Java List add UnsupportedOperationException

Question

I try to add objects to a List lt String gt  instance but it throws an UnsupportedOperationException  Does anyone know why   My Java code   String   membersArray   request getParameterValues  members    List lt String gt  membersList   Arrays asList membersArray    for  String member   membersList        Person person   Dao findByName member       List lt String gt  seeAlso      seeAlso   person getSeeAlso        if   seeAlso contains groupDn            seeAlso add groupDn           person setSeeAlso seeAlso             The error message    java lang UnsupportedOperationException     java util AbstractList add Unknown Source      java util AbstractList add Unknown Source      javax servlet http HttpServlet service HttpServlet java 641      javax servlet http HttpServlet service HttpServlet java 722

User · Answer

You cannot modify a result from a LDAP query  Your problem is in this line   seeAlso add groupDn     The seeAlso list is unmodifiable

User · Answer

You must initialize your List seeAlso     List lt String gt  seeAlso   new Vector lt String gt       or     List lt String gt  seeAlso   new ArrayList lt String gt

User · Answer

instead of using add   we can use addall      seeAlso addall groupDn       add adds a single item  while addAll adds each item from the collection one by one  In the end  both methods return true if the collection has been modified  In case of ArrayList this is trivial  because the collection is always modified  but other collections  such as Set  may return false if items being added are already there

User · Answer

Form the Inheritance concept  If some perticular method is not available in the current class it will search for that method in super classes  If available it executes      It executes AbstractList lt E gt  class add   method which throws UnsupportedOperationException      When you are converting from an Array to a Collection Obejct  i e   array-based to collection-based API then it is going to provide you fixed-size collection object  because Array s behaviour is of Fixed size      java util Arrays asList  T    a     Souce samples for conformation    public class Arrays       public static  lt T gt  List lt T gt  asList T    a            return new java util Arrays ArrayList ArrayList lt  gt  a      Arrays Inner Class ArrayList                     private static class ArrayList lt E gt  extends AbstractList lt E gt  implements RandomAccess  java io Serializable                         public abstract class AbstractList lt E gt  extends AbstractCollection lt E gt  implements List lt E gt        public void add int index  E element            throw new UnsupportedOperationException              public E set int index  E element            throw new UnsupportedOperationException              public E remove int index            throw new UnsupportedOperationException               public Iterator lt E gt  iterator             return new Itr              private class Itr implements Iterator lt E gt                             public ListIterator lt E gt  listIterator             return listIterator 0             private class ListItr extends Itr implements ListIterator lt E gt                            Form the above Source you may observe that java util Arrays ArrayList class doesn t  Override add index  element   set index  element   remove index   So  From inheritance it executes super AbstractList lt E gt  class add   function which throws UnsupportedOperationException   As AbstractList lt E gt  is an abstract class it provides the implementation to iterator   and listIterator    So  that we can iterate over the list object   List lt String gt  list of Arrays   Arrays asList new String      a    b    c      try       list of Arrays add  Yashwanth M      catch java lang UnsupportedOperationException e        System out println  List Interface executes AbstractList add   fucntion which throws UnsupportedOperationException       System out println  Arrays   List       list of Arrays    Iterator lt String gt  iterator   list of Arrays iterator    while  iterator hasNext    System out println  Iteration       iterator next       ListIterator lt String gt  listIterator   list of Arrays listIterator    while  listIterator hasNext       System out println  Forward  iteration       listIterator next      while listIterator hasPrevious    System out println  Backward iteration       listIterator previous       You can even create Fixed-Size array form Collections class Collections unmodifiableList list     Sample Source    public class Collections       public static  lt T gt  List lt T gt  unmodifiableList List lt   extends T gt  list            return  list instanceof RandomAccess                   new UnmodifiableRandomAccessList lt  gt  list                    new UnmodifiableList lt  gt  list                A Collection     sometimes called a container     is simply an object that groups multiple elements into a single unit  Collections are used to store  retrieve  manipulate  and communicate aggregate data    see also   HashMap vs HashTable Object Serialization uses the Serializable and Externalizable interfaces

User · Answer

Many of the List implementation support limited support to add remove  and Arrays asList membersArray  is one of that  You need to insert the record in java util ArrayList or use the below approach to convert into ArrayList   With the minimal change in your code  you can do below to convert a list to ArrayList  The first solution is having a minimum change in your solution  but the second one is more optimized  I guess       String   membersArray   request getParameterValues  members        ArrayList lt String gt  membersList   new ArrayList lt  gt  Arrays asList membersArray      OR      String   membersArray   request getParameterValues  members        ArrayList lt String gt  membersList   Stream of membersArray  collect Collectors toCollection ArrayList  new

User · Answer

Not every List implementation supports the add   method   One common example is the List returned by Arrays asList    it is documented not to support any structural modification  i e  removing or adding elements   emphasis mine       Returns a fixed-size list backed by the specified array    Even if that s not the specific List you re trying to modify  the answer still applies to other List implementations that are either immutable or only allow some selected changes   You can find out about this by reading the documentation of UnsupportedOperationException and List add    which documents this to be an   optional operation    The precise meaning of this phrase is explained at the top of the List documentation   As a workaround you can create a copy of the list to a known-modifiable implementation like ArrayList   seeAlso   new ArrayList lt  gt  seeAlso

User · Answer

List membersList   Arrays asList membersArray     returns immutable list  what you need to do is   new ArrayList lt   Arrays asList membersArray    to make it mutable

[java] Java List.add() UnsupportedOperationException

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