How can I calculate the difference between two ArrayLists

Question

I have two ArrayLists   ArrayList A contains     2009-05-18   2009-05-19   2009-05-21     ArrayList B contains     2009-05-18   2009-05-18   2009-05-19   2009-05-19   2009-05-20   2009-05-21   2009-05-21   2009-05-22     I have to compare ArrayList A and ArrayList B  The result ArrayList  should contain the  List which does not exist in ArrayList A   ArrayList result should be     2009-05-20   2009-05-22     how to compare

User · Answer

EDIT  Original question did not specify language  My answer is in C    You should instead use HashSet for this purpose  If you must use ArrayList  you could use the following extension methods   var a   arrayListA Cast lt DateTime gt     var b   arrayListB Cast lt DateTime gt         var c   b Except a    var arrayListC   new ArrayList c ToArray       using HashSet     var a   new HashSet lt DateTime gt           and fill it var b   new HashSet lt DateTime gt           and fill it b ExceptWith a      removes from b items that are in a

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Although this is a very old question in Java 8 you could do something like   List lt String gt  a1   Arrays asList  2009-05-18    2009-05-19    2009-05-21     List lt String gt  a2   Arrays asList  2009-05-18    2009-05-18    2009-05-19    2009-05-19    2009-05-20    2009-05-21   2009-05-21    2009-05-22      List lt String gt  result   a2 stream   filter elem - gt   a1 contains elem   collect Collectors toList

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Hi use this class this will compare both lists and shows exactly the mismatch b w both lists   import java util ArrayList  import java util List    public class ListCompare                    param args             public static void main String   args            List lt String gt  dbVinList          dbVinList   new ArrayList lt String gt             List lt String gt  ediVinList          ediVinList   new ArrayList lt String gt                         dbVinList add  A            dbVinList add  B            dbVinList add  C            dbVinList add  D             ediVinList add  A            ediVinList add  C            ediVinList add  E            ediVinList add  F              ediVinList add  G            ediVinList add  H            ediVinList add  I            ediVinList add  J                 List lt String gt  dbVinListClone   dbVinList          List lt String gt  ediVinListClone   ediVinList           boolean flag          String mismatchVins   null          if dbVinListClone containsAll ediVinListClone                flag   dbVinListClone removeAll ediVinListClone                  if flag                   mismatchVins   getMismatchVins dbVinListClone                          else              flag   ediVinListClone removeAll dbVinListClone               if flag                   mismatchVins   getMismatchVins ediVinListClone                                   if mismatchVins    null               System out println  mismatch vins     mismatchVins                                private static String getMismatchVins List lt String gt  mismatchList           StringBuilder mismatchVins   new StringBuilder            int i   0          for String mismatch   mismatchList               i                if i  lt  mismatchList size    amp  amp  i  5                   mismatchVins append mismatch  append                      else                  mismatchVins append mismatch                             if i  5                                  break                                  String mismatch1          if mismatchVins length    gt  100               mismatch1   mismatchVins substring 0  99            else              mismatch1   mismatchVins toString                             return mismatch1

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I guess you re talking about C   If so  you can try this      ArrayList CompareArrayList ArrayList a  ArrayList b                ArrayList output   new ArrayList            for  int i   0  i  lt  a Count  i                          string str    string a i               if   b Contains str                                 if  output Contains str      check for dupes                     output Add str                                   return output

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THIS WORK ALSO WITH Arraylist         Create a couple ArrayList objects and populate them        with some delicious fruits      ArrayList lt String gt  firstList   new ArrayList lt String gt                                         private static final long serialVersionUID   1L                 add  apple            add  orange            add  pea                 ArrayList lt String gt  secondList   new ArrayList lt String gt                                               private static final long serialVersionUID   1L                 add  apple            add  orange            add  banana            add  strawberry                    Show the  before  lists     System out println  First List      firstList       System out println  Second List      secondList           Remove all elements in firstList from secondList     secondList removeAll firstList           Show the  after  list     System out println  Result      secondList

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In Java  you can use the Collection interface s removeAll method      Create a couple ArrayList objects and populate them    with some delicious fruits  Collection firstList   new ArrayList          add  apple        add  orange         Collection secondList   new ArrayList          add  apple        add  orange        add  banana        add  strawberry            Show the  before  lists System out println  First List      firstList   System out println  Second List      secondList       Remove all elements in firstList from secondList secondList removeAll firstList       Show the  after  list System out println  Result      secondList     The above code will produce the following output   First List   apple  orange  Second List   apple  orange  banana  strawberry  Result   banana  strawberry

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You are just comparing strings   Put the values in ArrayList A as keys in HashTable A  Put the values in ArrayList B as keys in HashTable B   Then  for each key in HashTable A  remove it from HashTable B if it exists     What you are left with in HashTable B are the strings  keys  that were not values in ArrayList A   C   3 0  example added in response to request for code   List lt string gt  listA   new List lt string gt   2009-05-18   2009-05-19   2009-05-21     List lt string gt  listB   new List lt string gt   2009-05-18   2009-05-18   2009-05-19   2009-05-19   2009-05-20   2009-05-21   2009-05-21   2009-05-22     HashSet lt string gt  hashA   new HashSet lt string gt     HashSet lt string gt  hashB   new HashSet lt string gt      foreach  string dateStrA in listA  hashA Add dateStrA   foreach  string dateStrB in listB  hashB Add dateStrB    foreach  string dateStrA in hashA        if  hashB Contains dateStrA   hashB Remove dateStrA      List lt string gt  result   hashB ToList lt string gt

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You already have the right answer  And if you want to make more complicated and interesting operations between Lists  collections  use apache commons collections  CollectionUtils  It allows you to make conjuction disjunction  find intersection  check if one collection is a subset of another and other nice things

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In Java 8 with streams  it s pretty simple actually  EDIT  Can be efficient without streams  see lower   List lt String gt  listA   Arrays asList  2009-05-18   2009-05-19   2009-05-21    List lt String gt  listB   Arrays asList  2009-05-18   2009-05-18   2009-05-19   2009-05-19                                       2009-05-20   2009-05-21   2009-05-21   2009-05-22     List lt String gt  result   listB stream                               filter not new HashSet lt  gt  listA   contains                               collect Collectors toList       Note that the hash set is only created once  The method reference is tied to its contains method  Doing the same with lambda would require having the set in a variable  Making a variable is not a bad idea  especially if you find it unsightly or harder to understand   You can t easily negate the predicate without something like this utility method  or explicit cast   as you can t call the negate method reference directly  type inference is needed first    private static  lt T gt  Predicate lt T gt  not Predicate lt T gt  predicate        return predicate negate        If streams had a filterOut method or something  it would look nicer     Also   Holger gave me an idea  ArrayList has its removeAll method optimized for multiple removals  it only rearranges its elements once  However  it uses the contains method provided by given collection  so we need to optimize that part if listA is anything but tiny   With listA and listB declared previously  this solution doesn t need Java 8 and it s very efficient   List lt String gt  result   new ArrayList listB   result removeAll new HashSet lt  gt  listA

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I have used Guava Sets difference   The parameters are sets and not general collections  but a handy way to create sets from any collection  with unique items  is Guava ImmutableSet copyOf Iterable     I first posted this on a related dupe question  but I m copying it here too since I feel it is a good option that is so far missing

[java] How can I calculate the difference between two ArrayLists?

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