Why do I get an UnsupportedOperationException when trying to remove an element from a List

Question

I have this code   public static String SelectRandomFromTemplate String template int count       String   split   template split          List lt String gt  list Arrays asList split      Random r   new Random       while  list size    gt  count           list remove r nextInt list size              return StringUtils join list             I get this   06-03 15 05 29 614  ERROR AndroidRuntime 7737   java lang UnsupportedOperationException 06-03 15 05 29 614  ERROR AndroidRuntime 7737       at java util AbstractList remove AbstractList java 645    How would be this the correct way  Java 15

User · Answer

Following is snippet of code from Arrays

public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

    /**
     * @serial include
     */
    private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

so what happens is that when asList method is called then it returns list of its own private static class version which does not override add funcion from AbstractList to store element in array. So by default add method in abstract list throws exception.

So it is not regular array list.

User · Answer

Creating a new list and populating valid values in new list worked for me   Code throwing error -   List lt String gt  list   new ArrayList lt  gt        for  String s  list         if s is null or blank            list remove s               desiredObject setValue list     After fix -    List lt String gt  list   new ArrayList lt  gt      List lt String gt  newList  new ArrayList lt  gt      for  String s  list       if s is null or blank          continue          newList add s       desiredObject setValue newList

User · Answer

This UnsupportedOperationException comes when you try to perform some operation on collection where its not allowed and in your case  When you call Arrays asList it does not return a java util ArrayList  It returns a java util Arrays ArrayList which is an immutable list  You cannot add to it and you cannot remove from it

User · Answer

You can t remove  nor can you add to a fixed-size-list of Arrays   But you can create your sublist from that list   list   list subList 0  list size   -  list size   - count     public static String SelectRandomFromTemplate String template  int count       String   split   template split            List lt String gt  list   Arrays asList split      Random r   new Random       while  list size    gt  count           list   list subList 0  list size   -  list size   - count            return StringUtils join list              Other way is   ArrayList lt String gt  al   new ArrayList lt String gt  Arrays asList template      this will create ArrayList which is not fixed size like Arrays asList

User · Answer

I ve got another solution for that problem   List lt String gt  list   Arrays asList split   List lt String gt  newList   new ArrayList lt  gt  list     work on newList

User · Answer

The issue is you re creating a List using Arrays asList   method with fixed Length meaning that  Since the returned List is a fixed-size List  we can   t add remove elements   See the below block of code that I am using  This iteration will give an Exception Since it is an iteration list Created by asList   so remove and add are not possible  it is a fixed array  List lt String gt  words   Arrays asList  quot pen quot    quot pencil quot    quot sky quot    quot blue quot    quot sky quot    quot dog quot     for  String word   words        if   quot sky quot  equals word             words remove word                This will work fine since we are taking a new ArrayList we can perform modifications while iterating  List lt String gt  words1   new ArrayList lt String gt  Arrays asList  quot pen quot    quot pencil quot    quot sky quot    quot blue quot    quot sky quot    quot dog quot     for  String word   words        if   quot sky quot  equals word             words remove word

User · Answer

Just read the JavaDoc for the asList method      Returns a   code List  of the objects   in the specified array  The size of   the   code List  cannot be modified    i e  adding and removing are   unsupported  but the elements can be   set  Setting an element modifies the   underlying array    This is from Java 6 but it looks like it is the same for the android java   EDIT  The type of the resulting list is Arrays ArrayList  which is a private class inside Arrays class   Practically speaking  it is nothing but a List-view on the array that you ve passed with Arrays asList  With a consequence  if you change the array  the list is changed too  And because an array is not resizeable  remove and add operation must be unsupported

User · Answer

Probably because you re working with unmodifiable wrapper   Change this line   List lt String gt  list   Arrays asList split     to this line   List lt String gt  list   new LinkedList lt  gt  Arrays asList split

User · Answer

Arraylist narraylist Arrays asList       Returns immutable arraylist To make it mutable solution would be  Arraylist narraylist new ArrayList Arrays asList

User · Answer

Replace   List lt String gt  list Arrays asList split     to  List lt String gt  list   New ArrayList lt  gt     list addAll Arrays asList split      or  List lt String gt  list   new ArrayList lt  gt  Arrays asList split      or  List lt String gt  list   new ArrayList lt String gt  Arrays asList split      or  Better for Remove elements   List lt String gt  list   new LinkedList lt  gt  Arrays asList split

User · Answer

The list returned by Arrays asList   might be immutable  Could you try  List lt String gt  list   new ArrayList Arrays asList split

User · Answer

Yes  on Arrays asList  returning a fixed-size list   Other than using a linked list  simply use addAll method list   Example   String idList    123 222 333 444    List lt String gt  parentRecepeIdList   new ArrayList lt String gt      parentRecepeIdList addAll Arrays asList idList split           parentRecepeIdList add  555

User · Answer

Quite a few problems with your code   On Arrays asList returning a fixed-size list  From the API      Arrays asList  Returns a fixed-size list backed by the specified array    You can t add to it  you can t remove from it  You can t structurally modify the List   Fix  Create a LinkedList  which supports faster remove   List lt String gt  list   new LinkedList lt String gt  Arrays asList split        On split taking regex  From the API      String split String regex   Splits this string around matches of the given regular expression      is a regex metacharacter  if you want to split on a literal    you must escape it to     which as a Java string literal is         Fix   template split            On better algorithm  Instead of calling remove one at a time with random indices  it s better to generate enough random numbers in the range  and then traversing the List once with a listIterator    calling remove   at appropriate indices  There are questions on stackoverflow on how to generate random but distinct numbers in a given range   With this  your algorithm would be O N

User · Answer

This one has burned me many times  Arrays asList creates an unmodifiable list  From the Javadoc  Returns a fixed-size list backed by the specified array   Create a new list with the same content   newList addAll Arrays asList newArray      This will create a little extra garbage  but you will be able to mutate it

User · Answer

I think that replacing   List lt String gt  list   Arrays asList split     with  List lt String gt  list   new ArrayList lt String gt  Arrays asList split      resolves the problem

User · Answer

Arrays asList   returns a list that doesn t allow operations affecting its size  note that this is not the same as  unmodifiable      You could do new ArrayList lt String gt  Arrays asList split    to create a real copy  but seeing what you are trying to do  here is an additional suggestion  you have a O n 2  algorithm right below that    You want to remove list size   - count  lets call this k  random elements from the list  Just pick as many random elements and swap them to the end k positions of the list  then delete that whole range  e g  using subList   and clear   on that   That would turn it to a lean and mean O n  algorithm  O k  is more precise    Update  As noted below  this algorithm only makes sense if the elements are unordered  e g  if the List represents a Bag  If  on the other hand  the List has a meaningful order  this algorithm would not preserve it  polygenelubricants  algorithm instead would    Update 2  So in retrospect  a better  linear  maintaining order  but with O n  random numbers  algorithm would be something like this   LinkedList lt String gt  elements          to avoid the slow ArrayList remove   int k   elements size   - count    elements to select delete int remaining   elements size      elements remaining to be iterated for  Iterator i   elements iterator    k  gt  0  amp  amp  i hasNext    remaining--      i next      if  random nextInt remaining   lt  k           or  random nextDouble    lt   double k remaining       i remove         k--

User · Answer

Arrays asList    uses fixed size array internally  You can t dynamically add or remove from thisArrays asList    Use this   Arraylist lt String gt  narraylist new ArrayList Arrays asList       In narraylist you can easily add or remove items

[java] Why do I get an UnsupportedOperationException when trying to remove an element from a List?

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