Question

Really basic OO comprehension issue I am running into, any help is greatly appreciated.

I’m trying to add instances of “Thing” to an arraylist every-time I press a button, I can’t wrap my head around how to create unique instances to add to the list. A different button press should remove the most recent object from the list.

ArrayList myList = new ArrayList<Thing>();
if(input.isKeyPressed(Input.KEY_A)){
            Thing myThing = new Thing();
            myThing.setNumber(myList.size());
            myList.add(myThing);
        }

if(input.isKeyPressed(Input.KEY_R)){
            if(myList.size()>0){
                myList.remove(myList.size()-1);
            }
        }

If I plan on making lots of “things” and I don’t care about what they are called (nor do I want to keep track of unique thing-object names). How can I create a unique ‘thing’ object on each button press with minimal pain.

UPDATE:
Thanks for the comments, please let me try to articulate my question better…
When I create an ArrayList full of ‘Thing’, each instance of which is called “myThing”, every instance has the same instance variables values.

If I wanted some of the ‘Thing”s to have boolean isVisable = true, and other’s to have boolean isVisable = false. I get stuck because each element of the list has the same name.

Answer 1

Make sure that Thing implements equals and hashCode correctly and then store the instances in a Set collection (i.e. HashSet). With the implementation of hashCode() and equals() it will be completely up to you which two instances of Thing are the same and hence you will be able to enforce uniqueness any way you need.

Now the trick here is that implementing hashCode() and equals() is not entirely trivial, but you need to know how to do it if you plan to use Java. So read the appropriate chapter of Effective JAva (or better yet read the entire book).