The situation in the field of Java decompilers is sad - most are over 10 years old and only support bytecode of JDK 1.3.
This thesis would consist of finding as many Java decompilres as can be found and compare their functionality, stability, performance, extensibility etc.
For diploma thesis, the comparison would have to be really rigorous - investigating all constructs up to JDK 7.
Some information and overview:
Similar Thesis (old): http://www.mcs.gold.ac.uk/~mas01sd/decompilers.pdf
Possible leaders: Ondrej Zizka