Hey so I am new to Xcode and I have a few questions.
I want to make a Grade-book Application, that implements quite a few classes. For example, I created a class called "AssignmentList" which basically stores a list of assignment grades in the form of a double.
Another class, called "Course" has a NSMutableArray of AssignmentList objects.
If I wanted to retrieve an AssignmentList parameter for a course, my Java instinct would tell me to make a getter method like this:
-(AssignmentList)getAssignmentList(Course);
However, I can't seem to do that in Objective C, and only works when I do this:
-(id)getAssignmentList(Course);
How do I go about this? I wanted to create a UITableView listing all the AssignmentList objects of a Course, but I can't even set a UITableViewCell with the name of the AssignmentList because it won't let me use the getName method I made, because getName only works for an AssignmentList object, but the getAssignmentList method returns an (id).
Assuming an AssignmentList is a class with instances and not some kind of collection (e.g. an NSArray of Assignments), then your declaration needs to be something like:
-(AssignmentList *) getAssignmentList:(Course *) course;
The * is important: in Objective-C you don't really pass around objects, you pass around pointers to objects. (Well, that's true in Java, too, but you don't have to be explicit about it.)
When you declare something id that means pointer to something of unknown type so you don't need the *.
I know that for an instance variable all I have to do is put it inside the initialise method in the instance side and assign it a default value. But how I do this for class variable ? I tried to create an initialise method at class side but it did not give my variable a default value so I had to do this in one of my methods
pythonString ifNil:[pythonString := '']
But I don't like this approach.
I also found this for squeak , http://forum.world.st/Howto-initialize-class-variables-td1667813.html again I don't like this approach either. Is there a proper way of doing this. In Python it was fairly simple case of assignment why is it so cryptic for Pharo ?
First of all I hope that you are talking about instance variable of a class object (not a thing that you define on instance side as "class variable").
initialize is working, but it's being run upon instance creation. And instance (a class object) exists already when you define initialize method.
So when you define your class for the first time, you should run it by yourself e.g. YourClass initialize, bun later each time you load your class into system it should be initialised.
i have a class at smalltalk in pharo.
i create an instance in global dictionary which is smalltak,
Smalltalk at:#asd put: myobject new.
then i write this code to workspace, mistakely.
Smalltalk at:asd put: myobject new.
and this gives to me error, continuously,
when a press a key from keyboard, i get error my object does not understand, is this byteArray, like that.
how can i solve this issue?
or how can i reset global dictionary?
Ok, this is a fun question.
First of all I suggest you not to do things like that, because globals are usually used for storing classes in them.
Now the cause of the problem is that you don't have to declare vars in workspace, it will create a new war for every name you use. By default new var is nil, so what you've done was putting a nil key in the dictionary
To remove it execute this thing:
Smalltalk globals removeKey: nil ifAbsent: [ ]
Now, if you want to have a unique instance of you class, I'd suggest you using a singleton pattern. This implies defining class instance variable (e.g. default) and a class method:
default
^ default ifNil: [default := self new ].
This will create new instance first time you run it, and then keep it and return it whenever you want it later.
I have a custom class in Obj-C called RouteManager which contains an array of NSStrings. Each string is a bus stop name which is used as a key for a dictionary to get the rest of the information for the bus stop (basically, just [busStopDictionary allkeys]). In one of the situations where my app uses this array, I want to return the array sorted by the distance from the user. I've started setting up the code to be able to call sortedArrayUsingSelector on my array with the following method:
- (NSComparisonResult)compareByDistance:(NSString*) otherStop
{
// Return appropriate NSOrdered enum here based on comparison of
// self and otherStop
}
My problem is that in the case where compareByDistance is a method of RouteManager, self refers to the instance of RouteManager. However, I need self to refer to the NSString that the compare is being called on. So, I assumed I needed to setup a category, as such:
#interface NSString (Support)
-(NSComparisonResult) compareByDistance:(NSString*)otherStop;
#end
This got my self reference correct, however this comparison uses values from the RouteManager class. When implemented as seen above, the NSString (Support) implementation obviously complains that those values are undeclared.
That should provide enough background info for my question. How do I go about doing this? I would like my category of NSString, which consists solely of the method compareByDistance, to be able to use values from the current instance of my class, RouteManager, which inherits from NSObject. Ideally, I feel as though the category should somehow be within RouteManager. I feel there has to be some way to accomplish this that is cleaner than passing the necessary values into compareByDistance. Thanks in advance for any and all help.
Your best bet would be to define a custom class for a bus stop, instead of storing them as strings and dictionaries.
Make the BusStop class have properties for Name, Location and whatever else. Implement the compareByDistance: method on the BusStop class.
You can still use a dictionary if you need to look them up by name. Just store them with the name as the dictionary's key, and the BusStop object as the dictionary's value.
I've been hearing these two words used in Microsoft tutorials for VB.NET. What is the difference between these two words when used in reference to variables?
Value vis-a-vis Reference Types
Variables in C# are in 1 of 2 groups. Value types or Reference types. Types like int and DateTime are value types. In contrast, any class you create is a reference type. C# strings are also a reference type. Most things in the .NET framework are reference types.
Parts of a Variable
There is the variable name and its value. Two parts.
The variable's name is what you declare it to be. The value is what you assign to it.
Variables are Initialized
All variables are always given an initial value at the point the variable is declared. Thus all variables are initialized.
For value types, like int the compiler will give them a valid value if you do not do so explicitly. int's initialize to zero by default, DateTime's initialize to DateTime.MinValue by default.
Reference type variables initialize to the object you give it. The compiler will not assign an object (i.e. a valid value) if you don't. In this case the value is null - nothing. So we say that the reference is initialized to null.
Objects are Instantiated
Humans are born. Objects are instantiated. A baby is an instance of a Human, an object is an instance of some Class.
The act of creating an instance of a Class is called instantiation (Ta-Da!)
So declare, initialize, and instantiate come together like this
MyClass myClassyReference = new MyClass();
In the above, it is wrong to say "... creating an instance of an object..."
edit - inspired by comments discussion
Three distinct things are going on (above) using distinct terminology and that terminology is not interchangeable :
A reference variable is declared - MyClass myClassyReference
An object is instantiated (...from/of a given class, implied) - new MyClass()
The object is assigned to the variable. =.
Restating the facts:
A reference-type variable is also called simply "a reference". A "value-type variable" is not a reference.
This: "objectA is an instance of an object" is profoundly wrong. If objectA was "an instance of objectB" then it must be that objectA begins life with objectB's type - whatever that is - and current state - whatever that is. What about creating objects D, E, and F as objectB changes? Nay, nay! It is the conceptual and technical case the "objectA is an instance of a Class". "Instantiation" and "instance of" have precise meaning - an object gets its type, definitions, and values from a Class.
MyClass myClassyReference = null Generally we don't say "the variable is assigned to null" and we never say "the variable is referencing null", No. instead we say "the variable is null"; or "the variable is not referencing anything", or "the reference is null"
Practical Application:
I jab my finger at your code and say "this instance has an invalid property. Maybe that's why the loop fails. You gotta validate parameters during instantiation." (i.e. constructor arguments).
I see this in your code ,
MyClass myClassyReference;
myClassyReference.DoSomething();
"You declared the variable but never assigned it. it's null so it's not referencing anything. That's why the method call throws an exception."
end edit
The Unbearable Lightness of Being
A reference type variable's name and value exists independently. And I do mean independent.
An instantiated object may or may not have a reference to it.
An instantiated object may have many references to it.
A declared reference may or may not be pointing to an object.
A variable is initialized with a value. An object is instantiated when memory is allocated for it and it's constructor has been run.
For instance here is a variable:
Dim obj as Object
This variable has not been initialized. Once I assign a value to the obj variable, the variable will be initialized. Here are examples of initialization:
obj = 1
obj = "foo"
Instantiation is a very different thing but is related since instantiation is usually followed by initialization:
Dim obj As New Object()
In the preceding line of code, the obj variable is initialized with the reference to the new Object that was instantiated. We say that the new Object was instantiated because we have created a new instance of it.
Now I believe that VB.NET makes this a lot more confusing than C# because it is not clear that an assignment is taking place in the code above. In C# it is much clearer that there is both an instantiation of an instance and an initialization of a variable:
Object obj = new Object();
To initialize something is to set it to its initial value. To instantiate something is to create an instance of it.
Often this is the more or less same thing. This:
SqlConnection conn = new SqlConnection();
instantiates a SqlConnection object, and initializes the conn variable by setting it to the that instance.
Since an object's constructor also sets the object's properties to their default values, it's often correct to say that instantiating an object initializes it. (Misleading, if the object exposes a method that you have to explictly call to initialize it after it's instantiated, as is sometimes the case.)
*Instantiation means to create an instance for a class or object.Initialization means to *initiate the same object or class for any purpose.**
Instantiated means that an instance of the object has been created. Initiated means that that same object has done some initialization.
When you instantiate a class or object, you're creating a new instance of it, or allocating memory to "hold" one. Initializing that object would be the instructions that are performed during instantiation.
Instantiation is when you create an instance of a class. That instance is then an object, and you can set its properties, or call methods on it (tell it to do things).
Initiation is when you set up a set of initial conditions for something. That something might be an object, where you tell it to initiate itself, or just a variable to which you assign a value.
An object might initialise some other things, or even instantiate other objects as part of its initiation.
The difference is that instantiation is creation of a thing that can do stuff; initiation is stuff that gets done.
See the Java docs:
https://docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html
"Point originOne = new Point(23, 94);
Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
Instantiation: The new keyword is a Java operator that creates the object.
Initialization: The new operator is followed by a call to a constructor, which initializes the new object."
We can see it this way. For a line of code below:
var p = new Person();
The above line can be read as following two ways:
The variable p has been initialized as a person class
Person class has been instantiated in variable p
The subject of reference or context matters. When talking in terms of variable, we use the word initialize. When talking in terms of class/type, we use the word instantiate.
Instantiation refers to the allocation of memory to create an instance of a class whereas initialization refers to naming that instance by assigning the variable name to that instance.
Eg: SqlConnection conn = new SqlConnection();
Here new is a keyword which allocates memory for an instance and conn is a variable name assigned for that instance.
Others have explained the difference, so I wont go into detail. But there are cases where instantiation does not properly initialize an object. When you instantiate an object you also initialize it with some data. The class/type will have the initialization logic, whereas the instantiation logic is typically carried out by thenew keyword (basically memory allocation, reference copying etc). But instantiation need not necessarily result in a valid state for objects which is when we can say the object is uninitialzed. Here's a practical example where an object can be instantiated but not initialized (sorry e.g. in C#).
class P { string name = "Ralf"; }
WriteLine(new P().name); // "Ralf";
WriteLine((FormatterServices.GetUninitializedObject(typeof(P)) as P).name); // null
GetUninitializedObject doesn't call the constructor to instantiate object there (but some internal magic).
One could also argue value types are not instantiated but only initialized as it doesn't need new allocation when you do new.. but that's up to one's definition of instantiation.
In object-oriented parlance:
To instantiate means creating an object of some class, which initial state may be undefined.
The class is a blueprint which is used by the program to create objects. Objects created are compliant with the blueprint and can be manipulated by the program. E.g. variables current_client and previous_client can be assigned objects of class Customer. An instance of class X is an object instantiated from class X.
In the code the class is a permanent static description of what an object can do, but the objects themselves are temporary and dynamic. They have an individual state which can be changed (e.g. the Customer name, the associated orders). Instantiation can be done like this:
dim current_client as new Customer (VB)
Customer* current_client = new Customer() (C++)
current_client = Customer() (Python)
new Customer, new Customer() and Customer() are equivalent forms in different languages to trigger the instantiation.
In the end objects are destructed to release memory and other resources required for their existence and working.
To initialize means assigning an initial state to the object before it is used.
This initialization can be part of the instantiation process, in that case values are explicitly assigned to object attributes in the constructor of the object. Alternatively it can be left to the user who can decide whether it is required or not. The latter method allows faster instantiation, but requires the user's code to not read the value of any attribute before this code has explicitly assigned a value to this attribute. E.g. this code:
current_client.count = current_client.count + 1
is not allowed before the attribute count has been set by the user, since it can contain any initial value, including an invalid value which would trigger an execution error.