We Keep Coding

Static default method for not initialized Classes

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.

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:
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).
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.
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?)
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.

You can write a static test() method like this:
static <T> boolean test(T object, Predicate<T> validation) {
return object != null && validation.test(object);
}
and
static class Foo {
public boolean isValid() {
return true;
}
}
static Foo dosomething() {
return new Foo();
}
public static void main(String[] args) {
Foo a = dosomething();
if (test(a, Foo::isValid))
System.out.println("OK");
else
System.out.println("NG");
}
output:
OK
If dosomething() returns null, it prints NG

Not exactly, but take a look at Optional:
Optional.ofNullable(dosomething())
.filter(Foo::isValid)
.ifPresent(a -> ...);

Cleanest way to shorten two similar methods

down bellow you can see two example methods, which are structured in the same way, but have to work with completely different integers.
You can guess if the code gets longer, it is pretty anoying to have a second long method which is doing the same.
Do you have any idea, how i can combine those two methods without using "if" or "switch" statements at every spot?
Thanks for your help
public List<> firstTestMethod(){
if(blabla != null){
if(blabla.getChildren().size() > 1){
return blabla.getChildren().subList(2, blabla.getChildren().size());
}
}
return null;
}
And:
public List<> secondTestMethod(){
if(blabla != null){
if(blabla.getChildren().size() > 4){
return blabla.getChildren().subList(0, 2);
}
}
return null;
}

Attempting to isolate common ground from 2 or more places into its own Helper method is not a good idea if you're just looking at what the code does without any context.
The right approach is first to define what you're actually isolating. It's not so much about the how (the fact that these methods look vaguely similar suggests that the how is the same, yes), but the why. What do these methods attempt to accomplish?
Usually, the why is also mostly the same. Rarely, the why is completely different, and the fact that the methods look similar is a pure coincidence.
Here's a key takeaway: If the why is completely different but the methods look somewhat similar, you do not want to turn them into a single method. DRY is a rule of thumb, not a commandment!
Thus, your question isn't directly answerable, because the 2 snippets are so abstractly named (blabla isn't all that informative), it's not possible to determine with the little context the question provides what the why might be.
Thus, answer the why question first, and usually the strategy on making a single method that can cater to both snippets here becomes trivial.
Here is an example answer: If list is 'valid', return the first, or last, X elements inside it. Validity is defined as follows: The list is not null, and contains at least Z entries. Otherwise, return null.
That's still pretty vague, and dangerously close to a 'how', but it sounds like it might describe what you have here.
An even better answer would be: blabla represents a family; determine the subset of children who are eligible for inheriting the property.
The reason you want this is twofold:
It makes it much easier to describe a method. A method that seems to do a few completely unrelated things and is incapable of describing the rhyme or reason of any of it cannot be understood without reading the whole thing through, which takes a long time and is error-prone. A large part of why you want methods in the first place is to let the programmer (the human) abstract ideas away. Instead of remembering what these 45 lines do, all you need to remember is 'fetch the eligible kids'.
Code changes over time. Bugs are found and need fixing. External influences change around you (APIs change, libraries change, standards change). Feature requests are a thing. Without the why part it is likely that one of the callers of this method grows needs that this method cannot provide, and then the 'easiest' (but not best!) solution is to just add the functionality to this method. The method will eventually grow into a 20 page monstrosity doing completely unrelated things, and having 50 parameters. To guard against this growth, define what the purpose of this method is in a way that is unlikely to spiral into 'read this book to understand what all this method is supposed to do'.
Thus, your question is not really answerable, as the 2 snippets do not make it obvious what the common thread might be, here.
Why do these methods abuse null? You seem to think null means empty list. It does not. Empty list means empty list. Shouldn't this be returning e.g. List.of instead of null? Once you fix that up, this method appears to simply be: "Give me a sublist consisting of everything except the first two elements. If the list is smaller than that or null, return an empty list", which is starting to move away from the 'how' and slowly towards a 'what' and 'why'. There are only 2 parameters to this generalized concept: The list, and the # of items from the start that need to be omitted.
The second snippet, on the other hand, makes no sense. Why return the first 3 elements, but only if the list has 5 or more items in it? What's the link between 3 and 5? If the answer is: "Nothing, it's a parameter", then this conundrum has far more parameters than the first snippet, and we see that whilst the code looks perhaps similar, once you start describing the why/what instead of the how, these two jobs aren't similar at all, and trying to shoehorn these 2 unrelated jobs into a single method is just going to lead to bad code now, and worse code later on as changes occur.
Let's say instead that this last snippet is trying to return all elements except the X elements at the end, returning an empty list if there are fewer than X. This matches much better with the first snippet (which does the same thing, except replace 'at the end' with 'at the start'). Then you could write:
// document somewhere that `blabla.getChildren()` is guaranteed to be sorted by age.
/** Returns the {#code numEldest} children. */
public List<Child> getEldest(int numEldest) {
if (numEldest < 0) throw new IllegalArgumentException();
return getChildren(numEldest, true);
}
/** Returns all children except the {#code numEldest} ones. */
public List<Child> getAllButEldest(int numEldest) {
if (numEldest < 0) throw new IllegalArgumentException();
return getChildren(numEldest, false);
}
private List<Child> getChildren(int numEldest, boolean include) {
if (blabla == null) return List.of();
List<Child> children = blabla.getChildren();
if (numEldest >= children.size()) return include ? children : List.of();
int startIdx = include ? 0 : numEldest;
int endIdx = include ? numEldest : children.size();
return children.subList(startIdx, endIdx);
}
Note a few stylistic tricks here:
boolean parameters are bad, because why would you know 'true' matches up with 'I want the eldest' and 'false' matches up with 'I want the youngest'? Names are good. This snippet has 2 methods that make very clear what they do, by using names.
That 'when extracting common ground, define the why, not the how' is a hierarchical idea - apply it all the way down, and as you get further away from the thousand-mile view, the what and how become more and more technical. That's okay. The more down to the details you get, the more private things should be.
By having defined what this all actually means, note that the behaviour is subtly different: If you ask for the 5 eldest children and there are only 4 children, this returns those 4 children instead of null. That shows off some of the power of defining the 'why': Now it's a consistent idea. Returning all 4 when you ask for 'give me the 5 eldest', is no doubt 90%+ of all those who get near this code would assume happens.
Preconditions, such as what comprises sane inputs, should always be checked. Here, we check if the numEldest param is negative and just crash out, as that makes no sense. Checks should be as early as they can reasonably be made: That way the stack traces are more useful.

You can pass objects that encapsulate the desired behavior differences at various points in your method. Often you can use a predefined interface for behavior encapsulation (Runnable, Callable, Predicate, etc.) or you may need to define your own.
public List<> testMethod(Predicate<BlaBlaType> test,
Function<BlaBlaType, List<>> extractor)
{
if(blabla != null){
if(test.test(blabla)){
return extractor.apply(blabla);
}
}
return null;
}
You could then call it with a couple of lambdas:
testMethod(
blabla -> blabla.getChildren().size() > 1,
blabla -> blabla.getChildren().subList(2, blabla.getChildren().size())
);
testMethod(
blabla -> blabla.getChildren().size() > 4,
blabla -> blabla.getChildren().subList(0, 2)
);

Here is one approach. Pass a named boolean to indicate which version you want. This also allows the list of children to be retrieved independent of the return. For lack of more meaningful names I choose START and END to indicate which parts of the list to return.
static boolean START = true;
static boolean END = false;
public List<Children> TestMethod(boolean type) {
if (blabla != null) {
List<Children> list = blabla.getChildren();
int size = list.size();
return START ?
(size > 1 ? list.subList(0, 2) : null) :
(size > 4 ? list.subList(2, size) :
null);
}
return null;
}

How do I delete the middle node from a LinkedList?

I'm trying to delete the middle node from a linked list given access to that node. I'm wondering if there is a difference between the two methods below, or do they accomplish the same thing?
public boolean deleteMiddle(Node middle){
Node next = middle.next; //line 2
middle.data = next.data;
middle.next = next.next;
return true;
}
public boolean deleteMiddle(Node middle){
middle.data = middle.next.data;
middle.next = middle.next.next;
return true;
}
The first method is what the textbook recommended but it seems like creating the Node "next" in the first method(line 2) is an unnecessary line of code.

I think you're probably right that they are equivalent (or certainly look that way)
In both cases it looks like there is a null pointer exception (on next.next) if the item you're deleting is the last in the list (e.g. if next is null then next.next is an error).
And if you get passed null of course that's also going to be a NPE.

creating the Node "next" in the first method(line 2) is an unnecessary line of code.
Both snippets achieve the same result.
But what the extra variable assignment (it does not "create a Node", it just assigns a new name to an existing one) does is avoiding to call middle.next twice, avoiding doing the same "calculation" twice.
In this example that won't make any real difference, but in general, it can be a usual performance optimization to avoid redundant work (especially if method calls are involved that do some "heavy lifting"). Before engaging in these adventures, though, one should think about if it is worth making the code potentially less legible, especially given that the JVM will try to do all kinds of optimization automatically. So apply this mindset only to simple patterns or real bottlenecks.
On top of that, giving names to intermediate results can also lead to more self-explanatory code (also not much difference here).
Final remarks: Sometimes (but not here) it is necessary to introduce extra variables to store things temporarily that would otherwise be overwritten in the course of the operation, such as the famous int h = x; x = y; y = h; example.

Yes, they're equivalent. I prefer the first one because it avoids the repetition of the expression middle.next.
As #Rick pointed out, when middle is the last element, you'll get an NPE in both cases.

Call void on possible null object, in one line

I use Java8 in my project but i cannot solve this issue with a nice implementation.
UIInput textInput = ...;
if (textInput != null)
{
textInput.setValid(false);
}
Is there a solution to check if the object is null, and if not, then call the function on it, in one line ?!

Don't.
What you have is easily readable for anyone with some Java knowledge. Any one-liner misusing a construct intended for something else will take most people way more time to read and understand than this will. And likely some people will misread it and have to read it again later when it does not behave like they expect during a debugging session.
Brevity / Number of lines of code is not an ultimate measure for readability or quality.
What you can do, if this is at the wrong level of detail compared with the rest of your method, is abstract it away with a single speaking method call. Say, create a method 'ensureTextInputIsSet' that just contains this code and returns the potentially modified object.

Optional.ofNullable(textInput).ifPresent(x -> x.setValid(false));
But this is not what Optional was designed for...

If a variable may be null, an object is optional, then:
Optional<UIInput> textInput = ...;
With a circumstantial, but always safe usage:
textInput.ifPresent(ti -> ti.setValid(false));
And nice chaining, for instance for Optional<UIInput> to Optional<String> calling a method on the UIInput.
String s = textInput.map(UIInput::getText).orElse("");

Reverse the value of a boolean property

I have an (Java) object backed by a database table. In the database, a value of "N" in a (new)column implies something is forbidden, while a value of "Y" or null (default) implies that something is allowed.
In my object, I'm looking to have the same effect: by default forbidden = false, but it can be set to true, if desired. I'm working with existing code, so I don't want to modify the constructor.
However, the way the property will be used is likely to be (and I think is more understandable) is positive: isAllowed()/setAllowed(). I have therefore set up my code as:
public boolean isAllowed()
{ return !this.forbidden; }
public void setAllowed(boolean allowed)
{ this.forbidden = ! allowed; }
Does anyone else consider if ([!]isAllowed()) {...} to be more understandable than
if ([!]isForbidden()) {...}, or am I making an erroneous assumption?
Does my code make sense? (Yes, I'm trying to comment it properly, that's why I'm asking.)
My main question: Is there a conventional name for this kind of tweak? A boolean property backed by a field that carries the opposite value of the exposed property.
Thank you.

(1) Does anyone else consider if ([!]isAllowed()) {...} to be more understandable than if ([!]isForbidden()) {...}...
Yes. Positives are usually the way to go. Not always, but usually. Consider the case where code wants to branch on whether something is allowed:
if (something.isAllowed()) {
// do the thing
}
vs.
if (!something.isForbidden()) {
// do the thing
}
The first just reads a lot more fluidly. (But this is subjective.)
Just FWIW, I'd probably hold the data member as allowed rather than forbidden, too. So you wouldn't be inverting it in the accessor. Because the argument above applies just as much to code that uses the field directly (assuming you have code that does) as to code using the accessor.
(2) Does my code make sense?
Other than the bracing style, yes. :-) (And style is just style, and completely your own decision.)
(3) - My main question: Is there a conventional name for this kind of tweak?
I've never heard one, it's not really common enough. I suppose you're doing something vaguely facade or adapter-like, but it's a real stretch. :-)

It doesn't have a name, it's just reversing a boolean.
Wy not provide both isAllowed/isForbidden accessors? Which makes sense depends on context, and I don't see any reason to explicitly dis-allow using whatever is the most natural in context.