Even if Java allows to use characters from the entire Unicode set it seems like Eclipse only allows a few.
Since I couldn’t find a list of supported emojis I tried it out myself and found the following as working:
☺❤☝✌☀☁❄✈☎✉✂✒✏㊙㊗❇✳✴♈♉♊♋♌♍♎♏♐♑♒♓▶◀➡↗↘↙↖↕↔↪↩⤴⤵〰✖✔™©®〽‼⁉Ⓜ♨
♠♣♥♦☑▪▫☹♤♡♢♧☢☣☠☤†☯☮☭☥✠✙✞✟✧⋆★☆✪✫✬✭✮✯✰✡☫☬☸✵❂❀❃❁✼♫♪☃❅❆☂❦♕♛♔♖♜☾
→⇒⇨➩➪➫➬➭➮➯➲➳➵➸➻➺➼➽☜☟➹➷↶↷✆⌘⏎❥ツღ☻`
If you find more feel free to edit my answer and add them.
EDIT: I think there are now a lot more supported emojis than back then.
You can also use thousands of letters from the chinese alphabet.