It has a java bytecode interpreter and an initial implementation of a classfile loader. As said above it doesn't have a full implementation of the java runtime, just a few classes to be able to print out to standard output and su things with string and not much else (at least from what i've seen of the content of libs/java).
Regarding the loader, it can load a few sections from a classfile, the one containing methods bytecodes, the exceptions table and the constant pool (i.e. where all your string are stored with some other "constants").
How much it could take for a full implementation? It depends of what you mean by full. Even using one of the opensource runtime library like openjdk,classpath,harmony/android i'd say a few years done solo, and not much less with more people, to build something complete and stable enough for general use. Definitely not a simple project.
Regarding the loader, it can load a few sections from a classfile, the one containing methods bytecodes, the exceptions table and the constant pool (i.e. where all your string are stored with some other "constants"). How much it could take for a full implementation? It depends of what you mean by full. Even using one of the opensource runtime library like openjdk,classpath,harmony/android i'd say a few years done solo, and not much less with more people, to build something complete and stable enough for general use. Definitely not a simple project.