How can I add to List<? extends Number> data structures?

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I have a List which is declared like this :

 List<? extends Number> foo3 = new ArrayList<Integer>();

I tried to add 3 to foo3. However I get an error message like this:

The method add(capture#1-of ? extends Number) in the type List<capture#1-of ?
extends Number> is not applicable for the arguments (ExtendsNumber)

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You can't (without unsafe casts). You can only read from them.

The problem is that you don't know what exactly the list is a list of. It could be a list of any subclass of Number, so when you try to put an element into it, you don't know that the element actually fits into the list.

For example the List might be a list of Bytes, so it would be an error to put a Float into it.

Python Lists | Python Education, Python list is an ordered collection of elements. Python adding list elements We have three methods to add new elements to a list: append() , insert() , and  The List<T> class in C# and .NET represents a strongly typed list of objects. List<T> provides functionality to create a collection of objects, find list items, sort list, search list, and manipulate list items. In List<T>, T is the type of objects. The code examples in this article demonstrates how to add items to a List using C#.

"List '<' ? extends Number> is actually an upper bound wildcard !

The upper-bounded wildcard says that any class that extends Number or Number itself can be used as the formal parameter type: The problem stems from the fact that Java doesn’t know what type List really is. It has to be an EXACT and UNIQUE Type. I hope it helps :)

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You could do this instead:

  List<Number> foo3 = new ArrayList<Number>();      
  foo3.add(3);

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It has been confusing to me even though I read answers here, until I found the comment by Pavel Minaev:

Note that List < ? extends Number > does not mean "list of objects of different types, all of which extend Number". It means "list of objects of a single type which extends Number"

After this I was able to understand BertF awesome explanation. List < ? extends Number > means ? could be of any type extending Number(Integer, Double, etc) and its not clearified in declaration ( List < ? extends Number > list ) that which of them it is, so when u wanna use add method its not known if the input is of the same type or not; what is the type at all?

So the elements of List < ? extends Number > could only be set when constructing.

Also note this: When we're using templates we are telling the compiler what type we're messing with. T for example holds that type for us, but not ? does the same

I gotta say.. This is one of the dirty ones to explain/learn

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List<T>.Add(T) Method, Here are four different ways that data can be added to an existing list. The .​append() Method. Adding data to the end of a list is accomplished  Is there a short way to add List<> to List<> instead of looping in result and add new result one by one? Use List.AddRange (collection As IEnumerable (Of T)) method. It allows you to append at the end of your list another collection/list. Try using list.AddRange (VTSWeb.GetDailyWorktimeViolations (VehicleID2));

Add an item to a list in Python (append, extend, insert), Python list method append() appends a passed obj into the existing list. Syntax. Following is the syntax for append() method − list.append(obj). Parameters. obj −​ 

Five Ways to Add Data to a List in Python,

Comments
  • Note that List<? extends Number> does not mean "list of objects of different types, all of which extend Number". It means "list of objects of a single type which extends Number".
  • You better check PECS rule, Producer extends, Consumer super. stackoverflow.com/questions/2723397/…
  • Why are we allowed to write a sentence like ` List<? extends Number> foo3 = new ArrayList<Integer>();` if we can't an element to this list further?
  • @articlestack: Adding isn't the only useful thing to do with a list. Once a list is populated, reading from a list may be useful. Generic wildcards allows for writing code that works generically for a family of lists, e.g. works for both List<Integer> or List<Double>. For example, look at the signature of the Collection.copy(). The src List argument uses extends to read from the src list, while the the dest List argument uses super to write to the dest list. This allows one method that can copy from List<Integer> or List<Double> into List<Number> or List<Object>.
  • Bert F, sorry for a late comment. Is there a typo in your passage, "to add a value of type T (or subclass of T)" should read (or superclass of T)?
  • yea, i saw it too @Vortex Im not sure if it's right or I read it wrong.
  • @Vortex - or subclass of T is correct. For example, I cannot add a an Object (superclass of Number) to List<? super Number> foo3 because foo3 may have been assigned as: List<? super Number> foo3 = new ArrayList<Number> (which can contain only Number or subclasses of Number). <? super Number> refers to the types of List<>s that can be assigned to foo3 - not the kinds of things that can be added/read from it. The kinds of things that can be added/removed from foo3 must be things that can be added/removed from any kind of List<> that can be assigned to foo3.