Issue with Bigdecimal in java -

i have calculate following function:

f(x)=x^3-x-1 

i've created class, named "function" , inside of it, have method calculating above function.

here comes code:

double function(double num) {     bigdecimal first,second;     double a,b,c,b_copy;     = math.pow(num, 3);     b=(double)a-num;     first=new  bigdecimal(b);     second=new bigdecimal("1.0");     first.min(second);     b_copy=first.doublevalue();     return b_copy ; }  

i have problems these 2 lines of code:

    first.min(second);     b_copy=first.doublevalue(); 

for example when num 0 b_copy must -1 0. why that?

the min(...) method returns bigdecimal, doesn't modify it. try this:

first = first.min(second); 

also, if want subtract 1 value of first (as formula indicates), use subtract(...) method because min(...) returns smallest values of 2 bigdecimals.

first = first.subtract(second);