Difference between Arrays asList array and new ArrayList Integer Arrays asList array

Question

What is the difference between   1 List lt Integer gt  list1   new ArrayList lt Integer gt  Arrays asList ia       copy 2 List lt Integer gt  list2   Arrays asList ia     where ia is array of integers   I came to know that some operations are not allowed in list2  why is it so  how is it stored in memory  references   copy    When I shuffle the lists  list1 doesn t affect the original array but list2 does  But still list2 is somewhat confusing    How ArrayList being upcasted to list differs from creating new ArrayList  list1 differs from  1  ArrayList lt Integer gt  list1   new ArrayList lt Integer gt  Arrays asList ia

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List lt Integer gt  list1   new ArrayList lt Integer gt  Arrays asList ia       copy   In this case  list1 is of type ArrayList   List lt Integer gt  list2   Arrays asList ia     Here  the list is returned as a List view  meaning it has only the methods attached to that interface  Hence why some methods are not allowed on list2   ArrayList lt Integer gt  list1   new ArrayList lt Integer gt  Arrays asList ia      Here  you ARE creating a new ArrayList  You re simply passing it a value in the constructor  This is not an example of casting  In casting  it might look more like this   ArrayList list1    ArrayList Arrays asList ia

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Many people have answered the mechanical details already  but it s worth noting  This is a poor design choice  by Java   Java s asList method is documented as  Returns a fixed-size list       If you take its result and call  say  the  add method  it throws an UnsupportedOperationException   This is unintuitive behavior   If a method says it returns a List  the standard expectation is that it returns an object which supports the methods of interface List   A developer shouldn t have to memorize which of the umpteen util List methods create Lists that don t actually support all the List methods   If they had named the method asImmutableList  it would make sense   Or if they just had the method return an actual List  and copy the backing array   it would make sense   They decided to favor both runtime-performance and short names  at the expense of violating both the Principle of Least Surprise and the good-O O  practice of avoiding UnsupportedOperationExceptions    Also  the designers might have made a interface ImmutableList  to avoid a plethora of UnsupportedOperationExceptions

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First of all Arrays class is an utility class which contains no  of  utility methods to operate on Arrays  thanks to Arrays class otherwise we would have needed to create our own methods to act on Array objects   asList   method    asList method is one of the utility methods of Array class  it is static method thats why we can call this method by its class name  like Arrays asList T   a    Now here is the twist  please note that this method doesn t create new ArrayList object  it just returns a List reference to existing Array object so now after using asList method  two references to existing Array object gets created  and this is the reason  all methods that operate on List object   may NOT work on this Array object using List reference like for example  Arrays size is fixed in length  hence you obviously can not add or remove elements from Array object using this List reference  like list add 10  or list remove 10   else it will throw UnsupportedOperationException  any change you are doing using list reference will be reflected in exiting Arrays object   as you are operating on existing Array object by using list reference    In first case you are creating a new Arraylist object  in 2nd case only reference to existing Array object is created but not a new ArrayList object   so now there are two different objects one is Array object and another is ArrayList object and no connection between them   so changes in one object will not be reflected affected in another object   that is in case 2 Array and Arraylist are two different objects   case 1   Integer    ia    1 2 3 4   System out println  Array     Arrays toString ia    List lt Integer gt  list1   new ArrayList lt Integer gt  Arrays asList ia        new ArrayList object is created   no connection between existing Array Object list1 add 5   list1 add 6   list1 remove 0   list1 remove 0   System out println  list1     list1   System out println  Array     Arrays toString ia      case 2   Integer    ia    1 2 3 4   System out println  Array     Arrays toString ia    List lt Integer gt  list2   Arrays asList ia      creates only a  new   List reference to existing Array object  and NOT a new ArrayList Object      list2 add 5       it will throw java lang UnsupportedOperationException - invalid operation  as Array size is fixed  list2 set 0 10       making changes in existing Array object using List reference - valid  list2 set 1 11    ia 2  12         making changes in existing Array object using Array reference - valid System out println  list2     list2   System out println  Array     Arrays toString ia

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String names     new String    Avinash   Amol   John   Peter    java util List lt String gt  namesList   Arrays asList names     or  String names     new String    Avinash   Amol   John   Peter    java util List lt String gt  temp   Arrays asList names              Above Statement adds the wrapper on the input array  So the methods like add  amp  remove will not be applicable on list reference object  namesList    If you try to add an element in the existing array list then you will get  Exception in thread  main  java lang UnsupportedOperationException    The above operation is readonly or viewonly   We can not perform add or remove operation in list object  But  String names     new String    Avinash   Amol   John   Peter    java util ArrayList lt String gt  list1   new ArrayList lt  gt  Arrays asList names      or  String names     new String    Avinash   Amol   John   Peter    java util List lt String gt  listObject   Arrays asList names   java util ArrayList lt String gt  list1   new ArrayList lt  gt  listObject     In above statement you have created a concrete instance of an ArrayList class and passed a list as a parameter   In this case method add  amp  remove will work properly as both methods are from ArrayList class so here we won t get any UnSupportedOperationException  Changes made in Arraylist object  method add or remove an element in from an arraylist  will get not reflect in to original java util List object   String names     new String          Avinash        Amol        John        Peter      java util List  lt  String  gt  listObject   Arrays asList names   java util ArrayList  lt  String  gt  list1   new ArrayList  lt   gt   listObject   for  String string  list1        System out print         string     list1 add  Alex      Added without any exception list1 remove  Avinash      Added without any exception will not make any changes in original list in this case temp object    for  String string  list1        System out print         string     String existingNames     new String          Avinash        Amol        John        Peter     java util List  lt  String  gt  namesList   Arrays asList names   namesList add  Bob       UnsupportedOperationException occur namesList remove  Avinash      UnsupportedOperationException

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package com copy   import java util ArrayList  import java util Arrays  import java util Iterator  import java util List   public class CopyArray        public static void main String   args            List lt Integer gt  list1  list2   null          Integer   intarr     3  4  2  1            list1   new ArrayList lt Integer gt  Arrays asList intarr            list1 add 30           list2   Arrays asList intarr              list2 add 40   Here  we can t modify the existing list because it s a wrapper         System out println  List1            Iterator lt Integer gt  itr1   list1 iterator            while  itr1 hasNext                  System out println itr1 next                       System out println  List2            Iterator lt Integer gt  itr2   list2 iterator            while  itr2 hasNext                  System out println itr2 next

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Well this is because ArrayList resulting from Arrays asList   is not of the type java util ArrayList   Arrays asList   creates an ArrayList of type java util Arrays ArrayList which does not extend java util ArrayList but only extends java util AbstractList

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First  let s see what this does   Arrays asList ia    It takes an array ia and creates a wrapper that implements List lt Integer gt   which makes the original array available as a list  Nothing is copied and all  only a single wrapper object is created  Operations on the list wrapper are propagated to the original array  This means that if you shuffle the list wrapper  the original array is shuffled as well  if you overwrite an element  it gets overwritten in the original array  etc  Of course  some List operations aren t allowed on the wrapper  like adding or removing elements from the list  you can only read or overwrite the elements   Note that the list wrapper doesn t extend ArrayList - it s a different kind of object  ArrayLists have their own  internal array  in which they store their elements  and are able to resize the internal arrays etc  The wrapper doesn t have its own internal array  it only propagates operations to the array given to it  On the other hand  if you subsequently create a new array as   new ArrayList lt Integer gt  Arrays asList ia     then you create new ArrayList  which is a full  independent copy of the original one  Although here you create the wrapper using Arrays asList as well  it is used only during the construction of the new ArrayList and is garbage-collected afterwards  The structure of this new ArrayList is completely independent of the original array  It contains the same elements  both the original array and this new ArrayList reference the same integers in memory   but it creates a new  internal array  that holds the references  So when you shuffle it  add  remove elements etc   the original array is unchanged

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Note that  in Java 8   ia  above must be Integer   and not int    Arrays asList   of an int array returns a list with a single element  When using OP s code snippet  the compiler will catch the issue  but some methods  e g  Collections shuffle    will silently fail to do what you expect

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I am pretty late here  anyways felt an explanation with doc references would be better for someone looking for answer         java util Arrays       This is a utility class with bunch of static methods to operate on given array asList is one such static method that takes input array and returns an object of java util Arrays ArrayList which is a static nested class that extends AbstractList which inturn implements List interface  So Arrays asList inarray  returns a List wrapper around the input array but this wrapper is java util Arrays ArrayList and not java util ArrayList and it refers to the same array so adding more elements to List wrapped array would affect orignal one too and also we cannot change the length          java util ArrayList       ArrayList has bunch of overloaded constructors  public ArrayList   -   returns arraylist with default capacity 10  public ArrayList Collection c   public ArrayList int initialCapacity  So when we pass Arrays asList returned object i e  List AbstractList  to second constructor above  it will create a new dynamic array this array size increases as we add more elements than its capacity and also the new elements will not affect the orignal array  shallow copying the original array shallow copy means it copies over the references only and does not create a new set of same objects as in orignal array

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1 List lt Integer gt  list1   new ArrayList lt Integer gt  Arrays asList ia       copy 2 List lt Integer gt  list2   Arrays asList ia     In line 2  Arrays asList ia  returns a List reference of inner class object defined within Arrays  which is also called ArrayList but is private and only extends AbstractList  This means what returned from Arrays asList ia  is a class object different from what  you get from new ArrayList lt Integer gt    You cannot use some operations to line 2 because the inner private class within Arrays does not provide those methods   Take a look at this link and see what you can do with the private inner class  http   grepcode com file repository grepcode com java root jdk openjdk 6-b14 java util Arrays java Arrays ArrayList  Line 1 creates a new ArrayList object copying elements from what you get from line 2  So you can do whatever you want since java util ArrayList provides all those methods

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Summary of difference -  when list is created without using new operator Arrays asList   method it return Wrapper which means   1  you can perform add update Operation   2  the changes done in original array will be reflected to List as well     and vice versa

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Arrays asList    this method returns its own implementation of List It takes an array as an argument and builds methods and attributes on top of it  since it is not copying any data from an array but using the original array this causes alteration in original array when you modify list returned by the Arrays asList   method   on the other hand  ArrayList Arrays asList     is a constructor of ArrayList class which takes a list as argument and returns an ArrayList that is independent of list ie  Arrays asList   in this case passed as an argument  that is why you see these results

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In response to some comments asking questions about the behaviour of Arrays asList   since Java 8       int   arr1    1 2 3                  Arrays are objects in Java  internally int   will be represented by         an Integer Array object which when printed on console shall output        a pattern such as          I address for 1-dim int array           I address for 2-dim int array            F address for 2-dim float array etc             System out println Arrays asList arr1                     The line below results in Compile time error as Arrays asList int   array         returns List lt int   gt   The returned list contains only one element         and that is the int    1 2 3                 List lt Integer gt  list1   Arrays asList arr1                   Arrays asList arr1  is  Arrays ArrayList object whose only element is int   array        so the line below prints   I       where  I     is the array object             System out println Arrays asList arr1                   This prints  I      the actual array object stored as single element       in the Arrays ArrayList object              System out println Arrays asList arr1  get 0            prints the contents of array  1 2 3      System out println Arrays toString Arrays asList arr1  get 0          Integer   arr2    1 2 3                Arrays asList arr  is  Arrays ArrayList object which is       a wrapper list object containing three elements 1 2 3       Technically  it is pointing to the original Integer   array             List lt Integer gt  list2   Arrays asList arr2           prints the contents of list  1 2 3      System out println list2

[java] Difference between Arrays.asList(array) and new ArrayList<Integer>(Arrays.asList(array))

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