Find null Type's name in Optional method chain(through map operation) for multilevel POJO

Question

I have similar class structure.

class Request {
    Level1 level1;
}

class Level1 {
    Level2 level2;
    String data;
}

class Level2 {
    Level3 level3;
    String data;
}

class Level3 {
    Level4 level4;
    String data;
}

class Level4 {
    String data;
}







Request r = new Request();
        r.level1 = new Level1();
        r.level1.level2 = new Level2();
        r.level1.level2.level3 = null;//new Level3();
        //r.level1.level2.level3.level4 = new Level4();
        //r.level1.level2.level3.level4.data = "level4Data";

and to get data from nested fields I do following
Benefit of using Optional being I don't have to worry about checking null at each level in object hierarchy

String level4Data = Optional.ofNullable(r)
                            .map(req -> req.level1)
                            .map(l1 -> l1.level2)
                            .map(l2 -> l2.level3)
                            .map(l3 -> l3.level4)
                            .map(l4 -> l4.data)
                            .orElse(null);
System.out.println("level4Data: " + level4Data);

but again if I want to log reason behind level4Data being null, I have to do following/I don't any better

if (level4Data == null) {
    if (r == null) System.out.println("request was null");
    else if (r.level1 == null) System.out.println("level1 was null");
    else if (r.level1.level2 == null) System.out.println("level2 was null");
    else if (r.level1.level2.level3 == null) System.out.println("level3 was null");
    else if (r.level1.level2.level3.level4 == null) System.out.println("level4 was null");
    else if (r.level1.level2.level3.level4.data == null) System.out.println("level4.data was null");
}

is there more elegant/efficient way of doing this as it defeats benefits of using Optional in first place

Thank you for your time and inputs

Answer 1

Optional doesn't have a peek method like in Stream API.

For your use case, you can write a wrapper that will do the additional job:

// logging wrapper

static <T, R> Function<T, R> logIfNull(Function<? super T, ? extends R> function, String message) {
  return input -> {
      R result;
      if ((result = function.apply(input)) == null)
          System.out.println("logIfNull :: Null found. " + message);
      return result;
  };
}

// usage
String level4Data = Optional.ofNullable(r)
       .map(logIfNull(req -> req.level1, "req.level1"))
       .map(logIfNull(l1 -> l1.level2, "l1.level2"))
       .map(logIfNull(l2 -> l2.level3, "l2.level3"))
       .map(logIfNull(l3 -> l3.level4, "l3.level4"))
       .map(logIfNull(l4 -> l4.data, "l4.data"))
       .orElse(null);