Can't subtract one double from another?

Question

Im quite new to Java and programming in general, but Ive read up on it quite a bit. Im currently making my first real OOP- a calculator that can perform certain equations. However, while trying to program something that would calculate the variance of a distribution -Heres the code:

void variance() {
  System.out.println("The variance of a distribution with x values of " + a + b + "and mean" 
                + mean + "is " + a*a+b*b/2 - mean*mean); 
}

I get the error

Bad operand types for binary operator '-' First type = string Second type = double".

I had previously stated that a, b and mean were doubles and had also stated how mean is calculated. I also tried changing a*a+b*b/2 from a string to a double, but then realized that if i put any integers or doubles into where a*a+b*b/2 (e.g. 2) but i get the same error. Any help would be much appreciated :)

Answer 1

That's because the + is overloaded in Java for string concatenation. You need to put parentheses around your math expression.

System.out.println("The variance of a distribution with x values of " + a + b + "and mean" 
 + mean + "is " + (a*a+b*b/2 - mean*mean));