Getting the difference between two sets

Question

So if I have two sets   Set lt Integer gt  test1   new HashSet lt Integer gt     test1 add 1   test1 add 2   test1 add 3    Set lt Integer gt  test2   new HashSet lt Integer gt     test2 add 1   test2 add 2   test2 add 3   test2 add 4   test2 add 5     Is there a way to compare them and only have a set of 4 and 5 returned

User · Answer

You can use CollectionUtils disjunction to get all differences or CollectionUtils subtract to get the difference in the first collection   Here is an example of how to do that       var collection1   List of 1  2  3  4  5       var collection2   List of 2  3  5  6       System out println StringUtils join collection1               System out println StringUtils join collection2               System out println StringUtils join CollectionUtils subtract collection1  collection2                System out println StringUtils join CollectionUtils retainAll collection1  collection2                System out println StringUtils join CollectionUtils collate collection1  collection2                System out println StringUtils join CollectionUtils disjunction collection1  collection2                System out println StringUtils join CollectionUtils intersection collection1  collection2                System out println StringUtils join CollectionUtils union collection1  collection2

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Adding a solution which I ve recently used myself and haven t seen mentioned here  If you have Apache Commons Collections available then you can use the SetUtils difference method     Returns all the elements of test2 which are not in test1 SetUtils difference test2  test1    Note that according to the documentation the returned set is an unmodifiable view   Returns a unmodifiable view containing the difference of the given Sets  denoted by a   b  or a - b   The returned view contains all elements of a that are not a member of b   Full documentation  https   commons apache org proper commons-collections apidocs org apache commons collections4 SetUtils html difference-java util Set-java util Set-

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If you are using Java 8  you could try something like this   public Set lt Number gt  difference final Set lt Number gt  set1  final Set lt Number gt  set2       final Set lt Number gt  larger   set1 size    gt  set2 size     set1   set2      final Set lt Number gt  smaller   larger equals set1    set2   set1      return larger stream   filter n - gt   smaller contains n   collect Collectors toSet

User · Answer

Java 8  We can make use of removeIf which takes a predicate to write a utility method as      computes the difference without modifying the sets public static  lt T gt  Set lt T gt  differenceJava8 final Set lt T gt  setOne  final Set lt T gt  setTwo         Set lt T gt  result   new HashSet lt T gt  setOne        result removeIf setTwo  contains        return result      And in case we are still at some prior version then we can use removeAll as   public static  lt T gt  Set lt T gt  difference final Set lt T gt  setOne  final Set lt T gt  setTwo         Set lt T gt  result   new HashSet lt T gt  setOne        result removeAll setTwo        return result

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If you use Guava  former Google Collections  library there is a solution   SetView lt Number gt  difference   com google common collect Sets difference test2  test1     The returned SetView is a Set  it is a live representation you can either make immutable or copy to another set  test1 and test2 are left intact

User · Answer

Just to put one example here  system is in existingState  and we want to find elements to remove  elements that are not in newState but are present in existingState  and elements to add  elements that are in newState but are not present in existingState     public class AddAndRemove      static Set lt Integer gt  existingState   Set of 1 2 3 4 5     static Set lt Integer gt  newState   Set of 0 5 2 11 3 99      public static void main String   args         Set lt Integer gt  add   new HashSet lt  gt  newState       add removeAll existingState        System out println  Elements to add       add        Set lt Integer gt  remove   new HashSet lt  gt  existingState       remove removeAll newState        System out println  Elements to remove       remove            would output this as a result   Elements to add    0  99  11  Elements to remove    1  4

User · Answer

Yes   test2 removeAll test1    Although this will mutate test2  so create a copy if you need to preserve it   Also  you probably meant  lt Integer gt  instead of  lt int gt

User · Answer

You can make a union using  addAll    and an intersection using  retainAll    of the two sets and use  removeIf    to remove the intersection  or the duplicated element  from the union  HashSet union   new HashSet group1   union addAll group2            System out println  quot Union   quot    union            HashSet intersection   new HashSet group1   intersection retainAll group2            System out println  quot Intersection   quot    intersection            HashSet difference   new HashSet union   difference removeIf n - gt   difference contains intersection              System out println  quot Difference   quot    difference

User · Answer

Try this    test2 removeAll test1     Set removeAll     Removes from this set all of its elements that are contained in the specified collection  optional operation   If the specified collection is also a set  this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets

[java] Getting the difference between two sets

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