This is one of the question that is most unwelcomed here. For once it show lack of interest and creativity.
How are we supposed to pick up a topic for you if we do not have any background on your Java ability? Also if you took your time to browse this forum you would find there been many sugestion brought up for this matter.
We will try to help you once you have selected a topic and have a problem. For selecting the topic, introspect. Have an idea about the level of difficulty of you are willing to handle. Then try google for finalizing the topic.