Remove Elements from a HashSet while Iterating

Question

So  if I try to remove elements from a Java HashSet while iterating  I get a ConcurrentModificationException   What is the best way to remove a subset of the elements from a HashSet as in the following example   Set lt Integer gt  set   new HashSet lt Integer gt      for int i   0  i  lt  10  i        set add i       Throws ConcurrentModificationException for Integer element   set      if element   2    0          set remove element     Here is a solution  but I don t think it s very elegant   Set lt Integer gt  set   new HashSet lt Integer gt     Collection lt Integer gt  removeCandidates   new LinkedList lt Integer gt      for int i   0  i  lt  10  i        set add i    for Integer element   set      if element   2    0          removeCandidates add element    set removeAll removeCandidates     Thanks

User · Answer

Java 8 Collection has a nice method called removeIf that makes things easier and safer. From the API docs:

default boolean removeIf(Predicate<? super E> filter)
Removes all of the elements of this collection that satisfy the given predicate. 
Errors or runtime exceptions thrown during iteration or by the predicate 
are relayed to the caller.

Interesting note:

The default implementation traverses all elements of the collection using its iterator(). 
Each matching element is removed using Iterator.remove().

From: https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html#removeIf-java.util.function.Predicate-

User · Answer

you can also refactor your solution removing the first loop   Set lt Integer gt  set   new HashSet lt Integer gt     Collection lt Integer gt  removeCandidates   new LinkedList lt Integer gt  set    for Integer element   set     if element   2    0         removeCandidates add element    set removeAll removeCandidates

User · Answer

Here s the more modern streams approach  myIntegerSet stream   filter  it  - gt  it   2    0  collect Collectors toSet     However  this makes a new set  so memory constraints might be an issue if it s a really huge set  EDIT  previous version of this answer suggested Apache CollectionUtils but that was before steams came about

User · Answer

You can manually iterate over the elements of the set   Iterator lt Integer gt  iterator   set iterator    while  iterator hasNext          Integer element   iterator next        if  element   2    0            iterator remove              You will often see this pattern using a for loop rather than a while loop   for  Iterator lt Integer gt  i   set iterator    i hasNext           Integer element   i next        if  element   2    0            i remove              As people have pointed out  using a for loop is preferred because it keeps the iterator variable  i in this case  confined to a smaller scope

User · Answer

Like timber said -  Java 8 Collection has a nice method called removeIf that makes things easier and safer   Here is the code that solve your problem   set removeIf  Integer element  - gt        return  element   2    0         Now your set contains only odd values

User · Answer

An other possible solution   for Object it   set toArray         Create a copy        Integer element    Integer it      if element   2    0          set remove element       Or   Integer   copy   new Integer set size     set toArray copy    for Integer element   copy        if element   2    0          set remove element

User · Answer

The reason you get a ConcurrentModificationException is because an entry is removed via Set remove   as opposed to Iterator remove     If an entry is removed via Set remove   while an iteration is being done  you will get a ConcurrentModificationException   On the other hand  removal of entries via Iterator remove   while iteration is supported in this case   The new for loop is nice  but unfortunately it does not work in this case  because you can t use the Iterator reference   If you need to remove an entry while iteration  you need to use the long form that uses the Iterator directly   for  Iterator lt Integer gt  it   set iterator    it hasNext           Integer element   it next        if  element   2    0            it remove

[java] Remove Elements from a HashSet while Iterating

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