java - Why I can't return nothing in a static method this way -

i wrote piece of code train litte bit , wanted understand why cannot use notation.

public static double retanglearea (double c, double d) {   double area = c * d;   if ( c < 0 || d < 0) {     system.out.println("error.");     return;   }   return area; } 

if call method in main() shout "error." , still calculate (c * d) negative number wanted print show in. well, know how rid of problem want understand why can't way , what's problem that.

three ways round this:

1. throw exception rather writing error console. write console in catch block arguably in better position deal error.

2. change function return type double (note capital), , return null if don't want calculate value.

3. use java.lang.optional<t> return type.

of these, prefer , therefore recommend 1.