Which element has been added to the ArrayList as last?

Question

For example: I have an ArrayList composed of Points. Every point have an array of ints which defines his position. I can have few points in this same position and add them to my ArrayList at every index I want.

The question is:

I have given position and I want to check if any point of my ArrayList have this position. If yes, i return it. If there are few points with this position i need to return the last added (by time). Method equals won't work. I need to compare it with ==, so i need the correct the reference to the newest point.

How i should do it?

I should build another list of unique points, if in this list is another point with the same position I should replace it with newer?

public class Point {
    private final int DIMENSIONS;
    private final int[] position;

    public Point(int dimensions) {
        DIMENSIONS = dimensions;
        position = new int[DIMENSIONS];
    }

    public void setPosition(int dim, int value) {
        position[dim] = value;
    }

    public int getPosition(int dim) {
        return position[dim];
    }
}

Equals won't work because if i have p1 = new Point(0,0,0) and p2 = new Point(0,0,0) equals will tell that's the same object. And I need to have distinguish every instance. — Szymon Żak, Dec 06 '18 at 19:57

GhostCat · Accepted Answer · 2018-12-07T09:25:27.557

You should use another or at least, an additional data structure.

ArrayLists are just that a List implementation that gives a you a list that grows dynamically. It doesn't care about insertion order.

If you need that, you should rather look to a LinkedHashMap. Of course, that alone won't work: you want that two Point objects that have the same position contents are in fact equal. So you could only have one such point in your Map.

That is what can be said given the current information. For more specific guidance, you should rather show us the relevant parts of the Point class implementation. For example: which elements do really go into your equals() implementation? That aspect alone drives how you should organize your data.

Well, one remark: if you add multiple "equal" Points to your list, and you just keep appending them to the end of your list, then you could simply search from the rear end. The first matching Point is also then one that was added last.

Edit, given the Point class implementation. Basically, that class is pointless. Either it is missing other attributes or it is missing reasonable equals/hashCode methods. Meaning: if a Point is really just that: a list of positions ... or it should be more than that.

But when a Point only contains positions, then that means: two points with equal positions should be equal. In other words: a Point class like this should definitely have value semantics. A Point(0, 1) should be equal to any other Point(0, 1)!

I've to ask, what is importance of _additional data structure_? `equals` method can and should be applied to `Point` objects in the appropriate way. E.g. `point a.equals(b)` if **and** only if the coordinates are the same **and** the name of the points are the same... — zlakad, Dec 06 '18 at 19:43
Probably. But then points become more of values. Or should be. And then it is very dubious that equal points should be "different" because of their creation time... — GhostCat, Dec 06 '18 at 20:20
I added an class Point to the main post. I need to implement class GeometricShape which builds an structure from given 'Point'. Geometric Shape has a ArrayList ListOfPoints. It can add and remove points in any index of ListOfPoints. I have also method "getByPosition" which checks if point with given position is in the List and if yes, it returns the last added (by the time) Point with this positions. — Szymon Żak, Dec 06 '18 at 20:41

Which element has been added to the ArrayList as last?

1 Answers1