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What are the difference between ArrayList and HashSet? 

Difference between ArrayList and HashSet:

1: Duplicates

ArrayList allows duplicate elements.

HashSet does NOT allow duplicate elements.

2: Order

ArrayList maintains the order in which you add elements (insertion order).

HashSet does NOT guarantee any order.

3: Performance

ArrayList is better if you need to access elements by their index (like getting the 3rd item).

HashSet is faster for checking if an element exists.

In short:

- Use ArrayList when you want an ordered list that can have duplicates.

- Use HashSet when you want a collection with unique items and don’t care about the order.