How to do Boundary Value Checking (Unit Testing with JUnit5) for a ArrayList?

I need to do Worst Boundary Value Checking Unit testing for the following method. I am really at a loss as I am just learning Testing and new with JUnit5.

What should I consider as Boundary ?

public int add(List<Integer>a) 
{
        
        Integer temp=0;
        for (Integer num : a) 
        {
            temp+=num;
        }
        
        return temp;
        
}