Need urgent help in assignment of JAVA, any idea suggestion plz

I need help in my JAVA assignment plz. kindly guide me any technique/idea to do the following. m not at advance level of programming. i already have the initial part of my assignment completed which is to make a simple command line calculator. this is the next part.

The program should take function definitions and those definitions can be used in future expressions
for eg:

def add(x,y)=(x+y)
>>> ok
def inv(x)=1/x
>>> ok
inv(add(2,2))
>>> ans : 0.25

Also four additional commands:
show; to prints all de?nitions on screen
save flename; to save all defnitions in a fle
load flename; to load all definitions from a fle
delete funname; to delete the defnition corresponding to the function named funname

Its a good assignment, I hope I could help you on this.

Let me know what are the options you have tried. What is your approach? Did you start writing some code?

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well, actually there is also a first part in which we have already implemented a command line calculator. in that part we are taking input from command line and parsing it making tree, using grammer. in short a user inputs an expression like (2+2)/5 and our calculator calculated the output and shows it. now we are required to put more functionality which is in 2nd part, the one which i am asking for.