So I’ve been following a unity course on Udemy and they were making a new list. But why is it:
List<int> number = new List<int>();
what is the purpose of the second part of the line. Doesn’t the first half already say that the list is of type array? I also don’t really get what the ‘new’ part does in this code.
Hope you can help, thanks in advance
While @Galaxy-1-Studios in general already answers it, i’d like to expand it a bit:
When you only write:
List<int> number;
number.add(1);
this will throw an error. A NullreferenceException. Reason for this is, that the variable number has no value assigned. It is null.
In general you can divide types of variables into 2 groups: value-Types and reference-types.
A value type is something like: bool, int, float, double …
A reference type is always an Object which when created will always be null
If we create a value type variable it automatically get’s assigned its default value. (In case of numbers i think always 0).
int i;
Debug.Log(i); //should print "0"
if we create a reference type variable like a List:
List<int> numbers;
Debug.Log(numbers); //should print "null"
So here we see, we actually have to create an object for any reference type:
List<int> numbers;
numbers = new List<int>(); //actually fill the variable "numbers" with a new object
numbers.add(1); //now this works
Now the question “why do i have to write new List<int>() once more” is actually a good one: Do we really have to?
List<int> numbers = default; //this works and should create the `default` for the given variable type
using default can be a thing here to shorten this if you want, but imo it is sometimes good to write it twice, helps to find errors.
Apart from that there is one reason where specifying this is important. Consider the following class structure:
class BaseClass {}
class SubClass : BaseClass {}
Now i can create a variable of type BaseClass but can fill in values of both classes:
BaseClass someVar;
someVar = new SubClass(); //create new SubClass object
someVar = new BaseClass(); //create new BaseClass object
both lines will work and when using inheritance properly this is a crucial part to understand. Thus you can see that there are some cases where it actually is important to explicitly write the new List<int>() as to specify what you actually want to have in your variable.
Those were a lot of words, hope it is clear and not confusing.
To break it down for you, List number declares the list name and the list type, so for instance you are creating an int list.
The second part actually creates the list using the “new” keyword. Here is an example of where this takes place when creating other things, not just lists:
GameObject myObj = new GameObject();
As you can see we are creating a GameObject, declaring the name and type. “New” is actually creating a new GameObject, and I guess the reason we add GameObject(); or “List” in your case at the end is just to specify the object we are creating. It doesn’t really make since if we just had GameObject myObj = new; So we are just declaring the object type twice.