合成(组合)/聚合复用原则

1.定义：尽量使用对象组合/聚合，而不是继承关系达到软件复用的目的

2.聚合has-A和组合contains-A

3.优点：可以使系统更加灵活，降低类与类之间的耦合度，一个类的变化对其他类造成的影响相对较少

4.思考：何时使用合成/聚合、继承？聚合has-A、组合contains-A、继承is-A

5.实例目录package

6.实例UML类图

7.代码

1 package com.geely.design.principle.compositionaggregation;
2 
3 public abstract class DBConnection {
4     /*public String getConnection(){
5         return "MySQL数据库连接";
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数据库连接";
6     }
7 }

1 package com.geely.design.principle.compositionaggregation;
2 
3 public class PostgreSQLConnection extends DBConnection {
4     public String getConnection() {
5         return "PostgreSQL数据库连接";
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 + "增加产品");
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 }