Java Synchronized list

Question

I have a pre-populated array list  And I have multiple threads which will remove elements from the array list  Each thread calls the remove method below and removes one item from the list  Does the following code give me consistent behavior    ArrayList lt String gt  list   Collections synchronizedList new ArrayList lt String gt       void remove String item         do something   doesn t work on the list       list remove item       Thanks

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synchronized list        for  Object o   list

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It will give consistent behavior for add remove operations  But while iterating you have to explicitly synchronized  Refer this link

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From Collections synchronizedList List  javadoc     Returns a synchronized  thread-safe  list backed by the specified   list   In order to guarantee serial access  it is critical that   all access to the backing list is accomplished   through the returned list     It is imperative that the user manually synchronize on the returned list when iterating over it  Failure to follow this advice may result in non-deterministic behavior

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You can have 2 diffent problems with lists   1  If you do a modification within an iteration even though in a mono thread environment  you will have ConcurrentModificationException like in this following example    List lt String gt  list   new ArrayList lt String gt     for  int i 0 i lt 5 i       list add  Hello   i    for String msg list     list remove msg     So  to avoid this problem  you can do    for int i list size  -1 i gt  0 i--     list remove i     2 The second problem could be multi threading environment  As mentioned above  you can use synchronized list  to avoid exceptions

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That should be fine as long as you don t require the  remove  method to be atomic   In other words  if the  do something  checks that the item appears more than once in the list for example  it is possible that the result of that check will be wrong by the time you reach the next line   Also  make sure you synchronize on the list when iterating   synchronized list        for  Object o   list         As mentioned by Peter Lawrey  CopyOnWriteArrayList can make your life easier and can provide better performance in a highly concurrent environment

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Yes  Just be careful if you are also iterating over the list  because in this case you will need to synchronize on it   From the Javadoc      It is imperative that the user manually synchronize on the returned list when iterating over it    List list   Collections synchronizedList new ArrayList             synchronized  list        Iterator i   list iterator       Must be in synchronized block     while  i hasNext            foo i next         Or  you can use CopyOnWriteArrayList which is slower for writes but doesn t have this issue

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Yes  it will work fine as you have synchronized the list   I would suggest you to use CopyOnWriteArrayList   CopyOnWriteArrayList lt String gt  cpList new CopyOnWriteArrayList lt String gt  new ArrayList lt String gt           void remove String item                 do something   doesn t work on the list                   cpList  remove item

[java] Java Synchronized list

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