Cannot add element (set) into an arraylist

I want to add a set that satisfies some conditions into an arraylist, however it cannot be added.

public void insert(List<Set<Integer>> tran){
    Node left = new Node();
    Node right = new Node();
    List<Set<Integer>> insert_list = new ArrayList<Set<Integer>>();
    for(int i = 0; i < insert_list.size(); i++){
        Set<Integer> temp = insert_list.get(i);
        System.out.println(temp);
        Iterator itr = temp.iterator();
        int value = (int)itr.next();
        System.out.println(value);
        if(value%2 ==0){
            left.leaf.add(temp);
        }
        else{
            right.leaf.add(temp);
        }
    }
    System.out.println(left.leaf);
    System.out.println(right.leaf);

In the main function, I called as:

Node test = new Node();
test.insert(candidate2);

This is a function under Node class, which its constructor looks like:

private List<Set<Integer>> leaf = new ArrayList<Set<Integer>>();
private Node left; // the node that can point to the left
private Node right; // the node that can point to the right
public Node(){
    this.leaf = new ArrayList<Set<Integer>>();
    left = null;
    right = null;
 }

Also, System.out.println(value); and System.out.println(temp); cannot be printed as well, which is really strange.

Moreover, if I change the iterator codes to if((int)temp.toArray()[0]%2 == 0), it still does not work.

I have a List<Set<Integer>> insert_list with following attribute: [[1, 15], [1, 58], [1, 274], [1, 326], [58, 15], [274, 15], [326, 15], [274, 58], [326, 58], [274, 326]]. The desired output should be [[274, 15], [326, 15], [274, 58],[326, 58], [274, 326]] for the left.leaf and [1, 15], [1, 58], [1, 274], [1, 326]] for the right.leaf.

By the way, I tested these codes in the main function and it worked, I have totally no idea why it didn't work in this way.

Thanks in advance!

Edit:

The output of my function is:

[]
[]