How to check if a certain Index exists in an ArrayList in Java

Question

In Java, I'm trying to store a User object in an ArrayList and before storing / adding it - I'm asking the index number where this object should be stored in. When he enters the Index Number, I want to check if that Index is empty i.e no other User object is stored on that index - if it's empty, the new User will be added and if it's already occupied, I'll ask the user to enter a valid Index number again..

So the question is - how do I check if a specific Index in an ArrayList is empty?

I'm aware that there exists this method ArrayList.contains(Object e) but the issue here is, I want to see if a specific INDEX exists, not if an OBJECT exists.. So kindly let me know how I can achieve that, Thank You.

Amongalen · Accepted Answer · 2020-11-18 10:31:58Z

2

I think you misunderstood how Lists (or ArrayLists in this case) works. A list can't have empty slots in it. When a List have 1 element, it will always have index 0. If there are 3 elements they have indicies 0, 1 and 2. Then, if you remove element with index 1 from that list, it gets reorganized, and have elements at positions 0 and 1.

If you want to keep indecies of elements you could either use a simple array of Objects, User[], or a map of indecies and objects assigned to them, Map<Integer, User>.

answered Nov 18, 2020 at 10:31

Amongalen

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lukaszwrzaszcz · Accepted Answer · 2020-11-18 10:20:08Z

0

You can check it by:

boolean exists = index < list.size()

method, if the index >= size, then it doesn't exits.

answered Nov 18, 2020 at 10:20

lukaszwrzaszcz

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3 Comments

Amongalen Over a year ago

Keep in mind that you can't add element at index > list.size(), not with add(int index, E element)

VenoM Over a year ago

I've a question - if the previous user is added at 5th index, won't the size be 1? So the 2nd user will still be able to add another user at 5th index - or am I misunderstanding something here?

Kevin Anderson Over a year ago

You can't add an element at an index that is not <= the size of the list. Thus, there's no way to add an element at index 5 if the list is empty; the only place you can add in an empty list is at index 0.

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