sometimes it would be convenient to have an easy way of doing the following:

Foo a = dosomething();
if (a != null){
    if (a.isValid()){
    ...
    }
}

My idea was to have some kind of static “default” methods for not initialized variables like this:

class Foo{
    public boolean isValid(){
        return true;
    }
    public static boolean isValid(){
        return false;
    }
}

And now I could do this…

Foo a = dosomething();

if (a.isValid()){
    // In our example case -> variable is initialized and the "normal" method gets called
}else{
    // In our example case -> variable is null
}

So, if a == null the static “default” methods from our class gets called, otherwise the method of our object gets called.

Is there either some keyword I’m missing to do exactly this or is there a reason why this is not already implemented in programming languages like java/c#?

Note: this example is not very breathtaking if this would work, however there are examples where this would be - indeed - very nice.

CodePudding user response：

It's very slightly odd; ordinarily, x.foo() runs the foo() method as defined by the object that the x reference is pointing to. What you propose is a fallback mechanism where, if x is null (is referencing nothing) then we don't look at the object that x is pointing to (there's nothing its pointing at; hence, that is impossible), but that we look at the type of x, the variable itself, instead, and ask this type: Hey, can you give me the default impl of foo()?

The core problem is that you're assigning a definition to null that it just doesn't have. Your idea requires a redefinition of what null means which means the entire community needs to go back to school. I think the current definition of null in the java community is some nebulous ill defined cloud of confusion, so this is probably a good idea, but it is a huge commitment, and it is extremely easy for the OpenJDK team to dictate a direction and for the community to just ignore it. The OpenJDK team should be very hesitant in trying to 'solve' this problem by introducing a language feature, and they are.

Let's talk about the definitions of null that make sense, which definition of null your idea specifically is catering to (at the detriment of the other interpretations!), and how catering to that specific idea is already easy to do in current java, i.e. - what you propose sounds outright daft to me, in that it's just unneccessary and forces an opinion of what null means down everybody's throats for no reason.

Not applicable / undefined / unset

This definition of null is exactly how SQL defines it, and it has the following properties:

There is no default implementation available. By definition! How can one define what the size is of, say, an unset list? You can't say 0. You have no idea what the list is supposed to be. The very point is that interaction with an unset/not-applicable/unknown value should immediately lead to a result that represents either [A] the programmer messed up, the fact that they think they can interact with this value means they programmed a bug - they made an assumption about the state of the system which does not hold, or [B] that the unset nature is infectuous: The operation returns the notion 'unknown / unset / not applicable' as result.

SQL chose the B route: Any interaction with NULL in SQL land is infectuous. For example, even NULL = NULL in SQL is NULL, not FALSE. It also means that all booleans in SQL are tri-state, but this actually 'works', in that one can honestly fathom this notion. If I ask you: Hey, are the lights on?, then there are 3 reasonable answers: Yes, No, and I can't tell you right now; I don't know.

In my opinion, java as a language is meant for this definition as well, but has mostly chosen the [A] route: Throw an NPE to let everybody know: There is a bug, and to let the programmer get to the relevant line extremely quickly. NPEs are easy to solve, which is why I don't get why everybody hates NPEs. I love NPEs. So much better than some default behaviour that is usually but not always what I intended (objectively speaking, it is better to have 50 bugs that each takes 3 minutes to solve, than one bug that takes an an entire working day, by a large margin!) – this definition 'works' with the language:

• Uninitialized fields, and uninitialized values in an array begin as null, and in the absence of further information, treating it as unset is correct.
• They are, in fact, infectuously erroneous: Virtually all attempts to interact with them results in an exception, except ==, but that is intentional, for the same reason in SQL IS NULL will return TRUE or FALSE and not NULL: Now we're actually talking about the pointer nature of the object itself ("foo" == "foo" can be false if the 2 strings aren't the same ref: Clearly == in java between objects is about the references itself and not about the objects referenced).

A key aspect to this is that null has absolutely no semantic meaning, at all. Its lack of semantic meaning is the point. In other words, null doesn't mean that a value is short or long or blank or indicative of anything in particular. The only thing it does mean is that it means nothing. You can't derive any information from it. Hence, foo.size() is not 0 when foo is unset/unknown - the question 'what is the size of the object foo is pointing at' is unanswerable, in this definition, and thus NPE is exactly right.

Your idea would hurt this interpretation - it would confound matters by giving answers to unanswerable questions.

Sentinel / 'empty'

null is sometimes used as a value that does have semantic meaning. Something specific. For example, if you ever wrote this, you're using this interpretation:

if (x == null || x.isEmpty()) return false;

Here you've assigned a semantic meaning to null - the same meaning you assigned to an empty string. This is common in java and presumably stems from some bass ackwards notion of performance. For example, in the eclipse ecj java parser system, all empty arrays are done with null pointers. For example, the definition of a method has a field Argument[] arguments (for the method parameters; using argument is the slightly wrong word, but it is used to store the param definitions); however, for methods with zero parameters, the semantically correct choice is obviously new Argument[0]. However, that is NOT what ecj fills the Abstract Syntax Tree with, and if you are hacking around on the ecj code and assign new Argument[0] to this, other code will mess up as it just wasn't written to deal with this.

This is in my opinion bad use of null, but is quite common. And, in ecj's defense, it is about 4 times faster than javac, so I don't think it's fair to cast aspersions at their seemingly deplorably outdated code practices. If it's stupid and it works it isn't stupid, right? ecj also has a better track record than javac (going mostly by personal experience; I've found 3 bugs in ecj over the years and 12 in javac).

This kind of null does get a lot better if we implement your idea.

The better solution

What ecj should have done, get the best of both worlds: Make a public constant for it! new Argument[0], the object, is entirely immutable. You need to make a single instance, once, ever, for an entire JVM run. The JVM itself does this; try it: List.of() returns the 'singleton empty list'. So does Collections.emptyList() for the old timers in the crowd. All lists 'made' with Collections.emptyList() are actually just refs to the same singleton 'empty list' object. This works because the lists these methods make are entirely immutable.

The same can and generally should apply to you!

If you ever write this:

if (x == null || x.isEmpty())

then you messed up if we go by the first definition of null, and you're simply writing needlessly wordy, but correct, code if we go by the second definition. You've come up with a solution to address this, but there's a much, much better one!

Find the place where x got its value, and address the boneheaded code that decided to return null instead of "". You should in fact emphatically NOT be adding null checks to your code, because it's far too easy to get into this mode where you almost always do it, and therefore you rarely actually have null refs, but it's just swiss cheese laid on top of each other: There may still be holes, and then you get NPEs. Better to never check so you get NPEs very quickly in the development process - somebody returned null where they should be returning "" instead.

Sometimes the code that made the bad null ref is out of your control. In that case, do the same thing you should always do when working with badly designed APIs: Fix it ASAP. Write a wrapper if you have to. But if you can commit a fix, do that instead. This may require making such an object.

Sentinels are awesome

Sometimes sentinel objects (objects that 'stand in' for this default / blank take, such as "" for strings, List.of() for lists, etc) can be a bit more fancy than this. For example, one can imagine using LocalDate.of(1800, 1, 1) as sentinel for a missing birthdate, but do note that this instance is not a great idea. It does crazy stuff. For example, if you write code to determine the age of a person, then it starts giving completely wrong answers (which is significantly worse than throwing an exception. With the exception you know you have a bug faster and you get a stacktrace that lets you find it in literally 500 milliseconds (just click the line, voila. That is the exact line you need to look at right now to fix the problem). It'll say someone is 212 years old all of a sudden.

But you could make a LocalDate object that does some things (such as: It CAN print itself; sentinel.toString() doesn't throw NPE but prints something like 'unset date'), but for other things it will throw an exception. For example, .getYear() would throw.

You can also make more than one sentinel. If you want a sentinel that means 'far future', that's trivially made (LocalDate.of(9999, 12, 31) is pretty good already), and you can also have one as 'for as long as anyone remembers', e.g. 'distant past'. That's cool, and not something your proposal could ever do!

You will have to deal with the consequences though. In some small ways the java ecosystem's definitions don't mesh with this, and null would perhaps have been a better standin. For example, the equals contract clearly states that a.equals(a) must always hold, and yet, just like in SQL NULL = NULL isn't TRUE, you probably don't want missingDate.equals(missingDate) to be true; that's conflating the meta with the value: You can't actually tell me that 2 missing dates are equal. By definition: The dates are missing. You do not know if they are equal or not. It is not an answerable question. And yet we can't implement the equals method of missingDate as return false; (or, better yet, as you also can't really know they aren't equal either, throw an exception) as that breaks contract (equals methods must have the identity property and must not throw, as per its own javadoc, so we can't do either of those things).

Dealing with null better

There are a few things that make dealing with null a lot easier:

1. Annotations: APIs can and should be very clear in communicating when their methods can return null and what that means. Annotations to turn that documentation into compiler-checked documentation is awesome. Your IDE can start warning you, as you type, that null may occur and what that means, and will say so in auto-complete dialogs too. And it's all entirely backwards compatible in all senses of the word: No need to start considering giant swaths of the java ecosystem as 'obsolete' (unlike Optional, which mostly sucks).

2. Optional, except this is a non-solution. The type isn't orthogonal (you can't write a method that takes a List<MaybeOptionalorNot<String>> that works on both List<String> and List<Optional<String>>, even though a method that checks the 'is it some or is it none?' state of all list members and doesn't add anything (except maybe shuffle things around) would work equally on both methods, and yet you just can't write it. This is bad, and it means all usages of optional must be 'unrolled' on the spot, and e.g. Optional<X> should show up pretty much never ever as a parameter type or field type. Only as return types and even that is dubious - I'd just stick to what Optional was made for: As return type of Stream terminal operations.

Adopting it also isn't backwards compatible. For example, hashMap.get(key) should, in all possible interpretations of what Optional is for, obviously return an Optional<V>, but it doesn't, and it never will, because java doesn't break backwards compatibility lightly and breaking that is obviously far too heavy an impact. The only real solution is to introduce java.util2 and a complete incompatible redesign of the collections API, which is splitting the java ecosystem in twain. Ask the python community (python2 vs. python3) how well that goes.

3. Use sentinels, use them heavily, make them available. If I were designing LocalDate, I'd have created LocalDate.FAR_FUTURE and LocalDate_DISTANT_PAST (but let it be clear that I think Stephen Colebourne, who designed JSR310, is perhaps the best API designer out there. But nothing is so perfect that it can't be complained about, right?)

4. Use API calls that allow defaulting. Map has this.

Do NOT write this code:

String phoneNr = phoneNumbers.get(userId);
if (phoneNr == null) return "Unknown phone number";
return phoneNr;

But DO write this:

return phoneNumbers.getOrDefault(userId, "Unknown phone number");

Don't write:

Map<Course, List<Student>> participants;
void enrollStudent(Student student) {
  List<Student> participating = participants.get(econ101);
  if (participating == null) {
    participating = new ArrayList<Student>();
    participants.put(econ101, participating);
  }
  participating.add(student);
}

instead write:

Map<Course, List<Student>> participants;
void enrollStudent(Student student) {
  participants.computeIfAbsent(econ101,
    k -> new ArrayList<Student>())
    .add(student);
}

and, crucially, if you are writing APIs, ensure things like getOrDefault, computeIfAbsent, etc. are available so that the users of your API don't have to deal with null nearly as much.

CodePudding user response：

Not exactly, but take a look at Optional:

Optional.ofNullable(dosomething())
        .filter(Foo::isValid)
        .ifPresent(a -> ...);