JSTL - Using forEach to iterate over a user-defined class

Question

What methods do I need to add to a custom Java class so that I can iterate over the items in one of its members? I couldn't find any specifications about how the JSTL forEach tag actually works so I'm not sure how to implement this.

For example, if I made a generic "ProjectSet" class and I woud like to use the following markup in the JSP view:

<c:forEach items="${projectset}" var="project">
...
</c:forEach>

Basic class file:

public class ProjectSet {
    private ArrayList<Project> projects;
    public ProjectSet() {
        this.projects = new ArrayList<Project>();
    }
    // .. iteration methods ??
}

Is there any interface that I must implement like PHP's ArrayAccess or Iterator in order for this to work?

Edit: Without directly accessing the ArrayList itself, because I will likely be using some sort of Set class using generics, and the JSP view shouldn't have to know about the inner workings of the class.

Answer 1

The Iterable interface provides this functionality:

public class ProjectSet implements Iterable<Project> {
    private ArrayList<Project> projects;
    public ProjectSet() {
        this.projects = new ArrayList<Project>();
    }

    // .. iteration methods ??
   @Override
   public Iterator<Project> iterator() {
       return projects.iterator();
   }
}

You can exchange the iterator logic as you need.