Common elements in two lists

Question

I have two ArrayList objects with three integers each  I want to find a way to return the common elements of the two lists  Has anybody an idea how I can achieve this

User · Answer

Some of the answers above are similar but not the same so posting it as a new answer.

Solution:
1. Use HashSet to hold elements which need to be removed
2. Add all elements of list1 to HashSet
3. iterate list2 and remove elements from a HashSet which are present in list2 ==> which are present in both list1 and list2
4. Now iterate over HashSet and remove elements from list1(since we have added all elements of list1 to set), finally, list1 has all common elements
Note: We can add all elements of list2 and in a 3rd iteration, we should remove elements from list2.

Time complexity: O(n)
Space Complexity: O(n)

Code:

import com.sun.tools.javac.util.Assert;
import org.apache.commons.collections4.CollectionUtils;

    List<Integer> list1 = new ArrayList<>();
    list1.add(1);
    list1.add(2);
    list1.add(3);
    list1.add(4);
    list1.add(5);

    List<Integer> list2 = new ArrayList<>();
    list2.add(1);
    list2.add(3);
    list2.add(5);
    list2.add(7);
    Set<Integer> toBeRemoveFromList1 = new HashSet<>(list1);
    System.out.println("list1:" + list1);
    System.out.println("list2:" + list2);
    for (Integer n : list2) {
        if (toBeRemoveFromList1.contains(n)) {
            toBeRemoveFromList1.remove(n);
        }
    }
    System.out.println("toBeRemoveFromList1:" + toBeRemoveFromList1);
    for (Integer n : toBeRemoveFromList1) {
        list1.remove(n);
    }
    System.out.println("list1:" + list1);
    System.out.println("collectionUtils:" + CollectionUtils.intersection(list1, list2));
    Assert.check(CollectionUtils.intersection(list1, list2).containsAll(list1));

output:

list1:[1, 2, 3, 4, 5]
list2:[1, 3, 5, 7]
toBeRemoveFromList1:[2, 4]
list1:[1, 3, 5]
collectionUtils:[1, 3, 5]

User · Answer

List lt Integer gt  listA   new ArrayList lt  gt         listA add 1       listA add 5       listA add 3       listA add 4       List lt Integer gt  listB   new ArrayList lt  gt         listB add 1       listB add 5       listB add 6       listB add 7   System out println listA stream   filter listB  contains  collect Collectors toList        Java 1 8 Stream API Solutions   Output  1  5

User · Answer

In case you want to do it yourself    List lt Integer gt  commons   new ArrayList lt Integer gt      for  Integer igr   group1        if  group2 contains igr             commons add igr            System out println  Common elements are    -    for  Integer igr   commons        System out println     igr

User · Answer

public static  lt T gt  List lt T gt  getCommonElements              java util Collection lt T gt  a              java util Collection lt T gt  b                         if a  null  amp  amp  b  null  return new ArrayList lt  gt             if a  null  amp  amp  a size    0  return new ArrayList lt  gt  b                      if b  null  amp  amp  b size    0  return new ArrayList lt  gt  a                    Set lt T gt  set  a instanceof HashSet  HashSet lt T gt  a new HashSet lt  gt  a           return b stream   filter set  contains  collect Collectors toList            For better time performance  please use HashSet  O 1  look up  instead of List O n  look ups  Time complexity- O b  Space Complexity- O a

User · Answer

Use Collection retainAll     listA retainAll listB      listA now contains only the elements which are also contained in listB    If you want to avoid that changes are being affected in listA  then you need to create a new one   List lt Integer gt  common   new ArrayList lt Integer gt  listA   common retainAll listB      common now contains only the elements which are contained in listA and listB

User · Answer

consider two list L1 ans L2  Using Java8 we can easily find it out  L1 stream   filter L2  contains  collect Collectors toList

User · Answer

Below code Remove common elements in the list List lt String gt  result    list1 stream   filter item- gt   list2 contains item   collect Collectors toList      Retrieve common elements List lt String gt  result   list1 stream                    distinct                    filter list  contains                   collect Collectors toList

User · Answer

public  lt T gt  List lt T gt  getIntersectOfCollections Collection lt T gt  first  Collection lt T gt  second            return first stream                    filter second  contains                   collect Collectors toList

User · Answer

Create two collections      LinkedList lt String gt  listA    new LinkedList lt String gt         ArrayList lt String gt  listB    new ArrayList lt String gt             Add some elements to listA      listA add  A        listA add  B        listA add  C        listA add  D            Add some elements to listB      listB add  A        listB add  B        listB add  C            use       List lt String gt  common   new ArrayList lt String gt  listA          use common retainAll      common retainAll listB        System out println  The common collection is       common

User · Answer

You can get the common elements between two lists using the method   retainAll   This method will remove all unmatched elements from the list to which it applies   Ex   list retainAll list1     In this case from the list  all the elements which are not in list1 will be  removed and only those will be remaining which are common between list and  list1   List lt Integer gt  list   new ArrayList lt  gt     list add 10   list add 13   list add 12   list add 11    List lt Integer gt  list1   new ArrayList lt  gt     list1 add 10   list1 add 113   list1 add 112   list1 add 111     before retainAll System out println list   System out println list1     applying retainAll on list list retainAll list1     After retainAll System out println  list    list   System out println  list1    list1     Output     10  13  12  11   10  113  112  111  list   10  list1   10  113  112  111    NOTE  After retainAll applied on the list  the list contains common element between  list and list1

User · Answer

You can use set intersection operations with your ArrayList objects   Something like this   List lt Integer gt  l1   new ArrayList lt Integer gt      l1 add 1   l1 add 2   l1 add 3    List lt Integer gt  l2  new ArrayList lt Integer gt     l2 add 4   l2 add 2   l2 add 3    System out println  l1      l1   System out println  l2      l2    List lt Integer gt  l3   new ArrayList lt Integer gt  l2   l3 retainAll l1        System out println  l3      l3     Now  l3 should have only common elements between l1 and l2   CONSOLE OUTPUT l1     1  2  3  l2     4  2  3  l3     2  3

User · Answer

Using Java 8 s Stream filter   method in combination with List contains     import static java util Arrays asList  import static java util stream Collectors toList              List lt Integer gt  list1   asList 1  2  3  4  5   List lt Integer gt  list2   asList 1  3  5  7  9    List lt Integer gt  common   list1 stream   filter list2  contains  collect toList

User · Answer

Why reinvent the wheel  Use Commons Collections   CollectionUtils intersection java util Collection a  java util Collection b

User · Answer

List lt String gt  lista  new ArrayList lt String gt                 List lt String gt  listb  new ArrayList lt String gt                  lista add  Isabella                lista add  Angelina                lista add  Pille                lista add  Hazem                 listb add  Isabella                listb add  Angelina                listb add  Bianca                    Create an aplusb list which will contain both list  list1 and list2  in which common element will occur twice              List lt String gt  listapluslistb  new ArrayList lt String gt  lista                   listapluslistb addAll listb                   Create an aunionb set which will contain both list  list1 and list2  in which common element will occur once             Set lt String gt  listaunionlistb  new HashSet lt String gt  lista               listaunionlistb addAll listb                for String s listaunionlistb                                listapluslistb remove s                             System out println listapluslistb

[java] Common elements in two lists

Examples related to java

Examples related to arraylist

Examples related to element