Java Compare Two Lists

Question

I have two lists   not java lists  you can say two columns   For example    List 1                Lists 2     milan                 hafil   dingo                 iga   iga                   dingo   elpha                 binga   hafil                 mike   meat                  dingo   milan   elpha   meat   iga                      neeta peeta       I d like a method that returns how many elements are same  For this example it should be 3 and it should return me similar values of both list and different values too   Should I use hashmap if yes then what method to get my result    Please help  P S  It is not a school assignment    So if you just guide me it will be enough

User · Answer

Assuming hash1 and hash2  List lt  String  gt  sames   whatever List lt  String  gt  diffs   whatever  int count   0  for  String key   hash1 keySet          if  hash2 containsKey  key                 sames add  key            else            diffs add  key              sames size   contains the number of similar elements

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Of all the approaches  I find using org apache commons collections CollectionUtils isEqualCollection is the best approach  Here are the reasons -    I don t have to declare any additional list set myself  I am not mutating the input lists It s very efficient  It checks the equality in O N  complexity    If it s not possible to have apache commons collections as a dependency  I would recommend to implement the algorithm it follows to check equality of the list because of it s efficiency

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Simple solution  -      List lt String gt  list   new ArrayList lt String gt  Arrays asList  a    b    d    c         List lt String gt  list2   new ArrayList lt String gt  Arrays asList  b    f    c          list retainAll list2       list2 removeAll list       System out println  similiar     list       System out println  different     list2     Output  -  similiar  b  c  different  f

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Using java 8 removeIf  public int getSimilarItems        List lt String gt  one   Arrays asList  milan    dingo    elpha    hafil    meat    iga    neeta peeta        List lt String gt  two   new ArrayList lt  gt  Arrays asList  hafil    iga    binga    mike    dingo       Cannot remove directly from array backed collection     int initial   two size         two removeIf one  contains       return initial - two size

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I found a very basic example of List comparison at List Compare This example verifies the size first and then checks the availability of the particular element of one list in another

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If you are looking for a handy way to test the equality of two collections  you can use org apache commons collections CollectionUtils isEqualCollection  which compares two collections regardless of the ordering

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You can try intersection   and subtract   methods from CollectionUtils   intersection   method gives you a collection containing common elements and the subtract   method gives you all the uncommon ones   They should also take care of similar elements

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Are these really lists  ordered  with duplicates   or are they sets  unordered  no duplicates    Because if it s the latter  then you can use  say  a java util HashSet lt E gt  and do this in expected linear time using the convenient retainAll       List lt String gt  list1   Arrays asList           milan    milan    iga    dingo    milan             List lt String gt  list2   Arrays asList           hafil    milan    dingo    meat                 intersection as set     Set lt String gt  intersect   new HashSet lt String gt  list1       intersect retainAll list2       System out println intersect size        prints  2      System out println intersect      prints   milan  dingo           intersection union as list     List lt String gt  intersectList   new ArrayList lt String gt         intersectList addAll list1       intersectList addAll list2       intersectList retainAll intersect       System out println intersectList          prints   milan  milan  dingo  milan  milan  dingo           original lists are structurally unmodified     System out println list1      prints   milan  milan  iga  dingo  milan       System out println list2      prints   hafil  milan  dingo  meat

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public static boolean compareList List ls1  List ls2       return ls1 containsAll ls2   amp  amp  ls1 size      ls2 size     true  false          public static void main String   args         ArrayList lt String gt  one   new ArrayList lt String gt         one add  one        one add  two        one add  six         ArrayList lt String gt  two   new ArrayList lt String gt         two add  one        two add  six        two add  two         System out println  Output1        compareList one  two         two add  ten         System out println  Output2        compareList one  two

User · Answer

EDIT  Here are two versions  One using ArrayList and other using HashSet   Compare them and create your own version from this  until you get what you need    This should be enough to cover the      P S  It is not a school assignment    So if you just guide me it will be enough   part of your question   continuing with the original answer    You may use a java util Collection  and or  java util ArrayList for that    The retainAll method does the following      Retains only the elements in this collection that are contained in the specified collection   see this sample   import java util Collection  import java util ArrayList  import java util Arrays   public class Repeated       public static void main  String     args             Collection listOne   new ArrayList Arrays asList  milan   dingo    elpha    hafil    meat    iga    neeta peeta             Collection listTwo   new ArrayList Arrays asList  hafil    iga    binga    mike    dingo              listOne retainAll  listTwo            System out println  listOne              EDIT  For the second part   similar values   you may use the removeAll method      Removes all of this collection s elements that are also contained in the specified collection    This second version gives you also the similar values and handles repeated   by discarding them    This time the Collection could be a Set instead of a List   the difference is  the Set doesn t allow repeated values     import java util Collection  import java util HashSet  import java util Arrays   class Repeated         public static void main  String     args                Collection lt String gt  listOne   Arrays asList  milan   iga                                                        dingo   iga                                                        elpha   iga                                                        hafil   iga                                                        meat   iga                                                         neeta peeta   iga               Collection lt String gt  listTwo   Arrays asList  hafil                                                         iga                                                         binga                                                          mike                                                          dingo   dingo   dingo               Collection lt String gt  similar   new HashSet lt String gt   listOne              Collection lt String gt  different   new HashSet lt String gt               different addAll  listOne              different addAll  listTwo               similar retainAll  listTwo              different removeAll  similar               System out printf  One  s nTwo  s nSimilar  s nDifferent  s n   listOne  listTwo  similar  different               Output     java Repeated One  milan  iga  dingo  iga  elpha  iga  hafil  iga  meat  iga  neeta peeta  iga   Two  hafil  iga  binga  mike  dingo  dingo  dingo   Similar  dingo  iga  hafil   Different  mike  binga  milan  meat  elpha  neeta peeta    If it doesn t do exactly what you need  it gives you a good start so you can handle  from here    Question for the reader  How would you include all the repeated values

[java] Java Compare Two Lists

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