iterating over and removing from a map

Question

I was doing   for  Object key   map keySet        if  something          map remove key     which threw a ConcurrentModificationException  so i changed it to   for  Object key   new ArrayList lt Object gt  map keySet         if  something          map remove key     this  and any other procedures that modify the map are in synchronized blocks   is there a better solution   if no one comes up with a better solution  first to say no gets the tick

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An alternative  more verbose way  List lt SomeObject gt  toRemove   new ArrayList lt SomeObject gt     for  SomeObject key  map keySet          if  something            toRemove add key            for  SomeObject key  toRemove        map remove key

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is there a better solution    Well  there is  definitely  a better way to do so in a single statement  but that depends on the condition based on which elements are removed   For eg  remove all those elements where value is test  then use below   map values   removeAll Collections singleton  test       UPDATE It can be done in a single line using Lambda expression in Java 8   map entrySet   removeIf e- gt   lt boolean expression gt       I know this question is way too old  but there isn t any harm in updating the better way to do the things

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Set s map entrySet    Iterator iter   s iterator     while  iter hasNext          Map Entry entry   Map Entry iter next         if  value you need to remove  equals entry getKey                map remove

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You have to use Iterator to safely remove element while traversing a map

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Java 8 support a more declarative approach to iteration  in that we specify the result we want rather than how to compute it  Benefits of the new approach are that it can be more readable  less error prone   public static void mapRemove          Map lt Integer  String gt  map   new HashMap lt Integer  String gt                            put 1   one                put 2   two                put 3   three                          map forEach   key  value  - gt             System out println   Key      key     t      Value      value                    map keySet   removeIf e- gt  e gt 2        lt -- remove here      System out println  After removing element         map forEach   key  value  - gt             System out println   Key      key     t      Value      value                 And result is as follows   Key  1   Value  one Key  2   Value  two Key  3   Value  three After removing element Key  1   Value  one Key  2   Value  two

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And this should work as well    ConcurrentMap lt Integer  String gt  running       create and populate map  Set lt Entry lt Integer  String gt  gt  set   running entrySet         for  Entry lt Integer  String gt  entry   set       if  entry getKey   gt 600000          set remove entry getKey

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Use a real iterator   Iterator lt Object gt  it   map keySet   iterator     while  it hasNext        it next      if  something      it remove         Actually  you might need to iterate over the entrySet   instead of the keySet   to make that work

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As of Java 8 you could do this as follows  map entrySet   removeIf e - gt   lt boolean expression gt     Oracle Docs  entrySet    The set is backed by the map  so changes to the map are reflected in the set  and vice-versa

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Maybe you can iterate over the map looking for the keys to remove and storing them in a separate collection  Then remove the collection of keys from the map  Modifying the map while iterating is usually frowned upon  This idea may be suspect if the map is very large

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Here is a code sample to use the iterator in a for loop to remove the entry   Map lt String  String gt  map   new HashMap lt String  String gt              put  test    test123        put  test2    test456            for Iterator lt Map Entry lt String  String gt  gt  it   map entrySet   iterator    it hasNext            Map Entry lt String  String gt  entry   it next        if entry getKey   equals  test              it remove

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I agree with Paul Tomblin  I usually use the keyset s iterator  and then base my condition off the value for that key   Iterator lt Integer gt  it   map keySet   iterator    while it hasNext          Integer key   it next        Object val   map get key       if  val shouldBeRemoved              it remove

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ConcurrentHashMap  You can use java util concurrent ConcurrentHashMap   It implements ConcurrentMap  which extends the Map interface    E g    Map lt Object  Content gt  map   new ConcurrentHashMap lt Object  Content gt      for  Object key   map keySet          if  something            map remove key             This approach leaves your code untouched  Only the map type differs

[java] iterating over and removing from a map

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