Java - How to convert type collection into ArrayList

Question

public class MyGraph lt V E gt  extends SparseMultigraph lt V E gt       private ArrayList lt MyNode gt  myNodeList       public MyNode getNode int nodeId           myNodeList   new ArrayList lt MyNode gt             myNodeList    ArrayList lt MyNode gt  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

User · Answer

Try this code  Convert ArrayList to Collection    ArrayList lt User gt  usersArrayList   new ArrayList lt User gt        Collection lt User gt  userCollection   new HashSet lt User gt  usersArrayList     Convert Collection to ArrayList    Collection lt User gt  userCollection   new HashSet lt User gt  usersArrayList      List lt User gt  userList   new ArrayList lt User gt  userCollection

User · Answer

public  lt E gt  List lt E gt  collectionToList Collection lt E gt  collection        return  collection instanceof List     List lt E gt   collection   new ArrayList lt E gt  collection       Use the above method for converting the collection to list

User · Answer

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 lt MyNode gt  myNodeList   new ArrayList lt MyNode gt  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 lt V gt  myNodeList   new ArrayList lt V gt  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

User · Answer

More information needed for a definitive answer  but this code  myNodeList    ArrayList lt MyNode gt  this getVertices      will only work if this getVertices   returns a   subtype of  List lt MyNode gt   If it is a different collection  like your Exception seems to indicate   you want to use  new ArrayList lt MyNode gt  this getVertices      This will work as long as a Collection type is returned by getVertices

User · Answer

The following code will fail   List lt String gt  will fail     List lt String gt  Collections unmodifiableCollection new ArrayList lt String gt        This instead will work   List lt String gt  will work   new ArrayList lt String gt  Collections unmodifiableCollection new ArrayList lt String gt

[java] Java - How to convert type collection into ArrayList?

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