Java Generics Tutorial

To specify an upper bound for wildcards, use this syntax,

GenericType<? extends SuperClass>

This specifies that a wildcard argument can contain ‘SuperClass’ type or its 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
     }
}