1.定義：儘量使用物件組合/聚合，而不是繼承關係達到軟體複用的目的

2.聚合has-A和組合contains-A

3.優點：可以使系統更加靈活，降低類與類之間的耦合度，一個類的變化對其他類造成的影響相對較少

4.思考：何時使用合成/聚合、繼承？聚合has-A、組合contains-A、繼承is-A

5.例項目錄package

6.例項UML類圖

7.程式碼

1 package com.geely.design.principle.compositionaggregation;
2 
3 public abstract class DBConnection {
4     /*
 
public String getConnection(){
5         return "MySQL資料庫連線";
6     }*/
7     public abstract String getConnection();
8 }

1 package com.geely.design.principle.compositionaggregation;
2 
3 public class MySQLConnection extends DBConnection{
4     public String getConnection() {
5         return "MySQL資料庫連線";
 
6     }
7 }

1 package com.geely.design.principle.compositionaggregation;
2 
3 public class PostgreSQLConnection extends DBConnection {
4     public String getConnection() {
5         return "PostgreSQL資料庫連線";
6     }
7 }

 1 package com.geely.design.principle.compositionaggregation;
 2 
 3
 
 public class ProductDao{// extends DBConnection
 4     private DBConnection dbConnection;
 5 
 6     public void setDbConnection(DBConnection dbConnection) {
 7         this.dbConnection = dbConnection;
 8     }
 9 
10     public void addProduct(){
11         String conn = dbConnection.getConnection();
12         System.out.println("使用"+conn + "增加產品");
13     }
14 }

 1 package com.geely.design.principle.compositionaggregation;
 2 
 3 public class Test {
 4     public static void main(String[] args) {
 5         ProductDao productDao = new ProductDao();
 6         productDao.setDbConnection(new MySQLConnection());
 7         productDao.addProduct();
 8         productDao.setDbConnection(new PostgreSQLConnection());
 9         productDao.addProduct();
10     }
11 }