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

I need help in my assignment plz. kindly guide me any technique/idea to do the following. m not at advance level of programming.

The program should take function defnitions 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 defnitions from a fle
delete funname; to delete the defnition corresponding to the function named funname

 

start coding the easy part
create a class and ask the user to give you commands
post your code here for the rest

 

import java.io.BufferedReader;
import java.io.InputStreamReader;

Code:

public class Funcdefin 
{
    public static void main(String[] args)
    {
    String line=null;
    System.out.println("please enter commands");
    BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
    try
    {
        line = stdin.readLine();
    } catch (java.io.IOException e) 
    {
        System.out.println("Error: Invalid IO operation.");
    }
    System.out.println(line);
    
    }
}

THIS IS ALL I HAVE SO FAR. PLZ HELP ME OUT. FOLLOWING ARE THE DETAILS THROUGH WHICH IT IS REQUIRED TO BE DONE.

A function is a labelled tree
It is a tree with a name
It also contains unbound variables, i.e. the variables listed in the
parameter list (and only those ones)
When a function is used, it is necessary to bound the variables to
actual values, and then evaluate the tree as any other numerical
expression

AND THE REQUIREMENTS ARE

show prints all definitions on screen
save filename saves all definitions in a file
load filename loads all definitions from a file
delete funname deletes the definition corresponding to the function
named funname

 

read this
http://www.javaprogrammingforums.com/algorithms-recursion/605-java-calculator.html

and this

http://www.cs.plu.edu/~brink/Somepages/ExprEvaluator/InfixEvalDemo.HTML