如何在数组列表中交换具有相同值的两个对象的 position

Question

In List<MyObject> list = new ArrayList<>() i want to swap position of two (always) objects that have the same value in field name.

public class SiteDTO {
   private Long id;
   private String name;

// getters setters constructors

}

i know that best way to do that is using Collections.swap(list, 1, 2); where 1 and 2 are positions of objects to swap.

But how to find these indexes?

Answer 1

You could iterate over the indices of the list and find those groups that are equals, then swap the positions of those that have exactly two elements:

List<SiteDTO> sites = Arrays.asList(new SiteDTO(1L, "1"), new SiteDTO(2L, "2"), new SiteDTO(3L, "1"));
Map<String, List<Integer>> groups = IntStream.range(0, sites.size()).boxed().collect(groupingBy(o -> sites.get(o).getName()));

for (List<Integer> positions : groups.values()) {

    if (positions.size() == 2)
        Collections.swap(sites, positions.get(0), positions.get(1));

}

System.out.println(sites);

Output

[SiteDTO{id=3, name='1'}, SiteDTO{id=2, name='2'}, SiteDTO{id=1, name='1'}]

Note: This works for the case you have multiple groups to swap.

Answer 2

By simply iterating that list?

int firstIndex = -1;
int secondIndex = -1;

for (int i=0; i < yourList.size(); i++) {
  if (yourList.get(i).getName().equals(whatever)) {
    firstIndex = i;
    break;
  }

And then you could for example just iterate in reverse order to identify the second index.

Of course, the above only works when there are exactly two objects that have the same property value in your list.