Java Generics - Wildcards

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 wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

1. Wildcard Arguments With An Unknown Type:

The syntax for declaring this type of wildcard arguments is,
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.

Simple Wildcard With An Unknown Type 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

2. Upper Bounded Wildcards

To specify an upper bound for wildcards, use this syntax,
GenericType<? extends SuperClass>
This specifies that a wildcard argument can contain ‘SuperClass’ type or it’s subclasses. Remember that extends clause is an inclusive bound. i.e ‘SuperClass’ also lies in the bound.

Upper Bounded Wildcards Example

In the above example, if you want the processElements() method to work with only numbers, then you can specify an upper bound for wildcard argument.
import java.util.ArrayList;
import java.util.List;

/**
 * Wildcard Arguments With An Upper Bound Demo
 * @author javaguides.net
 *
 */

public class WildCardWithUpperBoundExample {
    static void processElements(List<? extends Number> a) {
        for (Object element : a) {
             System.out.println(element);
        }
    }

    public static void main(String[] args) {
         // ArrayList Containing Integers

         List<Integer> a1 = new ArrayList<>();

         a1.add(10);

         a1.add(20);

         a1.add(30);

         processElements(a1);

         // ArrayList Containing Longs

         List<Long> a2 = new ArrayList<>();

         a2.add(100L);

         a2.add(200L);

         a2.add(300L);

         processElements(a2);

          // Arraylist containing Doubles

         List<Double> a3 = new ArrayList<>();

         a3.add(21.35);

         a3.add(56.47);

         a3.add(78.12);

         processElements(a3);

         // Arraylist containing Strings

         List<String> a4 = new ArrayList<>();
 
         a4.add("One");

         a4.add("Two");

         a4.add("Three");

         // This will not work

         //processElements(a4); // Compile time error
    }
}
Output:
10
20
30
10
20
30
21.35
56.47
78.12

3. Lower Bounded Wildcards

A lower bounded wildcard is expressed using the wildcard character ('?'), following by the super keyword, followed by its lower bound: <? super A>.
GenericType<? super SubClass>

Lower Bounded Wildcards Example

import java.util.ArrayList;

import java.util.List;

/**
 * Wildcard Arguments With An Lower Bound Demo
 * @author javaguides.net
 *
 */

public class WildCardWithLoweroundExample {
    static void processElements(List<? super Integer> a) {
         for (Object element : a) {
             System.out.println(element);
         }
    }

    public static void main(String[] args) {
         // ArrayList Containing Integers

         List<Integer> a1 = new ArrayList<>();

         a1.add(10);

         a1.add(20);

         a1.add(30);

         processElements(a1);

         // ArrayList Containing Longs

         List<Long> a2 = new ArrayList<>();

         a2.add(100L);

         a2.add(200L);

         a2.add(300L);

         processElements(a2); // compiler error

     }
}

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