Java-tailcall-trampoline

@FunctionalInterface
public interface TailCall<T> {
    static <S> TailCall<S> done(S result) {
        return new TailCall<S>() {
            @Override
            public TailCall<S> apply() {
                throw new IllegalStateException("Last tailCall does not support apply method");
            }

            @Override
            public boolean isDone() {
                return true;
            }

            @Override
            public S result() {
                return result;
            }
        };
    }

    static <S> TailCall<S> invoke(final TailCall<S> tailCall) {
        TailCall<S> tailCallStage = tailCall;
        while (!tailCallStage.isDone()) {
            tailCallStage = tailCallStage.apply();
        }
        return tailCallStage;
    }

    TailCall<T> apply();

    default boolean isDone() {
        return false;
    }

    default T result() {
        throw new IllegalStateException("Intermediate tailCall does not have any result");
    }
}


class Factorial {
    public static TailCall<Long> factorial(long n) {
        return factorial(n, n - 1);
    }

    private static TailCall<Long> factorial(long fact, long n) {
        if (n == 0 || n == 1) {
            return TailCall.done(fact);
        } else {
            return () -> factorial(fact * n, n - 1);
        }
    }
}

class Test {
    public static void main(String[] args) {
        Long result = TailCall.invoke(Factorial.factorial(2)).result();
        System.out.println("Result " + result);
    }
}