cast a List to a Collection

Question

i have some pb  I want to cast a List to Collection in java  Collection lt T gt  collection   new Collection lt T gt  mylList      but i have this error     Can not instantiate the type Collection

User · Accepted Answer

List lt T gt  already implements Collection lt T gt  - why would you need to create a new one   Collection lt T gt  collection   myList    The error message is absolutely right - you can t directly instantiate an interface  If you want to create a copy of the existing list  you could use something like   Collection lt T gt  collection   new ArrayList lt T gt  myList

User · Answer

First Collection is class Interface and you can not instantiate  Collection API  List Ver APi is also an interface class   It may be so  List list   Collections synchronizedList new ArrayList           ver enter link description here  Collection collection  Collections synchronizedList new ArrayList

User · Answer

Not knowing your code  it s a bit hard to answer your question  but based on all the info here  I believe the issue is you re trying to use Collections sort passing in an object defined as Collection  and sort doesn t support that   First question  Why is client defined so generically  Why isn t it a List  Map  Set or something a little more specific    If client was defined as a List  Map or Set  you wouldn t have this issue  as then you d be able to directly use Collections sort client    HTH

User · Answer

Casting never needs a new   Collection lt T gt  collection   myList    You don t even make the cast explicit  because Collection is a super-type of List  so it will work just like this

User · Answer

There have multiple solusions to convert list to a collection  Solution 1  List lt Contact gt  CONTACTS   new ArrayList lt String gt        fill CONTACTS Collection lt Contact gt  c   CONTACTS    Solution 2  private static final Collection lt String gt  c   new ArrayList lt String gt                                                   Arrays asList  a    b    c       Solution 3  private static final Collection lt Contact gt    new ArrayList lt Contact gt                          Arrays asList new Contact  text1    name1                                        new Contact  text2    name2        Solution 4  List lt   extends Contact gt  col   new ArrayList lt Contact gt  CONTACTS

[java] cast a List to a Collection

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