I have a problem with Null Value.
I Explain. Here is the below code :
ChnToto is a chain
ChnToto = Null
On debugging, we find that Windev has stored a "0" (zero) in "ChnToto".
I totally lose the initial information (=> "Null").
How to force the compiler to keep this information without having to use the "Variant" type?
I feel that you have to go through a complex variable, or a class.
Thanks for your advices or ideas
You must use the variant type if you want to handle the null value.
In windev the string type handle 0 and null the same way.
https://doc.windev.com/en-US/?1511015&verdisp=210
To assign Null Values in DataBase you have to use this syntax:
ChnToto..Null = True
Then use HModify(), HSave(), HAdd(), Etc.
Avoid ChnToto = null. That is an incorrect assignment of null
Related
I have some code and when I run it produces an error, saying:
NoSuchMethod: the method 'XYZ' was called on null
What does that mean and how do I fix it?
Why do I get this error?
Example
As a real world comparison, what just happened is this conversation:
Hey, how much gas is left in the tank of the car?
What are you talking about, we don't have a car.
That is exactly what is happening in your program. You wanted to call a function like _car.getGasLevel(); but there is no car, the variable _car is null.
Obviously, in your program it might not be a car. It could be a list or a string or anything else really.
Technical explanation
You are trying to use a variable that is null. Either you have explicitly set it to null, or you just never set it at all, the default value is null.
Like any variable, it can be passed into other functions. The place where you get the error might not be the source. You will have to follow the leads from the actual null value to where it originally came from, to find what the problem is and what the solution might be.
null can have different meanings: variables not set to another value will be null, but sometimes null values are used by programmers intentionally to signal that there is no value. Databases have nullable fields, JSON has missing values. Missing information may indeed be the information itself. The variable bool userWantsPizzaForDinner; for example might be used for true when the user said yes, false when the user declined and it might still be null when the user has not yet picked something. That's not a mistake, it's intentionally used and needs to be handled accordingly.
How do I fix it?
Find it
Use the stack trace that came with the error message to find out exactly which line the error was on. Then set a breakpoint on that line. When the program hits the breakpoint, inspect all the values of the variables. One of them is null, find out which one.
Fix it
Once you know which variable it is, find out how it ended up being null. Where did it come from? Was the value never set in the first place? Was the value another variable? How did that variable got it's value. It's like a line of breadcrumbs you can follow until you arrive at a point where you find that some variable was never set, or maybe you arrive at a point where you find that a variable was intentionally set to null. If it was unintentional, just fix it. Set it to the value you want it to have. If it was intentional, then you need to handle it further down in the program. Maybe you need another if to do something special for this case. If in doubt, you can ask the person that intentionally set it to null what they wanted to achieve.
simply the variable/function you are trying to access from the class does not exist
someClass.xyz();
above will give the error
NoSuchMethod: the method 'xyz' was called on null
because the class someClass does not exist
The following will work fine
// SomeClass created
// SomeClass has a function xyz
class SomeClass {
SomeClass();
void xyz() {
print('xyz');
}
}
void main() {
// create an instance of the class
final someClass = SomeClass();
// access the xyz function
someClass.xyz();
}
I have some code and when I run it produces an error, saying:
NoSuchMethod: the method 'XYZ' was called on null
What does that mean and how do I fix it?
Why do I get this error?
Example
As a real world comparison, what just happened is this conversation:
Hey, how much gas is left in the tank of the car?
What are you talking about, we don't have a car.
That is exactly what is happening in your program. You wanted to call a function like _car.getGasLevel(); but there is no car, the variable _car is null.
Obviously, in your program it might not be a car. It could be a list or a string or anything else really.
Technical explanation
You are trying to use a variable that is null. Either you have explicitly set it to null, or you just never set it at all, the default value is null.
Like any variable, it can be passed into other functions. The place where you get the error might not be the source. You will have to follow the leads from the actual null value to where it originally came from, to find what the problem is and what the solution might be.
null can have different meanings: variables not set to another value will be null, but sometimes null values are used by programmers intentionally to signal that there is no value. Databases have nullable fields, JSON has missing values. Missing information may indeed be the information itself. The variable bool userWantsPizzaForDinner; for example might be used for true when the user said yes, false when the user declined and it might still be null when the user has not yet picked something. That's not a mistake, it's intentionally used and needs to be handled accordingly.
How do I fix it?
Find it
Use the stack trace that came with the error message to find out exactly which line the error was on. Then set a breakpoint on that line. When the program hits the breakpoint, inspect all the values of the variables. One of them is null, find out which one.
Fix it
Once you know which variable it is, find out how it ended up being null. Where did it come from? Was the value never set in the first place? Was the value another variable? How did that variable got it's value. It's like a line of breadcrumbs you can follow until you arrive at a point where you find that some variable was never set, or maybe you arrive at a point where you find that a variable was intentionally set to null. If it was unintentional, just fix it. Set it to the value you want it to have. If it was intentional, then you need to handle it further down in the program. Maybe you need another if to do something special for this case. If in doubt, you can ask the person that intentionally set it to null what they wanted to achieve.
simply the variable/function you are trying to access from the class does not exist
someClass.xyz();
above will give the error
NoSuchMethod: the method 'xyz' was called on null
because the class someClass does not exist
The following will work fine
// SomeClass created
// SomeClass has a function xyz
class SomeClass {
SomeClass();
void xyz() {
print('xyz');
}
}
void main() {
// create an instance of the class
final someClass = SomeClass();
// access the xyz function
someClass.xyz();
}
val args = Bundle()
args.putString("type", details.type)
navigator.navigate(context!!, findNavController(), Destination.TYPE, args)
I am quite confused as to why in the receiving fragment when I go to access the arguments I have passed through it is responding with...
val type: String = arguments.getString("type")
The arguments.getString is all underlined red and says "Required String Found String?" But how when I called method "putString"?!?
It is resulting in text not being rendered in the new fragment and I assume this is a nullability issue.
It's a matter of knowledge that is available in the receiving Fragment.
The Fragment is not aware of how its arguments were created (or modified) so it has to assume the "type" key you're looking for might not be in the arguments Bundle. That's why it returns a nullable (String?) result (the null value would mean absent in arguments).
Your fragment might be created in many places in your app and its arguments might have been modified in many places. We have no way of tracking that.
There are different solutions for this problem, depending on your approach in other parts of the code and how "confident" you are in creating of your Fragment.
I would usually choose a solution in which I assume setting the type is mandatory. Therefore if the type is absent - I fail fast. That would mean the Fragment was misused.
val type: String = arguments!!.getString("type")!!
The code above will crash if either:
a) arguments weren't set, or
b) String with type wasn't put in the arguments Bundle.
You are right, that is a : null ability issue.
First you should be sure if you are expecting a value, so try adding "?" or "!!", i would recommend "?", or go with the block of if {} else
To read the string safely you can use:
val type: String = arguments?.getString("type").orEmpty()
The orEmpty call at the end ensures that a valid String is returned even if either arguments or getString() returns null.
The method signature for getString() returns a nullable String. This is because at compile time, the compiler can't know if the value exists in the bundle or not. You will have the same issue when retrieving anything from any Map.
If you know for certain that the value in the bundle or map should exist at the time you call getString(), you can use the !! operator. That's what it's there for. When you know something should always be there, it is appropriate to want an exception to be thrown (in this case KNPE) if it's not there so you can easily find any programming error during testing.
isEmpty() or ?.let aren't helpful in this particular case because they would just be masking a programming error and making it harder to discover or debug.
I have a struct with a lot of fields and I have to check if any of those fields is null without having to type every field name by hand. The field's type is always a pointer so I can check without having to worry about zero-values.
I'm trying to to this using the reflection package, but it doesn't seem to be working properly and I can't figure out why.
Here is a playground replicating my problem:
http://play.golang.org/p/LOb6a8eklE
As you can see, if I check by hand everything works fine. When asked to print it prints null as well, but when comparing it evaluates to false.
Any thoughts on what is going on?
My main guess is because the return type of Interface() is, obviously, interface{}, and by storing "null" inside it, it doesn't make it "null" anymore. Any way around that?
Thanks!
Use IsNil() to determine if a pointer is nil.
playground
The expression v.Interface() == nil is always false because Interface() returns a value with an associated type.
See the nil error FAQ for more information on this topic.
I came across this interesting function in a piece of ASP Classic that I maintain. At first I laughed, shook my head, then cried (only a little). But then I started to wonder if there is any legitimate reason why 999999999999999 would apparently be considered NULL since VBScript has its quirks. As mentioned in comments, the values passed to this function are returned from the COM dll.
Can anyone confirm if there is some legitimate reason for this or is it ripe for submission to TheDailyWTF.
function NullNumberCheck(Value)
if IsNumeric(Value) then
if Value = 999999999999999 then
Value = ""
end if
end if
NullNumberCheck = Value
end function
That looks like a case of "magic null" in the source of the data - is there a column in the database that backs the values being passed to this function which is not nullable?
Why are people using magic values instead of null in their code?
The author is using this as a method of clearing or unsetting the variable. By setting the value of Value to an empty string, it clears any previous value. The author could have just as easily set it equal to 0, vbNull, or vbEmpty. It really depends on what you are doing with the value later in your script. If you go on to perform further checks on the variable, setting it to vbNull may not be advisable and setting it equal to vbEmpty may crash your script if you are using Option Explicit.
So to answer your question, no this is not a valid way to check for a "null" value, because it's not a comparison operation at all, it's performing a variable assignment.
To check for a null value you can use the IsNull function built into VBscript
http://www.w3schools.com/vbscript/func_isnull.asp