Wildcard (Java)

In the Java programming language, the wildcard  is a special kind of type argument that controls the type safety of the use of generic (parameterized) types. It can be used in variable declarations and instantiations as well as in method definitions, but not in the definition of a generic type. This is a form of use-site variance annotation, in contrast with the definition-site variance annotations found in C# and Scala.

Covariance for generic types
Unlike arrays (which are covariant in Java), different instantiations of a generic type are not compatible with each other, not even explicitly. For example, the declarations  will cause the compiler to report conversion errors for both castings   and.

This incompatibility can be softened by the wildcard if  is used as an actual type parameter. is a supertype of all parameterizarions of the generic type. This allows objects of type  and   to be safely assigned to a variable or method parameter of type. Using  allows the same, restricting compatibility to   and its children. Another possibility is, which also accepts both objects and restricts compatibility to   and all its parents.

Wildcard as parameter type
In the body of a generic unit, the (formal) type parameter is handled like its upper bound (expressed with ;   if not constrained). If the return type of a method is the type parameter, the result (e.g. of type ) can be referenced by a variable of the type of the upper bound (or  ). In the other direction, the wildcard fits no other type, not even : If   has been applied as the formal type parameter of a method, no actual parameters can be passed to it. However, objects of the unknown type can be read from the generic object and assigned to a variable of a supertype of the upperbound.

Sample code for the  class:

Sample code that uses the  class:

Bounded wildcards
A bounded wildcard is one with either an upper or a lower inheritance constraint. The bound of a wildcard can be either a class type, interface type, array type, or type variable. Upper bounds are expressed using the extends keyword and lower bounds using the super keyword. Wildcards can state either an upper bound or a lower bound, but not both.

Upper bounds
An upper bound on a wildcard must be a subtype of the upper bound of the corresponding type parameter declared in the corresponding generic type. An example of a wildcard that explicitly states an upper bound is:

This reference can hold any parameterization of  whose type argument is a subtype of. A wildcard that does not explicitly state an upper bound is effectively the same as one that has the constraint, since all reference types in Java are subtypes of Object.

Lower bounds
A wildcard with a lower bound, such as

can hold any parameterization of  whose any type argument is both a subtype of the corresponding type parameter's upper bound and a supertype of.

Object creation with wildcard
No objects may be created with a wildcard type argument: for example,  is forbidden. In practice, this is unnecessary because if one wanted to create an object that was assignable to a variable of type, one could simply use any arbitrary type (that falls within the constraints of the wildcard, if any) as the type argument.

However,  is allowed, because the wildcard is not a parameter to the instantiated type. The same holds for.

In an array creation expression, the component type of the array must be reifiable as defined by the Java Language Specification, Section 4.7. This entails that, if the component type of the array has any type arguments, they must all be unbounded wildcards (wildcards consisting of only a ). For example,  is correct, while   is not.

For both cases, using no parameters is another option. This will generate a warning since it is less type-safe (see Raw type).

Example: Lists
In the Java Collections Framework, the class  represents an ordered collection of objects of type. Upper bounds are specified using : A  is a list of objects of some subclass of , i.e. any object in the list is guaranteed to be of type  , so one can iterate over it using a variable of type However, it is not guaranteed that one can add any object of type  to that list:

The converse is true for lower bounds, which are specified using : A  is a list of objects of some superclass of , i.e. the list is guaranteed to be able to contain any object of type  , so one can add any object of type  : However, it is not guaranteed that one can iterate over that list using a variable of type :

In order to be able to do both add objects of type  to the list and iterate over it using a variable of type , a   is needed, which is the only type of   that is both   and.

The mnemonics PECS (Producer Extends, Consumer Super) from the book Effective Java by Joshua Bloch gives an easy way to remember when to use wildcards (corresponding to Covariance and Contravariance) in Java.