How do I iterate and modify Java Sets

Question

Let s say I have a Set of Integers  and I want to increment every Integer in the Set   How would I do this   Am I allowed to add and remove elements from the set while iterating it   Would I need to create a new set that I would  copy and modify  the elements into  while I m iterating the original set   EDIT  What if the elements of the set are immutable

User · Accepted Answer

You can safely remove from a set during iteration with an Iterator object; attempting to modify a set through its API while iterating will break the iterator. the Set class provides an iterator through getIterator().

however, Integer objects are immutable; my strategy would be to iterate through the set and for each Integer i, add i+1 to some new temporary set. When you are finished iterating, remove all the elements from the original set and add all the elements of the new temporary set.

Set<Integer> s; //contains your Integers
...
Set<Integer> temp = new Set<Integer>();
for(Integer i : s)
    temp.add(i+1);
s.clear();
s.addAll(temp);

User · Answer

I don t like very much iterator s semantic  please consider this as an option   It s also safer as you publish less of your internal state  private Map lt String  String gt  JSONtoMAP String jsonString         JSONObject json   new JSONObject jsonString       Map lt String  String gt  outMap   new HashMap lt String  String gt          for  String curKey    Set lt String gt   json keySet              outMap put curKey  json getString curKey               return outMap

User · Answer

You could create a mutable wrapper of the primitive int and create a Set of those   class MutableInteger       private int value      public int getValue                 return value            public void setValue int value                this value   value           class Test       public static void main String   args                Set lt MutableInteger gt  mySet   new HashSet lt MutableInteger gt                populate the set                          for  MutableInteger integer  mySet                        integer setValue integer getValue     1                       Of course if you are using a HashSet you should implement the hash  equals method in your MutableInteger but that s outside the scope of this answer

User · Answer

You can do what you want if you use an iterator object to go over the elements in your set  You can remove them on the go an it s ok  However removing them while in a for loop  either  standard   of the for each kind  will get you in trouble   Set lt Integer gt  set   new TreeSet lt Integer gt         set add 1       set add 2       set add 3          good way      Iterator lt Integer gt  iterator   set iterator        while iterator hasNext              Integer setElement   iterator next            if setElement  2                iterator remove                           bad way      for Integer setElement set            if setElement  2                  might work or might throw exception  Java calls it indefined behaviour              set remove setElement                      As per  mrgloom s comment  here are more details as to why the  bad  way described above is  well    bad    Without getting into too much details about how Java implements this  at a high level  we can say that the  bad  way is bad because it is clearly stipulated as such in the Java docs    https   docs oracle com javase 8 docs api java util ConcurrentModificationException html  stipulate  amongst others  that  emphasis mine        For example  it is not generally permissible for one thread to   modify a Collection while another thread is iterating over it  In   general  the results of the iteration are undefined under these   circumstances  Some Iterator implementations  including those of all   the general purpose collection implementations provided by the JRE    may choose to throw this exception if this behavior is detected              Note that this exception does not always indicate that an object   has been concurrently modified by a different thread  If a single   thread issues a sequence of method invocations that violates the   contract of an object  the object may throw this exception  For   example  if a thread modifies a collection directly while it is   iterating over the collection with a fail-fast iterator  the iterator   will throw this exception     To go more into details  an object that can be used in a forEach loop needs to implement the  java lang Iterable  interface  javadoc here   This produces an Iterator  via the  Iterator  method found in this interface   which is instantiated on demand  and will contain internally a reference to the Iterable object from which it was created  However  when an Iterable object is used in a forEach loop  the instance of this iterator is hidden to the user  you cannot access it yourself in any way    This  coupled with the fact that an Iterator is pretty stateful  i e  in order to do its magic and have coherent responses for its  next  and  hasNext  methods it needs that the backing object is not changed by something else than the iterator itself while it s iterating  makes it so that it will throw an exception as soon as it detects that something changed in the backing object while it is iterating over it    Java calls this  fail-fast  iteration  i e  there are some actions  usually those that modify an Iterable instance  while an Iterator is iterating over it   The  fail  part of the  fail-fast  notion refers to the ability of an Iterator to detect when such  fail  actions happen  The  fast  part of the  fail-fast   and  which in my opinion should be called  best-effort-fast    will terminate the iteration via ConcurrentModificationException as soon as it can detect that a  fail  action has happen

User · Answer

Firstly  I believe that trying to do several things at once is a bad practice in general and I suggest you think over what you are trying to achieve   It serves as a good theoretical question though and from what I gather the CopyOnWriteArraySet implementation of java util Set interface satisfies your rather special requirements   http   download oracle com javase 1 5 0 docs api java util concurrent CopyOnWriteArraySet html

[java] How do I iterate and modify Java Sets?

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