In the title I provided use-case for the following behavior: the field should be publicly mutable and immutable for the class's own methods. Consider also that I essentially want single line per field, because the number of fields may be great. Also I want to have no-arg constructor.
The nearest I can think of that could satisfy what you want is to have a trait which only has accessor methods, and an implementation class that allows mutations. Pass the class type around wherever you need to be able to alter values, but reference it only as the trait wherever the values should be unalterable.
Related
I have a class "Vertex" with 4 attributes and a class "Vertex_" with one attribute. The one attribute in Vertex_ is also in Vertex. Is it a good design to keep the two classes or is it better to program just the class Vertex, although there will be 3 variables, which are not used, when I instantiate an object which needs just the one attribute?
Class Vertex_ is actually somewhat a duplicate of Class Vertex.
I would suggest using inheritance and having Class Vertex inherit the attribute from the parent Class Vertex_ while having the 3 other attributes Class Vertex_ does not have.
TL;DR
This is a question that deserves a very long answer.There are two reasons for inheritance and the reason for doing it can depend on the language being used. One reason is for code reuse. Without knowing anything else about your situation, it would seem you are inheriting simply to reuse an attribute (but I suspect there could be more you will be reusing). But, there are other ways of getting code reuse without inheritance, for example containment, which is often a better way.
A powerful feature of object-oriented programming is the ability to substitute one type of object for another. When a message is sent to that object, the correct method implementation is invoked according the actual type of object receiving the message. This is one type of polymorphism. But in some languages the ability to substitute one object for another is constrained. In Java I can only substitute an instance of class B for an instance of class A if B is a descendant of A. So inheritance becomes important in Java to support polymorphism.
But what does it mean to be able to substitute a B instance for an A instance? Will it work? Class A has established a contract stating what each of its methods requires before you can successfully call it and at the same time states what each method promises to deliver. Will the methods of class B live up to that contract? If not, you really cannot substitute a B for an A and expect the program to run correctly. B may be a subclass of A but it is not a subtype of A (see Liskov substitution principle]).
In a language such as Python, inheritance is not required for polymorphism and coders are more apt to use it as code-reuse mechanism. Nevertheless, some people feel that subclassing should only be used to express subtyping. So, if Vertex_ is only using one of the four attributes it has inherited, I am doubtful that an instance of Vertex_ could be safely substituted for an instance of Vertex. I would not do the inheritance unless the language were C++ and then I would use private inheritance.
Scala case classes essentially capture a set of fields with helping methods.
How are case classes resolved? Are they expanded to different classes with fields or a generic class that contains, say, a HashMap<String, Field>?
If it's the latter, are case classes with single field more expensive than explicitly defined data objects?
Case classes are exactly the same as regular classes, except that they offer some additional convenience functions.
No, they are not backed by a map. What made you think that they are?
The documentation says this is allowed:
ClassMethod GetContacts() As %ListOfObjects(ELEMENTTYPE="ContactDB.Contact")
[WebMethod]
I want to do this:
Property Permissions As %ListOfObjects(ELEMENTTYPE="MyPackage.MyClass");
I get an error:
ERROR #5480: Property parameter not declared:
MyPackage.Myclass:ELEMENTTYPE
So, do I really have to create a new class and set the ELEMENTTYPE parameter in it for each list I need?
Correct syntax for %ListOfObjects in properties is this one
Property Permissions As list of MyPackage.MyClass;
Yes, a property does sometimes work differently than a method when it comes to types. That is an issue here, in that you can set a class parameter of the return value of a method declaration in a straightforward way, but that doesn't always work for class parameters on the class of a property.
I don't think the way it does work is documented completely, but here are some of my observations:
You can put in class parameters on a property if the type of the property is a data-type (which are often treated differently than objects).
If you look at the %XML.Adaptor class it has the keyword assignment statement
PropertyClass = %XML.PropertyParameters
This appears to add its parameters to all the properties of the class that declares it as its PropertyClass. This appears to be an example of Intersystems wanting to implement something (an XML adaptor) and realizing the implementation of objects didn't provide it cleanly, so they hacked something new into the class compiler. I can't really find much documentation so it isn't clear if its considered a usable API or an implementation detail subject to breakage.
You might be able to hack something this way - I've never tried anything similar.
A possibly simpler work around might be to initialize the Permissions property in %OnNew and %OnOpen. You will probably want a zero element array at that point anyway, rather than a null.
If you look at the implementation of %ListOfObjects you can see that the class parameter which you are trying to set simply provides a default value for the ElementType property. So after you create an instance of %ListOfObjects you could just set it's ElementType property to the proper element type.
This is a bit annoying, because you have to remember to do it every time by hand, and you might forget. Or a maintainer down the road might not now to do it.
You might hope to maybe make it a little less annoying by creating a generator method that initializes all your properties that need it. This would be easy if Intersystems had some decent system of annotating properties with arbitrary values (so you could know what ElementType to use for each property). But they don't, so you would have to do something like roll your own annotations using an XData block or a class method. This probably isn't worth it unless you have more use cases for annotations than just this one, so I would just do it by hand until that happens, if it ever does.
I'm defining a Scala class today, and I think "I need an equals method and a hashCode method; and a copy method would be handy too. I'll turn this into a case class." My class already has a bunch of other code, and is in no way trivial.
So fine, it all works and everything, but when the text books deal with case classes, all of the examples define them for use as value classes or 'data transfer objects'. Is it appropriate to define a non-trivial case class? Is the thought process described above OK, or do I need to think of case classes differently?
A case class provides, equals, hashCode and toString methods based on the main constructor parameters, all of which are turned into val too. In addition, the object companion gets an apply and an unapply methods, again based on the main constructor parameters.
Also, a case class inherits from Serializable and from Product, and should not be extended by other classes.
If all of these things are appropriate for your class, then feel free to declare it as a `case class'.
Feel free, provided it doesn't have descendants. Extending case classes is a bad idea.
As we know, Scala generates getters and setters automatically for any public field and make the actual field variable private. Why is it better than just making the field public ?
For one this allows swapping a public var/val with a (couple of) def(s) and still maintain binary compatibility. Secondly it allows overriding a var/val in derived classes.
First, keeping the field public allows a client to read and write the field. Since it's beneficial to have immutable objects, I'd recommend to make the field read only (which you can achieve in Scala by declaring it as "val" rather than "var").
Now back to your actual question. Scala allows you to define your own setters and getters if you need more than the trivial versions. This is useful to maintain invariants. For setters you might want to check the value the field is set to. If you keep the field itself public, you have no chance to do so.
This is also useful for fields declared as "val". Assume you have a field of type Array[X] to represent the internal state of your class. A client could now get a reference to this array and modify it--again you have no chance to ensure the invariant is maintained. But since you can define your own getter you can return a copy of the actual array.
The same argument applies when you make a field of a reference type "final public" in Java--clients can't reset the reference but still modify the object the reference points to.
On a related note: accessing a field via getters in Scala looks like accessing the field directly. The nice thing about this is that it allows to make accessing a field and calling a method without parameters on the object look like the same thing. So if you decide you don't want to store a value in a field anymore but calculate it on the fly, the client does not have to care because it looks like the same thing to him--this is known as the Uniform Access Principle
In short: the Uniform Access Principle.
You can use a val to implement an abstract method from a superclass. Imagine the following definition from some imaginary graphics package:
abstract class circle {
def bounds: Rectangle
def centre: Point
def radius: Double
}
There are two possible subclasses, one where the circle is defined in terms of a bounding box, and one where it's defined in terms of the centre and radius. Thanks to the UAP, details of the implementation can be completely abstracted away, and easily changed.
There's also a third possibility: lazy vals. These would be very useful to avoid recalculating the bounds of our circle again and again, but it's hard to imagine how lazy vals could be implemented without the uniform access principle.