Best data structure for this?

I want to keep a record of a load of Strings, and everytime a new String is added i want to know if it has previously been added.

Obviously a silly method would be to just store each String in an arraylist and then search through it each time but this will be too slow once the list starts getting big.

Could there be a better way of doing this?

[364 byte] By [Unconditionala] at [2007-11-27 2:02:23]