java面向对象设计原则之合成复用原则示例详解

public class SqlServerConnect {
    private Connection con = null;
    public Connection getCon() {
        System.out.println("创建数据库连接");
        return con;
    }
}
public class UserDao extends SqlServerConnect {
   //继承复用连接数据
    public void queryData()
    {
        Connection con =getCon();
        String sql = "select * from emp";
        try {
            Statement statement = con.createStatement();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

//增加一个接口
public interface DatabaseConnection {
    Connection getCon();
}
//SqlServerConnect实现该接口
public class SqlServerConnect implements DatabaseConnection {
    private Connection con = null;
    @Override
    public Connection getCon() {
        System.out.println("创建数据库连接");
        return con;
    }
}
//clsUserDaoNew和数据库连接接口呈现聚合关系，使用依赖倒置，可动态替换此类，复用了getCon()代码
public class UserDaoNew {
    private DatabaseConnection objCon;
    public UserDaoNew(DatabaseConnection conn){
        objCon = conn;
    }
    public void queryData() {
        Connection con = objCon.getCon();
        String sql = "select * from emp";
        try {
            Statement statement = con.createStatement();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}