scala functin is anonymous inner class which is the same as guava in byte
code level.
but writing anonymous inner class in java is just not that convinient.
for the case sin(a)+sin(b)+sin(c).
why you need to seperate into different lines and debug because you DO NOT
have enough test coverage for sin().
this is not a drawback of writing in one line, but a push to get better test
coverage.