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Generics in Java 126
The use of wildcards above is necessary since objects of one type parameter cannot be converted to objects of
another parameter. Neither List nor List is a subtype of the other, even though Integer is a
subtype of Number. So, code that deals with List does not work with List. If it did, it would be
possible to insert a Number that is not a Integer into it, which violates type safety. Here is a sample code that
explains the contradiction it brings if List is a subtype of List:
List ints = new ArrayList();
ints.add(2);
List nums = ints; //valid if List is a subtype of List accordin
substitution rule.
nums.add(3.14);
Integer x=ints.get(1); // now 3.14 is assigned to an Integer variable!
The solution with wildcards works because it disallows operations that would violate type safety.
To specify the lower bounding class of a generic element, the super keyword is used. This keyword indicates that the
aforementioned generic type is a super-type of said bounding class. So, List