I am looking for syntactic sugar with null-safe-operators to do this:
map["key"] == null ? default : (int.tryParse(map["key"]!) ?? default)
This however accesses map twice and requires default twice.
Better, but verbose code that also compiles:
String? val = map["key"];
int? res;
if (val != null) { res = int.tryParse(val) }
res ??= default;
In essence I am looking for a way to only call a function if the parameter is not null. E.g. in kotlin you could do (pseudocode)
map["key"].let{ int.tryParse(it) } ...
I found this related question, however writing helper methods is even more verbose and I cannot edit the function to take nullable parameters.
What i would love to see for dart (but afaik this does not exist):
int.tryParse(map["key"]?) ?? default;
I.e. func(null?) => null, no matter what func is, or sth similar.
Is there a smooth way to do that or is the verbose way for now the "accepted" way?
EDIT
I am aware of the int.tryParse("") or int.tryparse(default.toString() hacks, however both are somewhat naughty as they will call tryParse either way instead of skipping it if the value is null anyways. (imagine replacing tryParse with a very expensive factory method)
There is two things you can do:
int.parse(map["key"] ?? default.toString());
This would work if you are sure that map["key"] can be parsed if it is not null.
If you have to do this operation a lot you can also write your own extension function and then use the null-safe operator:
extension NullSaveParser on String {
int? tryToInt() {
return int.tryParse(this);
}
}
And then use this:
map["key"]?.tryToInt() ?? default;
This extension is neccessary because there is currently no way in dart to not call a method if an argument would be null, only if the object that you are calling it on is null can be caught.
I hope this helps.
It's sort of a hack and maybe ugly but for this case you might want to write something like
int.tryParse(map["key"] ?? '') ?? default
That is, put a fallback value inside the tryParse that you know will make the tryParse return null
Is it bad practice to change my getter method like version 2 in my class.
Version 1:
public String getMyValue(){
return this.myValue
}
Version 2:
public String getMyValue(){
if(this.myValue == null || this.myValue.isEmpty()){
this.myValue = "N/A";
}
return this.myValue;
}
I think it is actually quite a bad practice if your getter methods change the internal state of the object.
To achieve the same I would suggest just returning the "N/A".
Generally speaking this internal field might be used in other places (internally) for which you don't need to use the getter method. So in the end, the call to foo.getMyValue() could actually change the behaviour of foo.
Alternatively, the translation from null to "N/A" could be done in the setter, i.e. the internal value could be set to "N/A" if null is passed.
A general remark:
I would only add states such as "N/A" if they are expected by some API or other instance relying on your code. If that is not the case you should rely on the standard null types that are available to you in your programming language.
In my opinion, unless you are doing lazy-loading (which you are not in that case), getters should not change the value. So I would either:
Put the change in the setter
public void setMyValue(String value) {
if(value == null || value.isEmpty()){
this.myValue = "N/A";
} else {
this.myValue = value;
}
}
Or make the getter return a default value if value not set properly:
public String getMyValue() {
if(this.myvalue == null || this.myvalue.isEmpty()){
return "N/A";
}
return this.myValue;
}
In the case of lazy-loading, where I would say that changing your members in a getter is fine, you would do something like:
public String getMyValue() {
if (this.myvalue == null) {
this.myvalue = loadMyValue();
}
return this.myValue;
}
No. You're doing two things here. Getting and setting.
Yes. It's a bad practice.
Why?
When the value is set (in a constructor or setter method), it should be validated, not when a getter method is called. Creating a private validate* method for this is also a good idea.
private boolean validateThisValue(String a) {
return this.myValue != null && !this.myValue.isEmpty();
}
public void setThisValue(String a) {
if (validateThisValue(a)) {
this.myValue = a;
}
else {
// do something else
// in this example will be
this.myValue = "N/A";
}
}
And, in the getter method, never ever change the state of the object. I have worked on some projects, and the getter often must be made const: "this method cannot change internal state".
At least, if you do not want to complicate things, in the getter method, you should return "N/A" rather than change internal state and set myValue to "N/A".
I usually define a specific getter.
Never alter original getter:
public String getMyValue(){
return this.myValue
}
And create a specific getter:
public String getMyValueFormatted(){
if(this.myvalue == null || this.myvalue.isEmpty()){
return "N/A";
}else{
return this.myValue;
}
}
I think it's better to initialize this.myValue = "N/A". And subsequent calls to setMyValue should modify the this.myValue according to your business conditions.
The getMyValue shouldn't modify in any way this.myValue. If your needs are to return a certain value, you should return that value (like "N/A") and not alter this.myValue . Getters must not modify member's value.
I would change better the setter method so, if the value is null or empty, the N/A is assigned to the attribute. So, if you use the attribute in other methods inside the class (v.g. toString()) you will have the intended value there.
Alternatively, change the setter method to launch an exception when the value being set is not right, so the programmer is forced to improve its handling prior to setting the value.
Other than that, it is ok.
I do feel this is a bad practice unless and until you explain the reason why it is so necessary for you modify the object inside the getter method instead of doing it inside the setter method.
Do you feel this cannot be done for some reason? Could you please elaborate?
Do what ever you like. After all getters and setters are just another public methods. You could use any other names.
But if you use frameworks like Spring, you are bound to use those standard names and you should never put your custom codes inside them.
absolutely yes, it's a bad pratice.
Imagine you communicate accross network with a third party (remoting, COM, ...), this will increase the round-trip and then hit application performance.
A setter could modify as part of validation, but a getter should return the value and let the validation be done by the caller. If you do validate, then how should be documented.
This actually highly depends on the contract you want to enforce with your get()-method. According to design-by-contract conventions the caller has to make sure that the preconditions are met (which means doing a validation in a setter method often is actually bad design) and the callee (I do not know if that's the correct english term for that, i.e., the called one) makes sure that the post conditions are met.
If you define your contract so that the get()-method is not allowed to change the object then you are breaking your own contract. Think about implementing a method like
public isValid() {
return (this.myvalue == null || this.myvalue.isEmpty());
}
Advantage of this approach is that you do not have to check wether the return of your get() is "N/A" or something else. This also can be called before calling set() to validate that you do not insert illegal values into your object.
If you want to set a default value you should do that during initialization.
State changes in getters should be a hanging offence. It means that client code must be careful about the order in which it accesses getters and setters and to do this it must have knowledge of the implementation. You should be able to call the getters in any order and still get the same results. A related problem occurs when the setter modifies the incoming value depending on the current state of the object.
You can use some value holder for this purpose. Like Optional class in guava library.
If I have a class:
class Haha
constructor: (#lolAmount = 1) ->
alert #lolAmount
And I want to check if an object is of the right class, Is it always safe to use constructor.name:
haha = new Haha()
unless haha.constructor.name is 'Haha'
throw Error 'Wrong type'
or is it better to use instanceof:
haha = new Haha()
unless haha instanceof Haha
throw Error 'Wrong type'
One argument I have for instanceof is when using extends:
class BigHaha extends Haha
bigHaha = new BigHaha
console.log bigHaha instanceof Haha #true
but how safe is it, being a JavaScript operator - I feel like I should be sceptical about it.
On the other hand, with constructor.name it is very clear what is happening. Is it guaranteed that constructor.name will be set on all objects?
Thanks for any info.
First of all, constructor is also straight JavaScript:
Returns a reference to the Object function that created the instance's prototype.
So when you say o.constructor, you're really doing straight JavaScript, the name constructor for the CoffeeScript object initialization function is a separate matter.
So now you have a choice between using JavaScript's constructor property or JavaScript's instanceof operator. The constructor just tells you what "class" was used to create the object, instanceof on the other hand:
[...] tests whether an object has in its prototype chain the prototype property of a constructor.
So instanceof is the right choice if you want to allow for subclassing.
I come from a C#/Java background which use a class based OO system and I don't get the JavaScript/CoffeeScript prototype OO system yet. I've written a CoffeeScript class below which allows me to display names for contacts according to a system-side preference. I can only get the class to work by making the joinNonEmpty(stringList, joinText) method belong to the prototype and calling it the way I would call a static method in Java/C# land.
Is there a way I can make this method call using this.joinNonEmpty(...)?
Can you shed some light on why I can reference the firstLastRender, lastFirstRender and firstOrNickThenLast methods in the constructor with this. but it doesn't work from those methods when calling the joinNonEmpty helper?
Does this have something to do with how I'm locating the appropriate method via the preference map?
prefs = displayNameFormat: "FirstOrNickThenLast"
class DisplayNameRenderer
constructor: ->
#prefToRenderMap =
FirstLast: this.firstLastRender
LastFirst: this.lastFirstRender
FirstOrNickThenLast: this.firstOrNickThenLast
# Why does this method have to be static (a class method)?
#joinNonEmpty: (stringList, joinText) ->
nonEmptyStrings = []
for s in stringList
nonEmptyStrings.push(s) if s isnt null and s isnt ""
nonEmptyStrings.join(joinText)
firstLastRender: (contact) ->
# TypeError: Object expected.
joinNonEmpty([contact.firstName, contact.lastName], ' ')
lastFirstRender: (contact) ->
# TypeError: Object doesn't support this method or property
this.joinNonEmpty([contact.lastName, contact.firstName], ', ')
firstOrNickThenLast: (contact) ->
# Works correctly.
DisplayNameRenderer.joinNonEmpty([(if contact.nickname isnt null and contact.nickname isnt "" then contact.nickname else contact.firstName), contact.lastName], ' ')
render: (contact) ->
#prefToRenderMap[prefs.displayNameFormat](contact)
contact = firstName: "Jonathan", nickname: "Jonny", lastName: "Appleseed"
dnr = new DisplayNameRenderer()
# => "Jonny Appleseed"
console.log dnr.render(contact)
Thanks for taking the time to answer.
this (AKA #) is determined when the function is called (with exceptions as below). So when you do this:
#prefToRenderMap =
FirstLast: this.firstLastRender
LastFirst: this.lastFirstRender
FirstOrNickThenLast: this.firstOrNickThenLast
You're storing unbound references to the three functions in the #prefToRenderMap instance variable and #prefToRenderMap is itself an object. Then you try to call the methods in your DisplayNameRenderer instance like this:
#prefToRenderMap[prefs.displayNameFormat](contact)
and everything falls apart because the methods are called in the wrong context and # isn't what they're expecting it to be. If prefs is 'FirstOrNickThenLast' then you're effectively doing this:
#prefToRenderMap.FirstOrNickThenLast(contact)
and # (AKA this) will be #prefToRenderMap inside the firstOrNickThenLast method. But, of course, #prefToRenderMap doesn't have any of the methods that you're trying to call so you get various errors.
One solution is to use the fat arrow (=>) to define the methods:
The fat arrow => can be used to both define a function, and to bind it to the current value of this, right on the spot.
So you could have things like this:
joinNonEmpty: (stringList, joinText) ->
#...
firstLastRender: (contact) =>
#joinNonEmpty([contact.firstName, contact.lastName], ' ')
and everything will work out. Here's a stripped down demo that will also show you your this problem:
http://jsfiddle.net/ambiguous/RAPJw/1/
You could also avoid this problem by referring to the methods by their names. Given a method name in a string, m = 'some_method', you can call that method like this o[m]() in both JavaScript and CoffeeScript and the result will be the same as if you said o.some_method(). A better structure would look more like this:
class DisplayNameRenderer
constructor: ->
#prefToRenderMap =
FirstOrNickThenLast: 'firstOrNickThenLast'
joinNonEmpty: (stringList, joinText) ->
#...
firstOrNickThenLast: (contact) ->
#joinNonEmpty([(if contact.nickname isnt null and contact.nickname isnt "" then contact.nickname else contact.firstName), contact.lastName], ' ')
render: (contact) ->
#[#prefToRenderMap['FirstOrNickThenLast']](contact)
Note the change to the structure of #prefToRenderMap and how it is used in render. And a demo of this approach: http://jsfiddle.net/ambiguous/DFYwu/
As an aside, instead of saying ClassName.class_method() inside an instance method, you can use the constructor property instead: #constructor.class_method(). Also, you usually say #method() or #property rather than this.method() and this.property in CoffeeScript.
I'm trying to build a way to do PHPUnit-style test case classes in CoffeeScript. I got very close to achieving my goal with these two classes:
QUnitTestCase.coffee:
class QUnitTestCase
constructor: (name) ->
module name
#setUp()
#runAllTests()
setUp: ->
return null
runAllTests: ->
for funcName, func of #
if funcName.substr(0, 4) is 'test' and typeof func is 'function'
testName = funcName.substr(4).charAt(0).toLowerCase() + funcName.substr(5)
#setUp()
test testName, func()
return null
CircleTest.coffee:
class CircleTest extends QUnitTestCase
constructor: ->
super "Circle"
setUp: ->
#mockCanvas = mock(Canvas)
#testObj = new Circle(#mockCanvas)
$('canvas').css display: 'none'
testDrawReturnsNull: =>
returned = #testObj.draw()
strictEqual null, returned, 'returns null'
However, QUnit blows up with this error: "Uncaught TypeError: Cannot read property 'assertions' of undefined (qunit.js line 666)".
Looking at QUnit, I believe somehow the Test object inside QUnit becomes mis-scoped when the function that defines the assertions is bound to the object. If I move the call to "test()" out to the child class, like this...
testDrawReturnsNull: =>
test "drawReturnsNull", =>
returned = #testObj.draw()
strictEqual null, returned, 'returns null'
... then QUnit runs fine without any errors, but this creates a kind of duplication that I don't really feel comfortable with. Perhaps someone who understands QUnit better can point me in the right direction.
Thanks.
You wrote
test testName, func()
instead of
test testName, func
That would seem to be a problem. You want to pass the function to test rather than running it first and passing the returned value.