Recalling a Big-O notation in (CS) Computer Science major course I have taken in College:

O(n)	O(1)	O(log n)	O(n log n)	O(n^2)	O(n^3)
1	1	1	1	1	1
2	1	1	2	4	8
4	1	2	8	16	64
8	1	3	24	64	512
16	1	4	64	265	4096
1024	1	10	10240	1048576	1073741824

In Biz apps, you need to use List object all the time. And often has performance issue. It’s better to use some other data structure, such as Map object because HashSet is much faster compared to ArrayList:

	get	contains	add	remove
ArrayList	O(1)	O(n)	O(1)	O(n)
HashSet	N/A	O(1)	O(1)	O(1)

Note: HashSet does not provide “get” method, but as long as “contains” method is available, we know what element we need

Here is how to work on:
First, converting ArrayList to HashSet

private Set<String> convertToHashSet(List<Map<String, Object>> listToDelete) {
	Set<String> set = new HashSet<String>();
	for (Map<String, Object> obj : listToDelete) {
		set.add(new String("test"));
	}
	return set;
}

Or just simply set a list as constructor’s parameter:

private Set<String> convertToHashSetDirectory(List<String> list) {
	return new HashSet<String>(list);
}

“contains” with for loop version:

private void hashSetDemoFor(Set<String> setOfMdseOnScreen, Set<String> setOfMdseAcctTo) {
	Map<String, Long> resultMap = new HashMap<String, Long>();
	for (String mdseAcctTo : setOfMdseAcctTo) {
		if (setOfMdseOnScreen.contains(mdseAcctTo)) {
			// Do something
			continue;
		}
	}
}

“contains” with while loop version:

private void hashSetDemoWhile(Set<String> setOfMdseOnScreen, Set<String> setOfMdseAcctTo) {
	Iterator<String> itr = setOfMdseOnScreen.iterator();
	while (itr.hasNext()) {
		String mdseOnScreen = itr.next();
		if (setOfMdseAcctTo.contains(mdseOnScreen)) {
			// Do something
			continue;
		}
	}
}