Getting a java collection of objects in Alphabetical order

Question

I have a question that I dont really know where to start. So I thought i'd ask it here.

Basically, I have a drop down with names in it. I want these names to be in alphabetical order.

Populating the drop down happens as follows;

I query a database and pull down an Id and Name, make a object called "UserList", and set the name and id variables with what I get back. I then add this object to an ArrayList. I do this over and over.

I then convert this collection to an array, and pass it to my JSP page using

session.setAttribute("userList", UserList);

I then populate the drop down as below.

<c:forEach items="${userList}" var="c" >
`<html-el:option value="${c.id}"><c:out value="${c.name}"/></html-el:option> </c:forEach>

There probably is a simple answer but how to I sort these names?

Answer 1

You can try and override the compareTo method for UserList so that it compares the elements in an alphabetical way according to some string value. Then, call the sort method of the array list and it should sort them using the compareTo you overrode.

Answer 2

You usually do it by invoking public static Collections.sort(List<T> list) with your ArrayList as the parameter, but take care to implement the Comparable interface or it won't work (if they are Strings then it's already implemented):

Collections.sort(yourList);

Otherwise if you have a custom class but you want to sort just over some string field inside you can delegate the compareTo method:

public class User implements Comparable<User>
{
  public int compareTo(User other)
  {
    return userName.compareTo(other.userName);
  }
}

Finally if noone is your case just roll your own compareTo method, it should return -1, 0 or 1 if the the calling object is less, equal to or greater than the passed one.

Answer 3

You should use Collections.sort and either of
1) Make your User class implement Comparable interface
2) Create a UserListComparator implementing Comparator<UserList> interface
See Comparable interface documentation for details

Answer 4

I think the most ideal solution to this problem is to sort them at the database query level. If the table is well indexed, this will be the fastest and arguable the "best" practice.

That being said, if you just want to sort that list and not start mucking on the database side, use Collections.sort and have your UserList implement Comparable<UserList>

Be careful to use the case insensitive string comparison, or your users will likely not appreciate the sorting results.

Answer 5

I would change the database query to return the rows in the order you want them.

If you are using SQL the query could look like the following.

SELECT Id, Name FROM Person ORDER BY Name;

This has some advantages over doing it in code.

• It is faster.
• You do not have to change your code.
• It is less work.

Answer 6

Michael, you should be using a join and an order by to get this data from the database, not retrieving and sorting in Java:

 select person.id, person.name from person inner join person_company using(personid)
 order by person.name;

And not trying to sort and do this in java (the syntax above may not be perfect, my MySQL's a bit rusty).