Why calling this function recursively does not throw a NullPointerException

Question

My question comes from this thread.

Consider this code:

public class Test {    
    static Function<Integer, Integer> fibLambda = null;
    public static void main (String[] args) { 
        fibLambda = n -> n <= 2 ? 1 : fibLambda.apply(n - 1) + fibLambda.apply(n - 2); 
        System.out.println(fibLambda.apply(6));
    }
}

The output above is 8.

What I don't get is that how fibLamdba is initialized? It seems that I totally miss how the method invocation is done because I though that this code would produce a NPE.

Hope my question is clear

Answer 1

Your code is equivalent to

static Function<Integer, Integer> fibLambda = null;

public static void main(String[] args) {
    fibLambda = n -> n <= 2 ? 1 : Example.fibLambda.apply(n - 1) + Example.fibLambda.apply(n - 2);
    System.out.println(fibLambda.apply(6));
}

By the time the apply is called fibLambda is assigned a value. Basically, the lambda expression doesn't capture the value of fibLambda, it just registers that the variable needs to be evaluated at the appropriate moment to produce a value.

Remember that a lambda expression doesn't execute the code appearing in its body. It's just a declaration, similar to how you declare an anonymous class instance.