Processing has a pre-processor (changing the 'color' datatype to int, #AABBCC to 0xFFAABBCC, int() to a real function name, etc.) that must parse the Processing code, and is actually a Java parser modified to handle the above mentioned particularities of the syntax.
The problem is that the Java syntax is complex, and the changes to it are not trivial.

At one time in the Processing history, they jumped from the syntax of Java 1.4 to the one of 1.5, like supporting 'foreach' style loops, generics, annotations (?), although forgetting some stuff like enums.
Processing 2.0 is still shipped with Java 1.6, perhaps because precisely it doesn't support yet the new syntax of Java 7, like 0b0, 1_000_000, try with resources, multiple exception catch or switch on strings (probably).
Perhaps they will support it when Java 8 will be officially out...