Java Generics Wildcards Example

In generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific).

The syntax for declaring a simple type of wildcard arguments with an unknown type,

GenericType<?>

The arguments which are declared like this can hold any type of objects. For example, Collection> or ArrayList> can hold any type of objects like String, Integer, Double etc.

Java Generics simple Wildcards Example

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedList;

/**
 * Wildcard Arguments With An Unknown Type
 * @author javaguides.net
 *
 */
public class WildCardSimpleExample {
	public static void printCollection(Collection<?> c) {
	    for (Object e : c) {
	        System.out.println(e);
	    }
	}

	public static void main(String[] args) {
		Collection<String> collection = new ArrayList<>();
		collection.add("ArrayList Collection");
		printCollection(collection);
		Collection<String> collection2 = new LinkedList<>();
		collection2.add("LinkedList Collection");
		printCollection(collection2);
		Collection<String> collection3 = new HashSet<>();
		collection3.add("HashSet Collection");
		printCollection(collection3);
		
	}
}

Output:

ArrayList Collection
LinkedList Collection
HashSet Collection