What is the fastest way to know if a letter exists in a Character collection

Question

1. Should I define a hashmap with something like "A" , new Character("A")
2. should I use something like a sorted list ? or cast it into an int?
3. simple list? (that doesnt seem too efficient )

I need a fast retrieving - when I ask if the Char exists. adding time is almost none important.

Answer 1

As we're talking about a Collection : It depends on the collection type . A HashSet offers the best performance for contains operations, which is O(1).

Set<Character> chars = new HashSet<Character>();
chars.add(new Character('A'));    
chars.add(new Character('B'));
chars.add(new Character('C'));

if (chars.contains('A'))
  System.out.println("Lightning fast answer: TRUE");

Answer 2

A char is an unsigned 16-bit value and can take on only 65,536 values. Since you talk about existence, I am kind of assuming you have a Set and not a general Collection . Use a BitSet to represent this set; add and check the existence of a char as if it is an integer value (because it is). This is the fastest I can imagine, will be faster for sure than a regular HashSet of Character`, and it takes just 8K of memory.

Answer 3

If you don't process the whole range of unicode characters, a simple (bit/bool) array should be reasonably fast. Compared to a hash you won't have to worry about collisions, too.

Answer 4

What is the size of your alphabet? If it is small and all characters are ASCII characters you can use array.

boolean[] chars;
chars['A'] = true;