How can I return the difference between two lists

Question

I have two array lists e g    List lt Date gt  a  contains   10 10 2014  10 11 2016  List lt Date gt  b  contains   10 10 2016   How can i do a check between list a and b so the value that is missing in b is returned e g  10 10 2014

User · Answer

I was looking similar but I wanted the difference in either list (uncommon elements between the 2 lists).

Let say I have:

List<String> oldKeys = Arrays.asList("key0","key1","key2","key5");
List<String> newKeys = Arrays.asList("key0","key2","key5", "key6");

And I wanted to know which key has been added and which key is removed i.e I wanted to get (key1, key6)

Using org.apache.commons.collections.CollectionUtils

List<String> list = new ArrayList<>(CollectionUtils.disjunction(newKeys, oldKeys));

Result

["key1", "key6"]

User · Answer

First convert list to sets      create an empty set  Set lt T gt  set   new HashSet lt  gt          Add each element of list into the set  for  T t   list       set add t      You can use Sets difference Set1  Set2   which returns extra items present in Set1  You can use Sets difference Set2  Set1   which returns extra items present in Set2

User · Answer

With Stream API you can do something like this  List lt String gt  aWithoutB   a stream        filter element - gt   b contains element        collect Collectors toList      List lt String gt  bWithoutA   b stream        filter element - gt   a contains element        collect Collectors toList

User · Answer

You can use filter in the Java 8 Stream library List lt String gt  aList   List of  quot l quot   quot e quot   quot t quot   quot   quot   quot s quot    List lt String gt  bList   List of  quot g quot   quot o quot   quot e quot   quot s quot   quot t quot     List lt String gt  difference   aList stream        filter aObject - gt            return   bList contains aObject                 collect Collectors toList        more reduced  no curly braces  no return List lt String gt  difference2   aList stream        filter aObject - gt    bList contains aObject        collect Collectors toList      Result of System out println difference      e  t  s

User · Answer

You may call U difference lists  method in underscore-java library  I am the maintainer of the project  Live example  import com github underscore U  import java util Arrays  import java util List   public class Main       public static void main String   args            List lt Integer gt  list1   Arrays asList 1  2  3           List lt Integer gt  list2   Arrays asList 1  2           List lt Integer gt  list3   U difference list1  list2           System out println list3               3

User · Answer

List lt String gt  l1   new ArrayList lt String gt     l1 add  apple    l1 add  orange    l1 add  banana    l1 add  strawberry     List lt String gt  l2   new ArrayList lt String gt     l2 add  apple    l2 add  orange     System out println l1   System out println l2    for  String A  l2      if  l1 contains A       l1 remove A      System out println  output    System out println l1     Output    apple  orange  banana  strawberry   apple  orange  output  banana  strawberry

User · Answer

You can use CollectionUtils from Apache Commons Collections 4 0   new ArrayList lt  gt  CollectionUtils subtract a  b

User · Answer

I was looking for a different problem and came across this  so I will add my solution to a related problem  comparing two Maps          make a copy of the data     Map lt String String gt  a   new HashMap lt String String gt  actual       Map lt String String gt  e   new HashMap lt String String gt  expected          check  every  expected value     for Map Entry lt String  String gt  val   e entrySet                check for presence         if  a containsKey val getKey                  System out println String format  Did not find expected value   s   val getKey                           check for equality         else              if 0    a get val getKey    compareTo val getValue                      System out println String format  Value does not match expected   s   val getValue                                   we have found the item  so remove it                 from future consideration  While it                 doesn t affect Java Maps  other types of sets                may contain duplicates  this will flag those                duplicates               a remove val getKey                            check to see that we did not receive extra values     for Map Entry lt String String gt  val   a entrySet             System out println String format  Found unexpected value   s   val getKey              It works on the same principle as the other solutions but also compares not only that values are present  but that they contain the same value  Mostly I ve used this in accounting software when comparing data from two sources  Employee and Manager entered values match  Customer and Corporate transactions match      etc

User · Answer

Here is a generic solution for this problem   public  lt T gt  List lt T gt  difference List lt T gt  first  List lt T gt  second        List lt T gt  toReturn   new ArrayList lt  gt  first       toReturn removeAll second       return toReturn

User · Answer

You can convert them to Set collections  and perform a set difference operation on them   Like this   Set lt Date gt  ad   new HashSet lt Date gt  a   Set lt Date gt  bd   new HashSet lt Date gt  b   ad removeAll bd

User · Answer

If you only want find missing values in b  you can do   List toReturn   new ArrayList a   toReturn removeAll b    return toReturn    If you want to find out values which are present in either list you can execute upper code twice  With changed lists

[java] How can I return the difference between two lists?

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