Question

I need to store certain id’s , and check if one exists there.
Either i can use concatenated string or array/List, which of them is a better and faster way.
This is how actually data is organized :

• Year 1
  • Month 1
    • Day 1
    • Day 2
    • Day 3
  • Month 2
    • Day 6
    • Day 2
    • Day 3
• Year 2
  • Month 3
    • Day 1
    • Day 3
    • Day 7
  • Month 6
    • Day 6
    • Day 2
    • Day 3

Answer 1

I would definitely use a collection of some form. If you only care about containment, you should use a Set<String> of some kind (e.g. HashSet<String> or LinkedHashSet<String>, which will both give O(1) complexity unless you have a significant number of hash collisions) but for goodness’ sake don’t use a concatenated string.

Your data isn’t naturally a concatenated string – it’s a collection of strings. Always keep your data in the most natural representation unless you have really good evidence that some alternative form (such as a single string) will bring you a meaningful benefit. Keeping your data in a natural representation almost always leads to clearer code which is easier to work with – and easier to optimize later, when you’ve found where the real bottlenecks are.