Java - How to convert type collection into ArrayList?

Try this code

Convert ArrayList to Collection

  ArrayList<User> usersArrayList = new ArrayList<User>();

  Collection<User> userCollection = new HashSet<User>(usersArrayList);

Convert Collection to ArrayList

  Collection<User> userCollection = new HashSet<User>(usersArrayList);

  List<User> userList = new ArrayList<User>(userCollection );

public <E> List<E> collectionToList(Collection<E> collection)
{
    return (collection instanceof List) ? (List<E>) collection : new ArrayList<E>(collection);
}

Use the above method for converting the collection to list

As other people have mentioned, ArrayList has a constructor that takes a collection of items, and adds all of them. Here's the documentation:

http://java.sun.com/javase/6/docs/api/java/util/ArrayList.html#ArrayList%28java.util.Collection%29

So you need to do:

ArrayList<MyNode> myNodeList = new ArrayList<MyNode>(this.getVertices());

However, in another comment you said that was giving you a compiler error. It looks like your class MyGraph is a generic class. And so getVertices() actually returns type V, not type myNode.

I think your code should look like this:

public V getNode(int nodeId){
        ArrayList<V> myNodeList = new ArrayList<V>(this.getVertices());
        return myNodeList(nodeId);
}

But, that said it's a very inefficient way to extract a node. What you might want to do is store the nodes in a binary tree, then when you get a request for the nth node, you do a binary search.

The following code will fail:

List<String> will_fail =  (List<String>)Collections.unmodifiableCollection(new ArrayList<String>());

This instead will work:

List<String> will_work = new ArrayList<String>(Collections.unmodifiableCollection(new ArrayList<String>()));