private static void convertoption() {//CHOOSE A CONVERTER TO USE
Scanner in = new Scanner(System.in);
To begin with I don't think a void method is going to work here. This method has to
return something to whoever calls it. What it returns will reflect what the user wants to do. It could as simple as returning 1 if the user wants to do the length conversion and return 2 if they want to do the mass conversion. (The other methods didn't have to return anything because they just did their job - they didn't produce a result.)
Overall the structure will look like:
// in main()...
int conversion = getConvertOption();
if(conversion == 1) {
ftToCmConverter();
} else if(//etc
// later...
/**
* Prompts for and returns the conversion required by the user.
* @return 1 for length conversion, 2 for mass conversion
*/
private static void getConvertOption() {
// prompt user: 1==convert lengths, etc. Like a menu.
// get user response. eg nextInt()
// ??? check that response is valid ie is 1 or 2
// return response
}