The sorts of things I would look for in a graduate "portfolio" would be one or more of the following:
1) A Blog - if you're an expert in something, then blog about it. But I'm actually more interested if you blog about the trouble you had learning stuff, if you document things you stumbled over and now understand (that helps other people enormously), if you link to videos or blogs of things that interest you. I can get a really good feel for who you are, what interests you, and maybe how you learn by reading your blog
2) Open Source - if you've contributed to open source this is awesome, because I can actually see your code and figure out if you'll fit in my team. Note that sometimes it's not about being brilliant at code or technology, especially as a graduate, but at having an approach to problem solving that works within the team
3) Stack Overflow - I hate this, personally, because I have about 3 points on SO. But seeing questions you've asked and answered also points me to your strengths and weaknesses, shows me what you're interested in and that you're committed to learning things.
Note that none of these things are Java-specific. When hiring graduates many companies know there will be a certain amount of technical training required. What is more difficult to train for is problem solving ability and team fit. If you can show who you are, what you're interested in and how your brain works, that's good enough, that way companies that want someone just like you can find you.