recursion - Implement recursive lambda function using Java 8

Question

Java 8 introduced lambda functions and I want to implement something like factorial:

 IntToDoubleFunction fact = x -> x == 0 ? 1 : x * fact.applyAsDouble(x-1);

Compilation returns

  error: variable fact might not have been initialized

How can I reference function itself. Class is anonymous but instance exists: It is called fact.

Answer 1

I usually use (once-for-all-functional-interfaces defined) generic helper class which wraps the variable of the functional interface type. This approach solves the problem with the local variable initialization and allows the code to look more clearly.

In case of this question the code will look as follows:

// Recursive.java
// @param <I> - Functional Interface Type
public class Recursive<I> {
    public I func;
}

// Test.java
public double factorial(int n) {

    Recursive<IntToDoubleFunction> recursive = new Recursive<>();
    recursive.func = x -> (x == 0) ? 1 : x * recursive.func.applyAsDouble(x - 1);

    return recursive.func.applyAsDouble(n);
}