Java - How to convert type collection into ArrayList?

Question

public class MyGraph<V,E> extends SparseMultigraph<V,E>{     private ArrayList<MyNode> myNodeList;      public MyNode getNode(int nodeId){         myNodeList = new ArrayList<MyNode>();         myNodeList = (ArrayList<MyNode>)this.getVertices();         int i; 

The following are the error msg:

Exception in thread "main" java.lang.ClassCastException: java.util.Collections$UnmodifiableCollection cannot be cast to java.util.ArrayList...

Can anyone help?

Answer 1

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.

Answer 2

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 );