使用Hibernate Transient Set会引发null异常

Question

I am trying to set up a menu with drop down children. I have created a regular table for menu items, and a set / array / list (tried each) into which child items will get moved to make it easier to render via the JSP. However, when i try to use .add() to add items into the list of children, I receive a java.lang.NullPointerException and it goes no further.

My system is set up using Spring / Hibernate, and everything works except this.

Model (excluding irrelevant fields and getters / setters):

public class Menu {
  @Transient
  private Set<Menu> pageChildren;

  @Column(name = "pageParent")
  private Integer pageParent;

  @Column(name = "pageId")
  private Integer pageId;

  ...

And the necessary snippet from the controller:

List<Menu> menuItems = new ArrayList<Menu>();
menuItems = menuService.getAll();
for (Menu item: menuItems) {
  if (item.getPageType() == 1 && item.getPageParent() > 0) {
    menuItems.get(item.getPageParent() - 1).getPageChildren().add(item);
    menuItems.remove(item);
  }
}

Answer 1

The transient variable needed initializing, and the for loop was configured wrong.

Model (excluding irrelevant fields and getters / setters):

public class Menu {
  @Transient
  private Set<Menu> pageChildren = new HashSet<Menu>();       //CHANGED

  @Column(name = "pageParent")
  private Integer pageParent;

  @Column(name = "pageId")
  private Integer pageId;

  ...

And the necessary snippet from the controller:

List<Menu> menuItems = new ArrayList<Menu>();
menuItems = menuService.getAll();
for(int i=0; i<menuItems.size(); i++){                        //CHANGED
  Menu item = menuItems.get(i);                               //CHANGED
  if (item.getPageType() == 1 && item.getPageParent() > 0) {
    menuItems.get(item.getPageParent() - 1).getPageChildren().add(item);
    menuItems.remove(item);
  }
}