Replacing a HashSet Java member

Question

I have Set of that structure. I do not have duplicates but when I call: set.add(element) -> and there is already exact element I would like the old to be replac

Answer 1

Try something as follows (this will only make sense if the equals and hashCode depends on one field, but the other fields could have different values):

if(!set.add(obj)) {
    //set already contains the element (not the same object though) 
    set.remove(obj); //remove the one in  the set
    set.add(obj); //add the new one
}

Check out the documentation for the Set.add method

If this set already contains the element, the call leaves the set unchanged and returns false.