Performance and Memory allocation comparison between List and Set

If you don't care about the ordering, and don't delete elements, then it really boils down to whether you need to find elements in this data structure, and how fast you need those lookups to be.

Finding an element by value in a HashSet is O(1). In an ArrayList, it's O(n).

If you are only using the container to store a bunch of unique objects, and iterate over them at the end (in any order), then arguably ArrayList is a better choice since it's simpler and more economical.

If you will compare, searching between List and Set, Set will be better because of the underline Hashing algorithm.

In the case of a list, in worst case scenario, contains will search till the end. In case of Set, because of hashing and bucket, it will search only subset.

Sample use case: Add 1 to 100_000 integer to ArrayList and HashSet. Search each integer in ArrayList and HashSet.

Set will take 9 milliseconds where as List will take 16232 seconds.

private static void compareSetvsList(){
    List<Integer> list = new ArrayList<>() ;
    Set<Integer> set = new HashSet<>() ;

    System.out.println("Setting values in list and set .... ");
    int counter = 100_000  ;

    for(int i =0 ; i< counter ; i++){            
        list.add(i);
        set.add(i);
    }

    System.out.println("Checking time .... ");
    long l1 = System.currentTimeMillis();
    for(int i =0 ; i< counter ; i++) list.contains(i);

    long l2 = System.currentTimeMillis();
    System.out.println(" time taken for list : "+ (l2-l1));

    for(int i =0 ; i< counter ; i++)set.contains(i);

    long l3 = System.currentTimeMillis();
    System.out.println(" time taken for set : "+ (l3-l2));

    //      for 10000   time taken for list : 123        time taken for set : 4
    //      for 100000  time taken for list : 16232          time taken for set : 9
    //      for 1000000 time taken for list : hung       time taken for set : 26

}

If you plan only to add elements and later iterate over them, your best bet is ArrayList as it's closest to the arrays you are replacing. It's more memory efficient than LinkedList or any Set implementation, has fast insertion, iteration, and random access.

HashSet consumes about 5.5 times more memory than ArrayList for the same number of elements (although they're both still linear), and has significantly slower iteration (albeit with the same asymptotics); a quick Google search suggests a 2-3x slowdown for HashSet iteration versus ArrayList.

If you don't care about uniqueness or the performance of contains, then use ArrayList.