Question

Is there a better approach to do the below in Java, without using external libraries.

I need to model group/child (tree like) structure of int (primitive). In Json

[{1,1}, {1,2}, {2,1},{3,1}]

I need to support addition/removal of elements (element is a pair {group, child} ) without duplication.

I am thinking of, keeping a data structure like.

ArrayList<HashMap<Integer,Integer>>

To add.

Iterate through ArrayList, check HashMap key and value against the value to insert, and insert if not exist.

To delete:

Iterate through ArrayList, check HashMap key and value against the value to delete, and delete if exist.

Is there a better data structure/approach with standard library.

---

As per one of the answer below, I made a class like this.
Please let me know anything to watchout. I am expecting (and going to try out) arraylist would handle add/remove correctly by using the equal method in KeyValue class. thanks.

 static class KeyValue {
        int groupPos;
        int childPos;

        KeyValue(int groupPos, int childPos) {
            this.groupPos = groupPos;
            this.childPos = childPos;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            KeyValue keyValue = (KeyValue) o;

            if (childPos != keyValue.childPos) return false;
            if (groupPos != keyValue.groupPos) return false;

            return true;
        }

        @Override
        public int hashCode() {
            int result = groupPos;
            result = 31 * result + childPos;
            return result;
        }
    }

Answer 1

If I understand what you’re trying to do, this may be simpler:

TreeMap<Integer,TreeSet<Integer>>
  or
HashMap<Integer,HashSet<Integer>>

So, rather than

[{1,1}, {1,2}, {2,1}, {3,1}]

you’d have

[{1, {1, 2}},
 {2, {1}},
 {3, {1}}]

Note that all 4 of the above classes automatically handles eliminating duplicates.

To add:

TreeMap<Integer, TreeSet<Integer>> map;
TreeSet<Integer> set = map.get(group);
if (set == null) // create it if it doesn't exist
{
  set = new TreeSet<Integer>();
  map.put(group, set);
}
set.add(child);

To remove:

TreeMap<Integer, TreeSet<Integer>> map;
TreeSet<Integer> set = map.get(group);
set.remove(child);
if (set.isEmpty()) // remove it if it is now empty
  map.remove(group);