Common elements in two lists

问题:

I have two arrayLists with 3 integer. I want to find a way to return the common elements of the two lists. Has anynody idea, how can I achieve this?

回答1:

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 common = new ArrayList(listA); common.retainAll(listB); // common now contains only the elements which are contained in listA and listB. 

回答2:

You can use set intersection operations with your ArrayList objects.

Something like this:

List l1 = new ArrayList();  l1.add(1); l1.add(2); l1.add(3);  List l2= new ArrayList(); l2.add(4); l2.add(2); l2.add(3);  System.out.println("l1 == "+l1); System.out.println("l2 == "+l2);  List l3 = new ArrayList(l2); l3.retainAll(l1);      System.out.println("l3 == "+l3);     System.out.println("l2 == "+l2); 

Now, l3 should have only common elements between l1 and l2.

CONSOLE OUTPUT l1 == [1, 2, 3] l2 == [4, 2, 3] l3 == [2, 3] l2 == [4, 2, 3] 

回答3:

Why reinvent the wheel? Use Commons Collections:

CollectionUtils.intersection(java.util.Collection a, java.util.Collection b) 

回答4:

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 list1 = asList(1, 2, 3, 4, 5); List list2 = asList(1, 3, 5, 7, 9);  List common = list1.stream().filter(list2::contains).collect(toList()); 

回答5:

            List lista =new ArrayList();             List listb =new ArrayList();              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 listapluslistb =new ArrayList(lista);                 listapluslistb.addAll(listb);              // Create an aunionb set which will contain both list (list1 and list2) in which common element will occur once             Set listaunionlistb =new HashSet(lista);             listaunionlistb.addAll(listb);              for(String s:listaunionlistb)             {                 listapluslistb.remove(s);             }             System.out.println(listapluslistb); 

回答6:

In case you want to do it yourself..

List commons = new ArrayList();  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); } 

回答7:

    // Create two collections:     LinkedList listA =  new LinkedList();     ArrayList listB =  new ArrayList();      // 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 common = new ArrayList(listA);     // use common.retainAll      common.retainAll(listB);      System.out.println("The common collection is : " + common); 

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