A type argument for a parameterized type is not limited to a concrete class or interface. Java allows the use of type wildcards to serve as type arguments for parameterized types. Wildcards are type arguments in the form "?", possibly with an upper or lower bound. Given that the exact type represented by a wildcard is unknown, restrictions are placed on the type of methods that may be called on object of the parameterized type.
As an example of an unbounded wildcard, List indicates a list which has an unknown object type. Methods which take such a list as a parameter will accept any type of list as argument. Reading from the list will return objects of type Object, and adding null elements to the list is not allowed, since the element type is not known.
To specify the upper bound of a type wildcard, the extends keyword is used, which indicates that the type argument is a subtype of the bounding class. So List means that the given list contains objects of some unknown type which extends the Number class. For example, the list could be List or List. Reading an element from the list will return a Number, while adding null elements is once again not allowed.
The use of wildcards above adds flexibility since there is not any inheritance relationship between any two parameterized types with concrete type as type argument. Neither List nor List is a subtype of the other, even though Integer is a subtype of Number. So, any method that takes List as parameter does not accept an argument of List. If it did, it would be possible to insert a Number that is not an Integer into it, which violates type safety. Here is sample code that explains the contradiction it brings if List were a subtype of List:
The solution with wildcards works because it disallows operations that would violate type safety.
List nums = ints; // it is OK nums.add(3.14); // it is prohibitedTo specify the lower bounding class of a type wildcard, the super keyword is used. This keyword indicates that the aforementioned parameterized type is with a type argument which is a super-type of said bounding type. So, List could represent List or List. Reading from a list defined as List returns elements of type Object. Adding to such a list requires elements of type Number or any super-type of Number.
Read more about this topic: Generics In Java
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