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

Just construct a new one based on it. The ArrayList has a constructor taking a Collection.

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

Answer 2

More information needed for a definitive answer, but this code

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

will only work if this.getVertices() returns a (subtype of) List<MyNode>. If it is a different collection (like your Exception seems to indicate), you want to use

new ArrayList<MyNode>(this.getVertices())

This will work as long as a Collection type is returned by getVertices.

Answer 3

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 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.