问题描述
我正在实现Composite Design模式,我需要在数据库中插入一个菜单,但是该菜单可能包含其他菜单和菜单项,因此当我尝试以递归方式插入它们时,出现错误,因为子菜单和子项需要知道尚未创建的父ID.
I am implementing the Composite Design pattern and I need to insert a menu in the database but the menu may consist of other menus and menu items so when I tried to insert them recursively, I got an error because the submenu and sub items need to know the parent ID which isn't created yet.
public boolean insertMenu(Menu menu) {
try {
PreparedStatement statement = connection.prepareStatement("INSERT INTO `menu` VALUES (NULL, ?, ?, ?)");
statement.setString(1, menu.getName());
statement.setDate(2, java.sql.Date.valueOf(menu.getLocalDate(menu.getDate())));
statement.setInt(3, menu.getParent_id()); // problem that this value always is null because it isn't created yet
for (MenuList child : menu.getChildren()) {
int x = child.getType();
if (x == 0) {
insertMenu((Menu) child);
} else {
MySqlMenuItemDAO a = new MySqlMenuItemDAO();
a.insertMenuItem((MenuItem) child);
}
}
int res = statement.executeUpdate();
if (res == 1) {
System.out.println("menu "+menu.getName()+" inserted");
return true;
}
} catch (SQLException e) {
e.printStackTrace();
}
return false;
}
推荐答案
menu.getParent_id()
对于最上面的Menu
始终返回null,因为暗示它没有父级.由于菜单遍历从最顶部的Menu
开始,因此您需要处理这种特殊情况.
menu.getParent_id()
will always return null for the top-most Menu
because by implication it doesn't have a parent. Since menu traversal begins with the top-most Menu
you need to handle this specific case.
首先,您需要一种表示Menu
的方法,该Menu
在数据库表中没有父级.例如,您可以选择NULL来代表这一点.然后处理此案:
Firstly, you need a way to represent a Menu
without a parent in your database table. For example, you may choose NULL to represent this. Then handle the case:
if(menu.getParent_id() == null) {
statement.setNull(3, java.sql.Types.INTEGER);
else {
statement.setInt(3, menu.getParent_id());
}
在递归调用该方法之前,您还需要在数据库中创建菜单.否则,menu.getParent_id()
将为每个 Menu
返回null,而不仅仅是最顶层的返回.因此,这需要在创建子菜单之前发生:int res = statement.executeUpdate();
You also need to create the menu in the database before calling the method recursively. Otherwise menu.getParent_id()
will return null for every Menu
and not just the top-most. So this needs to happen before creating the submenus: int res = statement.executeUpdate();
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