使用 addAll 在哈希集中添加列表

Question

In java im not able to add a list to a hashset using hash set addAll method

List a = new ArrayList();
a.add(20);

List b = new ArrayList();
b.add(30);

Set set = new HashSet ( a );

set.addAll( b);

Please help

Thanks

Answer 1

I tried your code and it works for me.

One thing though - it would be better to use the generic versions of the collections. This removes the warnings.

List<Integer> a = new ArrayList<Integer>();
a.add(20);

List<Integer> b = new ArrayList<Integer>();
b.add(30);

Set<Integer> set = new HashSet<Integer>(a);
set.addAll(b);

Answer 2

This works fine, just that if you add a list to the set, the repeated elements between the list and the set are added just once.

Say for example ArrayList arr has elements 2,3,4 and HashSet set has elements 2,5,7 now if you do set.addAll(arr), then set still includes 2,5,7,3,4.

Also Imagine a scenario where you have an ArrayList arr and HashSet set where T is a generic class containing several parameters, then common elements in the final set will be removed as per equals method's overridden definition in T class and the element added to set will be persisted in the final set over the element in the arraylist.

Answer 3

你可以简单地这样做：

Set<String> set = new HashSet<String>(list);

Answer 4

    ArrayList<Integer> arr = new ArrayList<>();
    arr.add(20);
    arr.add(30);
    arr.add(40);
    System.out.println(arr); //[20, 30, 40]

    ArrayList<Integer> arr2 = new ArrayList<>();
    arr2.add(10);
    arr2.add(70);
    arr2.add(40);
    
    System.out.println(arr2); //[10, 70, 40]
    arr2.addAll(arr);

    System.out.println(arr2); //[10, 70, 40, 20, 30, 40]

    HashSet<Integer> set = new HashSet<>(arr);
    System.out.println(set); //[20, 40, 30]
    set.addAll(arr2);
    ArrayList<Integer> dummy = new ArrayList<>(set);

    System.out.println(dummy); //[20, 70, 40, 10, 30]