RosettaCodeData/Task/Anonymous-recursion/Java/anonymous-recursion-2.java

import java.util.function.Function;

@FunctionalInterface
interface SelfApplicable<OUTPUT> {
    OUTPUT apply(SelfApplicable<OUTPUT> input);
}

class Utils {
    public static <INPUT, OUTPUT> SelfApplicable<Function<Function<Function<INPUT, OUTPUT>, Function<INPUT, OUTPUT>>, Function<INPUT, OUTPUT>>> y() {
        return y -> f -> x -> f.apply(y.apply(y).apply(f)).apply(x);
    }

    public static <INPUT, OUTPUT> Function<Function<Function<INPUT, OUTPUT>, Function<INPUT, OUTPUT>>, Function<INPUT, OUTPUT>> fix() {
        return Utils.<INPUT, OUTPUT>y().apply(Utils.<INPUT, OUTPUT>y());
    }

    public static long fib(int m) {
        if (m < 0)
            throw new IllegalArgumentException("n can not be a negative number");

        return Utils.<Integer, Long>fix().apply(
                f -> n -> (n < 2) ? n : (f.apply(n - 1) + f.apply(n - 2))
        ).apply(m);
    }
}