There exists one open-source Java software that constructs the de Bruijn graph (but it does not assemble). http://grafia.cs.ucsb.edu/msp/download.html
There does not appear to exist any open source de novo assembler written in Java.
Indeed, Contrail can be considered as a de novo assembler written in Java.
This is probably not exactly what you are looking for, but Michael Shatz's group is working on a Hadoop-based de Bruijn graph assembler called Contrail:
Since it is a native Hadoop application it is mostly written in Java, although it appears that they also invoke other non-Java programs (FLASH and QUAKE) as part of their workflow.
This gets around the memory management features of Java that make a traditional single-machine algorithm hard to implement by using Hadoop's mechanisms of streaming data to and from disks across a cluster.
I am not sure what its state of readiness/current ability to run on real data is.