How do I clone a generic List in Java

Question

I have an ArrayList lt String gt  that I d like to return a copy of   ArrayList has a clone method which has the following signature   public Object clone     After I call this method  how do I cast the returned Object back to ArrayList lt String gt

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This is the code I use for that   ArrayList copy   new ArrayList  original size     Collections copy copy  original     Hope is usefull for you

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This should also work   ArrayList lt String gt  orig   new ArrayList lt String gt     ArrayList lt String gt  copy    ArrayList lt String gt   orig clone

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I find using addAll works fine   ArrayList lt String gt  copy   new ArrayList lt String gt     copy addAll original     parentheses are used rather than the generics syntax

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ArrayList newArrayList    ArrayList  oldArrayList clone

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To clone a generic interface like java util List you will just need to cast it  here you are an example   List list   new ArrayList    List list2     List     ArrayList  list  clone       It is a bit tricky  but it works  if you are limited to return a List interface  so anyone after you can implement your list whenever he wants   I know this answer is close to the final answer  but my answer answers how to do all of that while you are working with List -the generic parent- not ArrayList

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I am not a java professional  but I have the same problem and I tried to solve by this method   It suppose that T has a copy constructor     public static  lt T extends Object gt  List lt T gt  clone List lt T gt  list          try              List lt T gt  c   list getClass   newInstance               for T t  list                 T copy    T  t getClass   getDeclaredConstructor t getclass    newInstance t                c add copy                           return c          catch Exception e               throw new RuntimeException  List cloning unsupported  e

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Be very careful when cloning ArrayLists  Cloning in java is shallow  This means that it will  only clone the Arraylist itself and not its members  So if you have an ArrayList X1 and clone it into X2 any change in X2 will also manifest in X1 and vice-versa  When you clone you will only generate a new ArrayList with pointers to the same elements in the original

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Why would you want to clone  Creating a new list usually makes more sense   List lt String gt  strs      List lt String gt  newStrs   new ArrayList lt  gt  strs     Job done

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If you want this in order to be able to return the List in a getter it would be better to do   ImmutableList copyOf list

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List lt String gt  shallowClonedList   new ArrayList lt  gt  listOfStrings     Keep in mind that this is only a shallow not a deep copy  ie  you get a new list  but the entries are the same  This is no problem for simply strings  Get s more tricky when the list entries are objects themself

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ArrayList first   new ArrayList     ArrayList copy    ArrayList  first clone

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Be advised that Object clone   has some major problems  and its use is discouraged in most cases  Please see Item 11  from  Effective Java  by Joshua Bloch for a complete answer  I believe you can safely use Object clone   on primitive type arrays  but apart from that you need to be judicious about properly using and overriding clone  You are probably better off defining a copy constructor or a static factory method that explicitly clones the object according to your semantics

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With Java 8 it can be cloned with a stream   import static java util stream Collectors toList         List lt AnObject gt  clone   myList stream   collect toList

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I think this should do the trick using the Collections API   Note  the copy method runs in linear time         assume oldList exists and has data in it  List lt String gt  newList   new ArrayList lt String gt     Collections copy newList  oldList

[java] How do I clone a generic List in Java?

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