两个列表中的通用元素

我有两个3整数的arrayLists。 我想find一种方法来返回两个列表的共同元素。 有任何想法,我怎么能做到这一点?

使用Collection#retainAll()

 listA.retainAll(listB); // listA now contains only the elements which are also contained in listB. 

如果您想避免listA这些更改受到影响,则需要创build一个新的更改。

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

你可以使用你的ArrayList对象的交集操作。

像这样的东西:

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

现在, l3l2之间只有共同的元素。

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

为什么重新发明轮子? 使用共享资源集合 :

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

将Java 8的Stream.filter()方法与List.contains()结合使用:

 import static java.util.Arrays.asList; import static java.util.stream.Collectors.toList; /* ... */ List<Integer> list1 = asList(1, 2, 3, 4, 5); List<Integer> list2 = asList(1, 3, 5, 7, 9); List<Integer> common = list1.stream().filter(list2::contains).collect(toList()); 

在这里输入图像描述

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

如果你想自己做

 List<Integer> commons = new ArrayList<Integer>(); 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); } 
  // Create two collections: LinkedList<String> listA = new LinkedList<String>(); ArrayList<String> listB = new ArrayList<String>(); // 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<String> common = new ArrayList<String>(listA); // use common.retainAll common.retainAll(listB); System.out.println("The common collection is : " + common);