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Is List a subtype of List<? extends Number> and why?

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逝去的感伤
逝去的感伤 2021-02-05 18:37

Here is what I know:

  1. Double is a subtype of Number and List is not a subtype of List
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  • 温柔的废话
    温柔的废话 (楼主)
    2021-02-05 18:49

    It helped me to see generics as constraints or contracts, not as types with subtypes. So a variable List var says: var is a list of some unknown type ?, which is constrained to be a subtype of Number.

    List listN;
List listD;
List listX;
...
Number n1 = ...;
Double d1 = ...;
...
listN.add(n1); // OK n1 is a Number
listN.add(d1); // OK d1 is a Double, which is a Number
listD.add(n1); // compile error, n1 is not a Double
listD.add(d1); // OK
listX.add(n1); // compile error, because the exact type of list is not known! (prevents putting a Dog in a Cat list)
listX.add(d1); // compile error, same cause

    So when you can't even put a Number into a List, whats the purpose of such a list? It allows you to work with lists of which the exact type does not matter for the task at hand:

    // instead of several exactly typed methods...
int count(List numberList) {...}
int count(List objectList) {...}
// ...etc. you can have one with a degree of freedom:
int count(List anyList) {...} // don't need to know the exact type of list

// instead of this...
Number sum(List numberList) {...}
Number sum(List doubleList) {...}
Number sum(List integerList){...}
// you can do this, with a little less freedom in the ?
Number sum(List list) {
  // the only thing we need to know about the list's type is that it is some number type
  ...
  Number ni = list.get(i);
  ...
}



    Using wildcards ? extends X allows to relax rigid contracts to weaker conditions.
    


    Using a named type parameter, you can establish constraints on allowed types between several variables:
    


    // works for any type T of list, whatever T is
// T is the same in the argument and in the return
 T pickObject(List list, int index) {
  return list.get(index);
}

// works for any type T of list, if T is a Number type
// T is the same in the argument and in the return
 T pickNumber(List list, int index) {
  return list.get(index);
}
...
List list;
Number n = pickNumber(list);

    
    
    
             
    
                                                        
    
            
            
    
              
                
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