java8 possibility to apply mathematical operators to functions

Assume, I’ve got two functions.

Function<Double, Double> function1 = (x) -> (3*x);
Function<Double, Double> function2 = (x) -> (Math.pow(x, 2));

I want to multiply these functions. So I want to get this lambda expression as a result:

(x) -> (3*Math.pow(x,3));

Is it possible in Java 8?

Answer

No, you won’t be able to do that with Function.

Suppose you have a Function.multiply(other) method, what would be the result of performing multiplication with the two following functions?

Function<String, String> f1 = s -> s.substring(1);
Function<String, Integer> f2 = s -> s.length();

However, if you rewrite your functions with DoubleUnaryOperator (or Function<Double, Double>), then you could have the following:

DoubleUnaryOperator f1 = x -> 3*x;
DoubleUnaryOperator f2 = x -> Math.pow(x, 2);

DoubleUnaryOperator f1xf2 = x -> f1.applyAsDouble(x) * f2.applyAsDouble(x);

The difference here is that we know that the operand of the function is a double and that it returns a double.

The f1xf2 function returns the multiplication of the result of f1 and f2. Since we know that both operands are double, the multiplication can be done safely.