Questions tagged [unbounded-wildcard]

An unbounded wildcard is the type argument ?, a feature of generics in the java language. This type argument represents some "unknown" type which was once present.

A wildcard is useful as a type argument in a situation where knowing the exact type is unnecessary:

public static void printElements(Iterable<?> anyIterable) {
    for (Object element : anyIterable) {
        System.out.println( element );
    }
}

A type parameterized with a wildcard places restrictions on how an object can be used:

List<String> listOfString =
    new LinkedList<>( Arrays.asList("a String") );

// We can assign any List to a List<?>, but, in
// doing so, we lose information about its original
// type argument.
List<?> listOfUnknown = listOfString;

// The wildcard therefore causes methods that once
// returned String to now return Object.
Object unknown0 = listOfUnknown.get( 0 );

// Compiler error:
//  The wildcard prevents us from passing arguments
//  to any method that once accepted String.
listOfUnknown.add( new Object() );

// Compiler error:
//  Since we have lost knowledge of the original
//  type, we can not pass String to the List either.
//  (This is the case, even though we, as the programmer,
//  can see this would be safe to do.)
listOfUnknown.add( "another String" );

The bounded-wildcard ? extends Object is equivalent to the unbounded wildcard.

See also:

• Type Arguments of Parameterized Types in the Java Language Specification
• Official wildcards tutorial