Java Generics Wildcards Example

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Java Generics Tutorial

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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

Related Java Generics Examples

Java Generics Upper Bounded Wildcards Example
Java Generics Lower Bounded Wildcards Example
Java Generic Methods Examples
Java Generic Interface Examples
Java Generic Class Examples
Java Generics Multiple Type Parameters Example

Java Generics Wildcards Example

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Java Generics Tutorial

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