How to compress a String in Java?

Compression algorithms almost always have some form of space overhead, which means that they are only effective when compressing data which is sufficiently large that the overhead is smaller than the amount of saved space.

Compressing a string which is only 20 characters long is not too easy, and it is not always possible. If you have repetition, Huffman Coding or simple run-length encoding might be able to compress, but probably not by very much.

When you create a String, you can think of it as a list of char's, this means that for each character in your String, you need to support all the possible values of char. From the sun docs

char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).

If you have a reduced set of characters you want to support you can write a simple compression algorithm, which is analogous to binary->decimal->hex radix converstion. You go from 65,536 (or however many characters your target system supports) to 26 (alphabetical) / 36 (alphanumeric) etc.

I've used this trick a few times, for example encoding timestamps as text (target 36 +, source 10) - just make sure you have plenty of unit tests!