Java Generics Wildcards Example

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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

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