兩個列表中的通用元素

Question

我有兩個3整數的arrayLists。我想找到一種方法來返回兩個列表的共同元素。有任何想法，我怎麼能做到這一點？

Answer 1

您可以使用與您的ArrayList對象設置的交集操作。

事情是這樣的：現在

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); 

，l3應該有l1和l2之間唯一的共同元素。

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

Answer 2

使用Collection#retainAll()。

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

如果你想避免這種變化正在影響在listA，那麼你需要創建一個新的。

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

Answer 3

爲什麼要重新發明輪子？使用Commons Collections：

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

Answer 4

如果你想自己做吧..

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); 
} 

Answer 5

使用Java 8的Stream.filter()方法結合List.contains()：

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()); 

Answer 6

// 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); 

Answer 7

  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);