I don't feel satisfied by avoiding null in OOP.
Is there any alternative solution?
I don't like to avoid it in this way either.
What is the best possible way to handle it?
Java 8 has the new Optional type which is what I think you are asking about
A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.
Although you don't like it, the Null Object Pattern is one of the best ways to avoid null. Also never return null when you could return an empty list.
I favour adding a static class property of Empty. This returns a specific value for the object which can be compared against - like string.Empty in .NET
Related
I am adding an element into a MutableList and I want to know its index.
mutableList.add(foo)
Will the index of the most recently added element always be the last index mutableList.size - 1 like it is with ArrayList?
I don't want to use mutableList.indexOf(foo) because I believe it takes O(n). I'm not finding lot of documentation on these.
Yes, MutableList.add() always adds to the end of the list (this is a requirement for any class implementing the interface). I've filed an issue to say this explicitly in the documentation.
MutableList is merely an interface. How are you initializing the list?
Using Kotlin methods like mutableListOf() creates ArrayList objects by default.
You raise a question that has never occurred to me since from testing and experience I've found that all new elements added with the add method are placed at the end.
I believe that this is the case and won't change.
Although not mentioned in the documentation, this case holds. If it didn't then for sure it would be in the documentation. Just like Set where it says
A generic unordered collection
On JVM, MutableList is equivalent to java.util.List whose add documentation does specify
Appends the specified element to the end of this list
I need to expose the "is mapped?" state of an instance of a class. The outcome is determined by a basic check. It is not simply exposing the value of a field. I am unsure as to whether I should use a read-only property or a method.
Read-only property:
public bool IsMapped
{
get
{
return MappedField != null;
}
}
Method:
public bool IsMapped()
{
return MappedField != null;
}
I have read MSDN's Choosing Between Properties and Methods but I am still unsure.
The C# standard says
§ 8.7.4
A property is a member that provides access to a characteristic of an object or a class. Examples of properties include the length of a string, the size of a font, the caption of a window, the name of a customer, and so on. Properties are a natural extension of fields. Both are named members with associated types, and the syntax for accessing fields and properties is the same. However, unlike fields, properties do not denote storage locations. Instead, properties have accessors that specify the statements to be executed when their values are read or written.
while as methods are defined as
§ 8.7.3
A method is a member that implements a computation or action that can be performed by an object or class. Methods have a (possibly empty) list of formal parameters, a return value (unless the method’s return-type is void ), and are either static or non-static.
Properties and methods are used to realize encapsulation. Properties encapsulate data, methods encapsulate logic. And this is why you should prefer a read-only property if you are exposing data. In your case there is no logic that modifies the internal state of your object. You want to provide access to a characteristic of an object.
Whether an instance of your object IsMapped or not is a characteristic of your object. It contains a check, but that's why you have properties to access it. Properties can be defined using logic, but they should not expose logic. Just like the example mentioned in the first quote: Imagine the String.Length property. Depending on the implementation, it may be that this property loops through the string and counts the characters. It also does perform an operation, but "from the outside" it just give's an statement over the internal state/characteristics of the object.
I would use the property, because there is no real "doing" (action), no side effects and it's not too complex.
I personally believe that a method should do something or perform some action. You are not performing anything inside IsMapped so it should be a property
I'd go for a property. Mostly because the first senctence on the referenced MSDN-article:
In general, methods represent actions and properties represent data.
In this case it seems pretty clear to me that it should be a property. It's a simple check, no logic, no side effects, no performance impact. It doesn't get much simpler than that check.
Edit:
Please note that if there was any of the above mentioned and you would put it into a method, that method should include a strong verb, not an auxiliary verb like is or has. A method does something. You could name it VerifyMapping or DetermineMappingExistance or something else as long as it starts with a verb.
I think this line in your link is the answer
methods represent actions and properties represent data.
There is no action here, just a piece of data. So it's a Property.
In situations/languages where you have access to both of these constructs, the general divide is as follows:
If the request is for something the object has, use a property (or a field).
If the request is for the result of something the object does, use a method.
A little more specifically, a property is to be used to access, in read and/or write fashion, a data member that is (for consuming purposes) owned by the object exposing the property. Properties are better than fields because the data doesn't have to exist in persistent form all the time (they allow you to be "lazy" about calculation or retrieval of this data value), and they're better than methods for this purpose because you can still use them in code as if they were public fields.
Properties should not, however, result in side effects (with the possible, understandable exception of setting a variable meant to persist the value being returned, avoiding expensive recalculation of a value needed many times); they should, all other things being equal, return a deterministic result (so NextRandomNumber is a bad conceptual choice for a property) and the calculation should not result in the alteration of any state data that would affect other calculations (for instance, getting PropertyA and PropertyB in that order should not return any different result than getting PropertyB and then PropertyA).
A method, OTOH, is conceptually understood as performing some operation and returning the result; in short, it does something, which may extend beyond the scope of computing a return value. Methods, therefore, are to be used when an operation that returns a value has additional side effects. The return value may still be the result of some calculation, but the method may have computed it non-deterministically (GetNextRandomNumber()), or the returned data is in the form of a unique instance of an object, and calling the method again produces a different instance even if it may have the same data (GetCurrentStatus()), or the method may alter state data such that doing exactly the same thing twice in a row produces different results (EncryptDataBlock(); many encryption ciphers work this way by design to ensure encrypting the same data twice in a row produces different ciphertexts).
If at any point you'll need to add parameters in order to get the value, then you need a method. Otherwise you need a property
IMHO , the first read-only property is correct because IsMapped as a Attribute of your object, and you're not performing an action (only an evaluation), but at the end of the day consistancy with your existing codebase probably counts for more than semantics.... unless this is a uni assignment
I'll agree with people here in saying that because it is obtaining data, and has no side-effects, it should be a property.
To expand on that, I'd also accept some side-effects with a setter (but not a getter) if the side-effects made sense to someone "looking at it from the outside".
One way to think about it is that methods are verbs, and properties are adjectives (meanwhile, the objects themselves are nouns, and static objects are abstract nouns).
The only exception to the verb/adjective guideline is that it can make sense to use a method rather than a property when obtaining (or setting) the information in question can be very expensive: Logically, such a feature should probably still be a property, but people are used to thinking of properties as low-impact performance-wise and while there's no real reason why that should always be the case, it could be useful to highlight that GetIsMapped() is relatively heavy perform-wise if it in fact was.
At the level of the running code, there's absolutely no difference between calling a property and calling an equivalent method to get or set; it's all about making life easier for the person writing code that uses it.
I would expect property as it only is returning the detail of a field. On the other hand I would expect
MappedFields[] mf;
public bool IsMapped()
{
mf.All(x => x != null);
}
you should use the property because c# has properties for this reason
If I got
Dim myRect As Rectangle = New Rectangle(0,0,100,100)
Is it necessary or just fine to later do this:
myRect = Nothing
Or it isn't necessary? Thank you.
IF it is necessary, are there other cases it isn't for my variables?
In general, as Joel said, it's unnecessary.
In your specific example, though, it is actually pointless. Rectangle is a value type, so setting it to Nothing is not even affecting an object's reference count; it's assigning a new value (the default value for Rectangle) to your myRect variable. This is analogous to having an Integer variable and setting it to 0 at the end of a method. Doesn't buy you anything.
I should point out that the claim "Setting any variable to Nothing [or null in C#] never accomplishes anything"* is a myth. It is entirely possible that you may have a field in a class which you might as well set to null if the object referenced is no longer needed but you still have a reference to the class instance itself.
As a simplistic example, suppose you had some container class which wraps a T[] array, and you give this container an Empty method. It might make sense to set the container's internal array to null in this method, which would result in zero references to the array object, qualifying it for garbage collection. (You would then create a new array when external code next tried to add a T to the collection.) If you did not set the field to null on Empty, then there would still be a reference to the array (i.e., the field), and so that would be a small amount of memory being used that you really don't need.
Like I said, that's a simplistic example. And honestly, it's rare that you ever need to consider scenarios like that. I just thought I would mention it so that you don't get the wrong impression that setting a field to Nothing literally never accomplishes anything.
*I'm not actually quoting anyone specific here; this is just an over-generalization I've heard stated more than once.
Don't do it. This was important to do in the vb6 days, but for .Net it's possible (though very unlikely) for it to actually throw the garbage collector off and have other unexpected side effects.
In the database you have a table with a bit field, let call that field Active
In the application you have a variable boolean, let call it NotActive
Everytime you get the field from the table, in the application you have switch the meaning of the variable.
NotActive = !mytable.active;
Another example would be a bit field in the database named Enable__yes__no and in the code you do
control.enabled = !mytable.Enable_yes_no
best practice would be to keep the same name and the same meaning, but the pattern above, how to you call that?
I wouldn't name boolean variables with a negative prefix.
Name the variable IsActive or Active, naming it NotActive is double negation.
Edit/Clarification:
If you need to check if the thing is active, you need a double negation:
If (!NotActive) { DoSomething() }
Positive boolean Variable names are much easier to understand:
If (isActive) { DoSomething() }
Obfuscation by design?
Backward compatibility with existing databases?
It's widely known as the "not-not-negative spaghetti confusion pattern" and was first mentioned 1972. ;-) SCNR
I think the problem here is in the architecture, not in the specific naming of the data. For instance, if you used an entity framework, then your entity for this table could declare a property called InActive, and it could use the Active column as the datastore. As far as the outside world is concerned, the translation back and forth is transparent.
Is it normal to modify setter arguments? Let's imagine that we have setString method. And we really want to keep a trimmed form of the string. So a string with trailing spaces is invalid, but we don't want to throw an exception.
What's the best solution? To trim the value in the setter e.g.
public void setString(String string) {
this.string = string.trim();
}
Or trim it in the caller (more than once) e.g.
object.setString(string.trim());
Or maybe something else?
Yes. After all, setters are designed for these kind of things! To control and sanitize the values written to fields ;)
Totally. Here's an example: suppose you have an engineering programs with different types of measurement units. You keep the internal values in one measurement system, but you convert from all others in the setter, and convert back in the getter, e.g.:
public double UserTemperature
{
get
{
return Units.Instance.ConvertFromSystem(UnitType.Temperature, temperature);
}
set
{
double v = Units.Instance.ConvertToSystem(UnitType.Temperature, value);
if (temperature != v)
{
temperature = v;
Changed("SystemTemperature");
Changed("UserTemperature");
}
}
}
Yes, sure. Just be careful to check for NULL before applying any method (such as trim()).
There are two schools: one says its ok to check param in setter(school style) and second one says beans should not contain any logic and just data(enterprise style).
I believe more in the second one. How often do you look at implementation of your beans? should getUser throw any exception or just return null?
When you put logic in your setter and getters you make it harder to understand whats going on since many people will never look at its implementation. If you disagree I urge you to look at every setter and getter implementation before you use it just to check if its not just a bean.
At first glance it seems like it violates the principle of least astonishment. If I'm a user of your class, I'd expect a setter to do exactly what I tell it to. I'd throw an exception in the setter to force users to trim the input.
The other (better?) alternative is to change the name of the method to trimAndSetString. That way, it's not surprising behavior to trim the input.
Correct me if I'm wrong, but it looks logical to me that the setter should hold this kind of logic.
If the setter is just assigning some value to an internal var without checking it, then why not expose the var itself?
This is exactly why you use setters rather than exposing the objects fields to the whole wide world.
Consider a class that holds an integer angle that's expected to be between 0 and 359 inclusive.
If you expose the field, calling functions can set it to whatever they want and this would break the contract specified by your API. It's also likely to break your functionality somewhere down the track because your code is written to assume a certain range for that variable.
With a setter, there's a number of things you can do. One is to raise an exception to indicate an invalid value was passed but that would be incorrect in my view (for this case). It's likely to be more useful if you modify the input value to something between 0 and 359 such as with:
actualVal = passedValue % 360;
As long as this is specified in your interface (API), it's perfectly valid. In fact, even if you don't specify it, you're still free to do whatever you want since the caller has violated the contract (by passing a value outside of range). I tend to follow the rule of "sanitize your input as soon as possible".
In your specific case, as long as you specify that the string is stored in trimmed format, there's no reason for callers to complain (you've already stated that such a string is invalid). It's better in terms of code size (not speed) to do it in the setter rather than at every piece of code that calls the setter. It also guarantees that the string is stored as you expect it to be - there's no guarantee a caller won't accidentally (or purposefully) store a non-trimmed string.
Yes. It is a feature of object oriented design is that the caller can treat your class as a black box. What you do inside is your own business, as long as the behavior of the interface is documented and logical.
While different people have different philosophies, I would suggest that property setters are only appropriate in cases where they will set an aspect of the object's state to match the indicated value and perhaps possibly notify anyone that cares about the change, but will not otherwise affect the object's state (it is entirely proper for a property setter to change the value of a read-only property if that property is defined in terms of the state associated with the property setter; for example, a control's read-only Right property may be defined in terms of its Bounds). Property setters should throw an exception if they cannot perform the indicated operation.
If one wishes allow a client to modify an object's state in some fashion not meeting the above description, one should use a method rather than a property. If one calls Foo.SetAngle(500) it would be reasonably expected that the method will use the indicated parameter in setting the angle, but the Angle property might not return the angle in the same form as it was set (e.g. it might return 140). On the other hand, if Angle is a read-write property, one would expect that writing a value of 500 would either be forbidden or else would cause the value to read back 500. If one wanted to have the object store an angle in the range 0 to 359, the object could also have a read-only property called BaseAngle which will always return an angle in that form.