Fast find exist in list or not
This strictly depends on the use case. If your list:


Doesn't change
Holds a lot of data
Is being searched many times
And you don't care about where the element is


Then it may be worth keeping a cached sorted copy of the list and using Arrays.binarySearch() on it.

If those conditions are not met, then you're better off just doing a linear search. Of course you can't go lower than O(n) time, because you have to check every element if the list is not sorted.