Variations on Generics - Wildcards

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Variations on Generics - Wildcards

Variations on Generics - Wildcards

In the documentation for a collections class, you may see some strange type parameters for methods or constructors, such as:

public boolean containsAll(Collection<?> c)
public boolean addAll(Collection<? extends E> c) 
public TreeSet(Comparator<? super E> comparator)

The question mark is a wildcard, indicating that the actual type is unknown. But, we at least know limits in the second two cases. The extends keyword in this usage actually means "is, extends, or implements" (which is the same criteria the instanceof operator applies). The super keyword means essentially the opposite: that the type parameter of the other class is, or is more basic than, this class's type. The usages with extends and super are called bounded wildcards.

This syntax only occurs when the variable is itself a generic class. The wildcards then state how that class's generic type relates to this class's type.

Why this is necessary leads down a long and winding path. To start, consider the following:

List<Employee> exEmps = new ArrayList<ExemptEmployee>();

This seems reasonable at first glance, but then consider if this line followed:

exEmps.add(new ContractEmployee());

Perfectly legal as far as the compiler is concerned, since ContractEmploye fits within the Employee type that the exEmps variable requires, but now we have a contract employee in a list instance that is supposed to hold only exempt employees. So, an instance of a class parameterized with a derived class is not an instance of that class parameterized with the base class, even though individual instances of the derived class can be used in the base-parameterized generic class; e.g., our List<Employee> can add individual exempt employees.

For the first wildcard case above, public boolean containsAll(Collection<?> c), it does no harm for us to see if our collection contains all the elements of some other collection that may contain an entirely unrelated type (but few, if any, of the items would compare as equal). Note that the contains method accepts an Object parameter, not an E, for this same reason.

The extends term in public boolean addAll(Collection<? extends E> c)means that the unknown class is, extends, or implements the listed type. For instance, we could add all the elements of an ArrayList<ExemptEmployee> to an ArrayList<Employee>. That makes sense, since we could add individual exempt employees to a basic employee collection. But, since we don't actually know what the parameterized type of the incoming collection is (it is the ? class), we cannot call any methods on that object that depend on its parameterized type. So we can't add to that collection; we can only read from it.

The super term is seen less often; it means that the parameterized type of the incoming collection must be of the same type or a more basic type. The TreeSet constructor can accept a Comparator for its actual type, or any type more basic (e.g., a Comparator<Object> can be used for a TreeSet of anything, since its compare method will accept any type of data). It is, however, likely that many "acceptable" comparators will end up throwing a ClassCastException at runtime if they can't actually compare the types involved. So, for example, if we had a Comparator<Employee> class that compared employee ids, we might still wish to use it in a TreeSet<ExemptEmployee>, where it would be perfectly valid (in fact, it would be an annoyance to have to write a special comparator for every employee type, if all the comparators did was compare the ids).