JPA - Tree structure with automatic order and tested operations for it

gistfile1.java

@Entity
public class TreeCategory extends IdentifiableEntity {
....

    @ManyToOne(optional = true)
    @JoinColumn(name = "parent_id", referencedColumnName = "id")
    public TreeCategory getParent() {
        return parent;
    }

    public void setParent(TreeCategory parent) {
        this.parent = parent;
    }
...

    @OrderColumn(name = "position")
    @OneToMany(
            cascade = {CascadeType.ALL},
            orphanRemoval = true
    )
    @JoinColumn(name = "parent_id")
    public List<TreeCategory> getChildren() {
        return children;
    }

    public void setChildren(List<TreeCategory> children) {
        this.children = children;
    }

...

// Operations:
public void move(TreeNode node, TreeNode newParent, TreeNode oldParent) {
    TreeCategory TreeCategory = dao.getById(Long.parseLong(node.getId()));
    TreeCategory newParentCategory = dao.getById(Long.parseLong(newParent.getId()));
    TreeCategory oldParentCategory = dao.getById(Long.parseLong(oldParent.getId()));
    
    TreeCategory.setParent(newParentCategory);
    newParentCategory.getChildren().add(TreeCategory);
    oldParentCategory.getChildren().remove(TreeCategory);
    
    dao.update(oldParentCategory);
    dao.update(newParentCategory);
    dao.update(TreeCategory);
}

public void delete(TreeCategory entity) {
    entity.getParent().getChildren().remove(entity);
    update(entity.getParent());
}

public TreeCategory addChild(TreeCategory TreeCategory, String name, TreeCategory.Type type) {
    TreeCategory child = new TreeCategory();
    child.setName(name);
    child.setType(type);
    child.setParent(TreeCategory);
    TreeCategory.getChildren().add(child);
    TreeCategory = update(TreeCategory);
    return TreeCategory.getChildren().get(TreeCategory.getChildren().size() - 1);
}

public void toIndex(String id, Long index) {
    TreeCategory treeCategory = dao.getById(Long.parseLong(id));
    TreeCategory parent = treeCategory.getParent();
    if (parent != null) {
      if (parent.getChildren().indexOf(treeCategory ) != index) {
        parent.getChildren().remove(treeCategory );
        parent.getChildren().add(index.intValue(), treeCategory );
        dao.update(parent);
      }
    }
}

// tested on Hibernate 4.1.4.Final