Sort JPA relational entity lists - jsf

Question

I'm using PrimeFaces to display information in a DataTable. The data in the datatable is actually a List of entity objects which is generated by OpenJPA. And the entities have relations to other entities. This means that there are Lists inside Lists.
For example, an entity called Authors, which has many Books. The data List<Authors> is listed in the datatable. And in order to sort the List i use Collections.sort() which of course doesn't work when i try to sort on the Authors book title. Because the title field is an instance of the Book class.
How do i go about to sort the Lists when there are relationships like this?
Thanks in advance.

Answer 1

Here is an example of sorting by books title, you need to sort your books list :
public static void main(String args[]) {
List<Author> list = new ArrayList<Author>();
for (int i=0;i<5;i++){
Author a = new Author();
a.setName("author"+i);
List<Book> books = new ArrayList<Book>();
for(int j=0;j<4;j++){
Random r = new Random();
char c = (char) (r.nextInt(26) + 'a');
Book b = new Book();
b.setTitle(c+"title");
books.add(b);
}
a.setBooks(books);
list.add(a);
}
/*
* At this point of time you have Authors list which you want to sort by book title.
* So you can do something like below if you want to do it through Collections.sort
*/
for(Author a : list){
Collections.sort(a.getBooks(), new Comparator<Book>(){
#Override
public int compare(Book o1, Book o2) {
return o1.getTitle().compareToIgnoreCase(o2.getTitle());
}});
}
System.out.println(list);
}
Author and Book used in above example:
import java.util.List;
public class Author {
private List<Book> books;
private String name;
#Override
public String toString() {
return "Author [books=" + books + ", name=" + name + "]";
}
public List<Book> getBooks() {
return books;
}
public void setBooks(List<Book> books) {
this.books = books;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Book.java:
public class Book {
private String title;
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
#Override
public String toString() {
return "Book [title=" + title + "]";
}
}