove(Employee e) {
      subordinates.remove(e);
   }

   public List<Employee> getSubordinates(){
     return subordinates;
   }

   public String toString(){
      return ("Employee :[ Name : "+ name 
      +", dept : "+ dept + ", salary :"
      + salary+" ]");
   }   
}
步驟 2
使用 Employee 類來建立和列印員工的層次結構。
CompositePatternDemo.java
public class CompositePatternDemo {
   public static void main(String[] args) {
      Employee CEO = new Employee("John","CEO", 30000);

      Employee headSales = new Employee("Robert","Head Sales", 20000);

      Employee headMarketing = new Employee("Michel","Head Marketing", 20000);

      Employee clerk1 = new Employee("Laura","Marketing", 10000);
      Employee clerk2 = new Employee("Bob","Marketing", 10000);

      Employee salesExecutive1 = new Employee("Richard","Sales", 10000);
      Employee salesExecutive2 = new Employee("Rob","Sales", 10000);

      CEO.add(headSales);
      CEO.add(headMarketing);

      headSales.add(salesExecutive1);
      headSales.add(salesExecutive2);

      headMarketing.add(clerk1);
      headMarketing.add(clerk2);

      //列印該組織的所有員工
      System.out.println(CEO); 
      for (Employee headEmployee : CEO.getSubordinates()) {
         System.out.println(headEmployee);
         for (Employee employee : headEmployee.getSubordinates()) {
            System.out.println(employee);
         }
      }     
   }
}
步驟 3
驗證輸出。
Employee :[ Name : John, dept : CEO, salary :30000 ]
Employee :[ Name : Robert, dept : Head Sales, salary :20000 ]
Employee :[ Name : Richard, dept : Sales, salary :10000 ]
Employee :[ Name : Rob, dept : Sales, salary :10000 ]
Employee :[ Name : Michel, dept : Head Marketing, salary :20000 ]
Employee :[ Name : Laura, dept : Marketing, salary :10000 ]
Employee :[ Name : Bob, dept : Marketing, salary :10000 ]
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