Combine 2 lists according to compared object's variable

So I have got 2 object lists A and B. I will call merged list c. That is from the class Flights.

   public class Flights{
   String airline;
   
   }

Where
The list looks like this

  list<Flights>a
    list<Flights>b 
    list<Flights>c 

The combined list should always have alternating element from A and B.
e.g. c = a(1), b(1), a(3), b(4). C also should have the following properties

1. The position of elements in C should be such that elements in A are compare to B where elements that have the same airline is placed beside one another. E.g. merged list = A.united, B.united, A.emirates, B.emirates and so on

By matching I mean placing the elements after the other

2. After completing step 1, it should then find the elements in A and B that were previously not added to C. Match all those elements to all elements from the other list. E.g. if a(5) wasn't added to the list previously, it will be matched with b(1-5)

3. Then, go through each element in A and make sure there is a match with all elements in B.

Example
Say A has 1,2,3 and B has 2,3,10, then the output as [2,2],[3,3],[1,2],[1,3],[1,10],[2,3],[2,10],[3,10]

The brackets mean each match.
My current code

List<Flights> secondaryFlights  // filled a
List<Flights> primaryFlights    // filled b
List<Flights> flightsList       // Empty  c
    for ( Flights p : primaryFlights) {
    			for ( Flights s : secondaryFlights) {
    				if ( p.getAirline().equals(s.getAirline())) {
    					flightsList.add(p);
    					flightsList.add(s);
    				}
    			}
    		}
    		
    		while( !flightsList.containsAll(primaryFlights) || !flightsList.containsAll(secondaryFlights)) {
    			
    		}

Get more understanding from

String a[] = { "A", "E", "I" };
  String b[] = { "O", "U" };
  List list = new ArrayList(Arrays.asList(a));
  list.addAll(Arrays.asList(b));
  Object[] c = list.toArray();
  System.out.println(Arrays.toString(c));

Fixed with something like this

List<String>list1 = new ArrayList<String>();
	list1.add("tommy");
	list1.add("Alex");
	list1.add("Bryan");
	list1.add("Tom");
	list1.add("William");
	list1.add("Willy");

	List<String>list2 = new ArrayList<String>();
	list2.add("Alex");
	list2.add("William");
	list2.add("Candice");
	list2.add("Jane");
	list2.add("Parker");
	list2.add("tommy");
	
	List<String>merged = new ArrayList<String>();
	
	for ( String str1 : list1) {
		for ( String str2 : list2) {
			if ( str1.equals(str2)) {
				merged.add("[ " + str1);
				merged.add(str2 + "]");
			}
		}
	}
	for ( String str1 : list1) {
		for ( String str2 : list2) {
			if ( !str1.equals(str2)) {
				merged.add("[ " + str1);
				merged.add(str2 + "]");

		}
	}
		}
	
	System.out.println(merged);

Looking at your examples, you need matched once first and all unmatched ones in their order, so you can do like below:

• Create a set of second list and loop over the first list. If element in first list exists in the set, add them.

• For unmatched ones, create a new set to avoid duplicating results. If the pair is un-visited, you can add it to the result. You can create the pair in a way it is convenient and doesn't overlap under any scenario. Taking your 1,2,3 and 2,3,10 example, I have separated integers by a pipe symbol(|). Java does seem to have Pair class too. You can use it if you like.

Snippet:

private static List<Integer> getData(List<Integer> l1,List<Integer> l2){
    List<Integer> res = new ArrayList<>();
    Set<Integer> set = new HashSet<>(l2);
    for(int i=0;i<l1.size();++i){
        int x = l1.get(i);
        if(set.contains(x)){
            res.add(x);
            res.add(x);
        }
    }
    Set<String> pairs = new HashSet<>();
    for(int i=0;i<l1.size();++i){
        int x = l1.get(i);
        for(int j=0;j<l2.size();++j){
            int y = l2.get(j);
            int min = Math.min(x,y);
            int max = Math.max(x,y);
            if(x != y && !pairs.contains(min + "|" + max)){
                res.add(x);
                res.add(y);
                pairs.add(min + "|" + max);
            }       
        }
    }        
    
    return res;
}

Driver code:

public static void main(String[] args) {
        List<Integer> l1 = Arrays.asList(1,2,3);
        List<Integer> l2 = Arrays.asList(2,3,10);
        System.out.println(getData(l1,l2).toString());
    }

Note: You can replicate the same logic for your Flight class dataset. To compare strings, you can use compareTo method.